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Cruz, Clebson dos Santos. "Propriedades Magnéticas de Magnetos Moleculares." Niterói, 2017. https://app.uff.br/riuff/handle/1/4020.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro
Com o desenvolvimento de novas tecnologias e os avanços nas técnicas de preparação de materiais, uma grande variedade de novos compostos puderam então ser sintetizados, dentre estes compostos estão os Magnetos Moleculares. Neste texto, apresentamos alguns fundamentos do magnetismo molecular, destacando o processo de construção de modelos para a descrição do comportamento magnético destes materiais através do ajuste dos dados da susceptibilidade magnética em função da temperatura. Buscamos compreender a maneira com que os diferentes parâmetros químicos e estruturais e afetam os mecanismos físicos que governam estes sistemas através do estudo de três séries de magnetos moleculares: um polímero bidimesional de Mn(II) sintetizado a partir do ácido 2,6-diclorobenzóico (C7H4Cl2O2), cujos parâmetros otimizados obtidos através do modelo sugerem que este composto possui um caráter global antiferromagnético; uma série de quatro compostos polinucleares de Cu(II) sintetizados com adenina (C5H5N5), cluster hexagonal heptanuclear ferromagnético e três cadeias antiferromagnéticas 2D dinucleares; uma série de quatro estruturas Metal-Orgânicas (Metal organic Frameworks-MOF) de Cu(II)-piperazina, onde foi feito um mapa das possíveis interações magnéticas para cada amostra. Por fim, como perspectiva para este trabalho pretendemos dar continuidade ao estudo de sistemas de magnetos moleculares dando ênfase à aplicação em informação quântica.
From the development of new technologies and advances in materials preparation techniques a wide variety of new compounds could be synthesized, among these compounds are the Molecular Magnets. In this paper, we present some fundamentals of molecular magnetism, highlighting the model-building process for the description of the magnetic behavior of these materials by fitting of the magnetic susceptibility as a function of temperature. Our goal is to understand how different chemical and structural parameters can affect the physical mechanisms that govern these systems . To achieve our aim we study three series of molecular magnets: a two-dimensional polymer Mn(II) synthesized from 2,6-acid dichlorobenzoic (C7H4Cl2O2), the optimized parameters obtained from the model suggest that this compound has an antiferromagnetic global character; a series of four polynuclear compounds of Cu(II) synthesized with adenine (C5H5N5), a ferromagnetic hexagonal cluster and three antiferromagnetic 2 D chains; a series of four Metal-Organic Frameworks (MOF) of Cu(II) -piperazine, where a magnetic interaction map was done for each sample. Finally, the perspective we intend to emphasize the study of molecular magnets systems with applications in quantum information.
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Dutra, Rangel Caio Quinino. "Magnetos moleculares de baixa dimensionalidade baseados em ligantes oxamato." Universidade Federal de Minas Gerais, 2008. http://hdl.handle.net/1843/BIRC-86EMC2.
Full textAbstract:
O presente trabalho trata da química e magnetismo de sistemas moleculares de baixa dimensionalidade. Inicialmente são descritos experimentos para a obtenção e caracterização de uma nova fase cristalina do composto [4-MeRad]2[Mn2{Cu(opba)}3], que apresenta fenômenos magnéticos não observados na literatura, como a Tc em torno de 13-15 K e um efeito de metamagnetismo induzido pela presença de excesso de solvente no composto. Foram estudados os compostos [4-MeRad]2[Mn2{Cu(opba)}3].5DMSO.2H2O e [4-MeRad]2[Mn2{Cu(opba)}3].4DMSO. 2H2O. Medidas de suscetibilidade AC nesse composto não apresentaram dependência com a freqüência. Apesar de ser verificada a abertura das curvas FCM e ZFCM em baixa temperatura, também verificou-se que curva da dependência da magnetização com o campo aplicado não apresenta histerese perceptível. Os valores de e da magnetização de saturação, se apresentaram próximos aos valores esperados para este sistema. Também é descrita a síntese com Fe2+, resultando na obtenção do composto Fe3Cu2(opba)3.9H2O, que apresenta um acoplamento antiferromagnético muito intenso revelado pelo valor de de 6,7 emu K mol-1 na tempertura do ambiente, indicando que nesta temperatura já há o efeito de cooperação antiferromagnética entre os spins do sistema. Na seqüência é estudada a influência da adição de cátions derivados do imidazol na química e magnetismo de sistemas constituídos por Cu(opba) e manganês. Foram obtidos e caracterizados os compostos inéditos IMes2Cu(opba).3H2O e IPr2Cu(opba).3H2O. O segundo apresentou um comportamento diamagnético, incomum para sistemas com o íon metálico Cu2+ em sua estrutura. Finalmente são descritos os procedimentos de inclusão de um segundo íon metálico (manganês e cobalto) que culminaram com a obtenção e caracterização do composto (IMes)2Mn{Cu(opba)}2.4H2O em que acredita-se que os grupos mesitil tenham sido capazes de evitar a coordenação nas carbonilas do opba, evitando assim o processo de polimerização.
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Alves, Álvaro Santos. "Estudo de magnetos moleculares através de cálculos de primeiros princípios." Niterói, 2017. https://app.uff.br/riuff/handle/1/3966.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Universidade Estadual de Feira de Santana
Neste trabalho, utilizamos cálculos de primeiros princípios baseados na Teoria do Funcional da Densidade (DFT - Density Functional Theory) para investigar a estabilidade energética e as propriedades magnéticas de compostos químicos. Foram estudados os complexos com metais de transição contendo radicais do tipo nitronilnitróxido [M(II)(Phtfac)2(NITpPy)2] [M = Co, Mn] e [M(II)(NITmPy)2(DMSO)2] [M = Cu, Ni, Co], bem como o silicato de cobre Na2Cu5(Si2O7)2. A partir do cálculo da energia total, determinamos a configuração de equilíbrio para os cinco complexos metálicos e os valores das constantes de acoplamento. Além disso, foram obtidas também as respectivas distribuições de densidade de magnetização, fundamentais para entender os mecanismos de acoplamento magnético nesse tipo de sistema. Para o silicato de cobre, cujos responsáveis pelas propriedades magnéticas são cadeias alternadas de trímeros e dímeros, calculamos a energia total das 32 configurações possíveis; determinamos o estado fundamental e as constantes de acoplamento magnético.
In this work we use first principles calculations based on density functional theory (DFT) to investigate the energy stability and the magnetic of chemical compounds. We studied the transition metal complexes with nitronyl nitroxide radical M(II)(P[htfac)2(NITpPy)2] [M = Co, Mn] and [M(NITmPy)2(DMSO)2] [M = Cu, Ni, Co], as well as the copper silicate Na2Cu5(Si2O7)2. By calculating the total energy, we determine the equilibrium configuration for the five metal complexes and the values of coupling constants. Moreover, we obtained their magnetization density distributions, fundamental for understanding the mechanisms of magnetic coupling in this type of system. For the copper silicate, whose magnetic properties are responsible for alternate chains of dimers and trimers, we calculate the total energy of the 32 possible configurations and determine the ground state and magnetic coupling constants.
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Almeida, Priscila Todero de 1988. "Estudo do tunelamento da magnetização em magnetos moleculares de Mn 12 via q-histerons." [s.n.], 2013. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278165.
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Orientador: Kleber Roberto PirotaDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: O principal objetivo desse trabalho consiste em uma nova abordagem para o tunelamento da magnetização do magneto molecular Mn12, embasado em uma ampliação do modelo de Preisach. Introduziremos novos operadores que levam em conta a possibilidade de efeitos quânticos. Implementamos esse novo modelo num programa de simulação que é capaz de simular curvas de histerese e curvas de relaxação magnética sem recorrer a resolução de hamiltonianas de spin. Além disso, este programa utiliza simulação estocástica, apresentando os resultados em poucos minutos. Os resultados obtidos concordam com os experimentos realizados de histerese e relaxação magnética. Apesar de ser um modelo de simulação simples, reproduz adequadamente a fenomenologia, pois introduz os dois ingredientes essenciais de um sistema com inversão da magnetização por efeito túnel termicamente ativado: a ativação térmica, descrita pela ocupação de níveis segundo a distribuição de Boltzmann e a possibilidade do efeito túnel descrita pelo modelo de Landau¿Zenner. A consistência física do modelo é estudada através da variação de parâmetros do modelo de forma sistemática
Abstract: The main objective of this work consists of a new approach concerning the tunneling of the magnetization of the molecular magnet Mn12, based on an extension of the Preisach model. We will introduce new operators that take into account the possibility of quantum effects. Thus, we have implemented this new model in a simulation software that is capable of simulating hysteresis curves and magnetic relaxation curves without utilizing resolution of spin Hamiltonians. Also, this program uses stochastic simulation, presenting the results in only a few minutes. The results obtained agree with the hysteresis and relaxation experiments. Despite being a simple simulation model, it adequately reproduces the phenomenology, because it introduces two key ingredients of a system with inversion of magnetization by thermally activated tunnel effect: the thermal activation, described by the occupation of levels according to the Boltzmann distribution and the possibility of tunnel effect described by the Landau-Zenner model. The physical consistency of the model is studied by systematically varying the model¿s parameters
Mestrado
Física
Mestra em Física
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Pires, Heber Silas. "Síntese e caracterização de precursores e magnetos moleculares contendo ligantes do tipo oxamato." Universidade Federal de Juiz de Fora (UFJF), 2008. https://repositorio.ufjf.br/jspui/handle/ufjf/5547.
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CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Este trabalho descreve a síntese e caracterização de três ligantes, N-(2-piridil)oxamato de etila (OPy), N-(3-piridil)oxamato de etila (MPy) e N-(4-piridil)oxamato de etila (PPy). Estes ligantes contêm pontes do tipo oxamato e encontram-se descritos na literatura; são isômeros e apresentam algumas propriedades em comum, como por exemplo, solubilidade. A partir destes ligantes, quatro novos precursores foram sintetizados e caracterizados, três deles contendo cátions tetrametilamônio, [Me4N]2[Cu(OPy)2].9H2O (MeCuOPy), [Me4N]2[Cu(MPy)2].2H2O (MeCuMPy) e [Me4N]2[Cu(PPy)2].4H2O (MeCuPPy), e um contendo o cátion sódio em sua composição, Na2[Cu(MPy)2].4H2O (NaCuMPy). Usando estes precursores, três cadeias inéditas foram obtidas, sendo elas CoCu(OPy)2.4H2O (CoCuOPy), CoCu(MPy)2.6H2O (CoCuMPy) e CoCu(PPy)2.7H2O (CoCuPPy). As cadeias são insolúveis na maioria dos solventes orgânicos e inorgânicos, decompondo-se em soluções ácidas. As propriedades magnéticas também foram investigadas sendo que os resultados indicaram que as cadeias comportam-se como sistemas magnéticos unidimensionais. Todos os compostos obtidos neste trabalho foram caracterizados por análise elementar, espectroscopia vibracional na região do IV e solubilidade. Os ligantes também foram caracterizados por ponto de fusão e ressonância magnética nuclear para carbono (RMN-13C) e hidrogênio (RMN-1H). Para os precursores, foi também utilizada espectroscopia de absorção na região do ultravioleta e visível. Para a determinação do número de moléculas de solvente, no caso dos precursores e das cadeias, foi utilizada análise térmica.
This work describes the synthesis and characterization of three ligands, ethyl N-(2-pyridyl)oxamate (OPy), ethyl N-(3-pyridyl)oxamate (MPy) and ethyl N-(4-pyridyl)oxamate (PPy). These ligands have oxamato bridges and are described in the literature; they are isomers and present some properties in common, for example, solubility. From these ligands, four novel precursors have been synthesized and characterized, three of them containing tetramethylamonium cations (Me4N+), [Me4N]2[Cu(OPy)2].9H2O (MeCuOPy), [Me4N]2[Cu(MPy)2].2H2O (MeCuMPy) and [Me4N]2[Cu(PPy)2].4H2O (MeCuPPy), and one containing the Na+ cation in its composition, Na2[Cu(MPy)2].4H2O (NaCuMPy). Using these precursors, three new chains have been obtained, CoCu(OPy)2.4H2O (CoCuOPy), CoCu(MPy)2.6H2O (CoCuMPy) and CoCu(PPy)2.7H2O (CoCuPPy). The chains are insoluble in most organic and inorganic solvents, decomposing in diluted acid solutions. The magnetic properties have been also investigated. The results indicated that the chains behave as onedimensional magnetic molecular systems. All compounds obtained in this work were characterized by elemental analysis, vibrational spectroscopy (IR) and solubility. The ligands were characterized also by melting point and nuclear magnetic resonance for carbon (NMR-13C) and hydrogen (NMR-1H). For the precursors, UV-Vis spectroscopy was used. To determine the number of solvent molecules in precursors and chains, thermal analysis has been used.
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Simoes, Tatiana Renata Gomes. "Síntese de magnetos moleculares à base de ligantes derivados de fenilenobis(oxamato) via estratégia metalosupramolecular." Universidade Federal de Minas Gerais, 2014. http://hdl.handle.net/1843/SFSA-9KDQ78.
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In this work, low and high dimensional molecular magnets were synthesized and characterized and their magnetic properties were investigated. Initially, reactions involving the precursor bis(2-pyridylcarbonyl) amidete of copper (II) [Cu(bpca)(H2O)2]+,
and its influence on species containing the ortho-phenylenebis(oxamate) group (opba) were investigated. Four compounds were obtained and characterized, being the X-ray structure of two of them determinated for the first time [Cu(bpca)(CF3SO3)(H2O)]·H2O and the chain {[Cu(bpca)(EtHopba)]}n . The magnetic study of {[Cu(bpca)]2[Cu(opba)(H2O)]}, [Cu2(bpca)2(H2opba)]2·3H2O and
{[Cu(bpca)(EtHopba)]}n showed antiferromagnetic coupling between the paramagnetic centers with J = -65.8, -2.36 and -0.17 cm-1, respectively. Afterwards, it was prepared a new spin-canted heterobimetallic 2-D network of formula (Bu4N)[Mn2{Cu(opba)}2ox], representing a unique example of a layered system containing copper(II) and
manganese(II) ions bridged by oxamate and oxalate groups. The intralayer antiferromagnetic interactions in the complex spin topology were evaluated through the analysis of the magnetic susceptibility data by the use of the QMC methodology which best-fit data was JCuMn = 32.5(3) cm1, e JMnMn = 2.7(3) cm1. On the third part
of this work three novel dinuclear compounds with the chemical formula
[Cu(opba)Cu(phen)(dmso)2].2H2O, [Cu(opba)Cu(dap)(H2O)].H2O and
[Cu(opba)Cu(dap-OH)(H2O)].2H2O with phen = 1,10 phenanthroline, dap = 1,3 diaminopropane and dap-OH = 2-hidroxi-1,3 diaminopropane have been synthesized and their crystal structures have been determined. The fiting of magnetic data revealed a strong antiferromagnetic coupling among the metal centers through
the oxamate bridge with J = -358.8, -366.0 e -392.1 cm-1 respectively. Finally, it was studied the solvent-driven preparation and structural characterization of the dinuclear compound [Cu(bipy)(H2mpba)]2·2H2O and the neutral chain [Cu(bipy)(H2mpba)]·dmso [bipy = 2,2'-bipyridine and H4mpba = N,N'-1,3-phenylenebis(oxamic acid)]. The
reversible interconversion between them is also presented and the magnetic studies reveals weak intradinuclear (J = -0.81 cm-1) and intrachain magnetic interactions (J = - 0.22 cm-1).Neste trabalho foram realizadas sínteses, caracterizações e estudos das propriedades magnéticas de magnetos moleculares de baixa e alta dimensionalidades. Primeiramente foram investigadas reações envolvendo o precursor bis(2-piridilcarbonil)amideto de cobre(II), [Cu(bpca)(H2O)2]+ e sua influência em espécies contendo o grupo orto-fenilenobis(oxamato) (opba). Quatro compostos foram obtidos e caracterizados, dentre eles dois cujas estruturas cristalinas são inéditas: [Cu(bpca)(CF3SO3)(H2O)]·H2O e a cadeia {[Cu(bpca)(EtHopba)]}n . O estudo magnético de {[Cu(bpca)]2[Cu(opba)(H2O)]}, [Cu2(bpca)2(H2opba)]2·3H2O e {[Cu(bpca)(EtHopba)]}n mostraram acoplamento antiferromagnético entre os centros paramagnéticos, com parâmetros J = -65,8, -2,36 e -0,17 cm-1, respectivamente. Posteriormente foi preparada a rede bidimensional heterometálica de fórmula (Bu4N)[Mn2{Cu(opba)}2ox], que representa o único exemplo de plano contendo íons de CuII, MnII conectados por grupos oxamato e oxalato. As interações antiferromagnéticas dos spins dentro das camadas foram analisadas através dos dados de suscetibilidade magnética, sendo caracterizado o spin-canting. O uso da metodologia QMC permitiu determinar os valores dos acoplamentos JCuMn = -32,5(3) cm-1, e JMnMn = -2,7(3) cm-1. Na terceira parte deste trabalho, foram sintetizados e caracterizados estruturalmente três novos compostos dinucleares: [Cu(opba)Cu(phen)(dmso)2]·2H2O, [Cu(opba)Cu(dap)(H2O)]·H2O e [Cu(opba)Cu(dap-OH)(H2O)]·2H2O; phen = 1,10-fenantrolina, dap = 1,3-diaminopropano e dapOH = 2-hidroxi-1,3-diaminopropano. O tratamento dos dados magnéticos mostrou forte acoplamento antiferromagnético intramolecular, com valores de J =-358,8, -366,0 e - 392,1 cm-1, respectivamente. Finalmente, estudou-se a síntese controlada pelos solventes do meio reacional para a preparação e caracterização estrutural de um composto dinuclear Cu(bipy)(H2mpba)]2·2H2O, e de uma cadeia neutra [Cu(bipy)(H2mpba)]·dmso [bipy = 2,2'-bipiridina, H4mpba = ácido N,N'-1,3- fenilenobis(oxâmico)]. Também é mostrada a interconversão reversível entre eles. O estudo magnético mostrou fracas interações magnéticas intradinuclear (J = -0,81 cm-1) e intracadeia (J = -0,22 cm-1).
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Rodrigues, Aline do Nascimento. "Estudo de sistemas magnéticos modeláveis mediante sub-redes." Universidade Federal de Sergipe, 2014. https://ri.ufs.br/handle/riufs/5362.
Full textAbstract:
We have modeled some magnetic systems, which consists of a number
of sublattices, in the mean field approximation. This is possible in crystalline
systems formed by two or more magnetic ions coupled by specific interactions
such as the crystal field, exchange, among others. The main idea is to solve the
microscopic Hamiltonian that models a given magnetic system in order to obtain
their magnetic equation of state: M (H, T). For this, we use the appropriate
arrangements to different magnetic (ferro-, ferri- and antiferro- magnetic)
scheme sublattices. From the solutions of the Hamiltonian (eigenvalues and
eigenvectors), physical quantities of interest were determined. In principle we
consider systems with localized magnetism due to 3d and 4f electrons with the
participation of non-magnetic ligands including 3d-4f systems with the presence
of crystal field. In this dissertation we use the model of two-and three sublattices
in order to obtain the equation of state for the following systems: RKKY
exchange in RNi2B2C, superexchange in (Y3-zRz)(T1xFe1-x)(T2yFe3-y)O12, LixFe3-
xO4 and (NixMn1-x)1.5[Cr(CN)6]. In these formulas, R represents a rare earth ion,
T1 and T2 represent non-magnetic ions. Some representative cases are
presented to illustrate the different equations of state and behavior of
sublattices, metamagnetism, temperature compensation, etc. The extension to
other similar systems can be direct or need to incorporate additional
phenomenological parameters.Consideramos neste estudo sistemas magnéticos modeláveis mediante sub- redes na aproximação do campo médio. Isto é possível em sistemas cristalinos formados por dois ou mais íons magnéticos acoplados por interações específicas como as do campo cristalino, troca, entre outros. A ideia central é resolver o hamiltoniano microscópico que modela um determinado sistema magnético de maneira a se obter sua equação de estado magnética: M(H,T). Para isto usamos o esquema de sub- redes adequado a diferentes arranjos magnéticos (ferro-, ferri- e antiferro- magnéticos). Com as soluções do hamiltoniano (autovalores e autovetores), grandezas físicas de interesse foram determinadas. Em princípio, consideramos sistemas com magnetismo localizado devido a elétrons 3d e 4f com participação de ligandos não magnéticos incluindo sistemas 3d-4f com presença de campo cristalino. Neste trabalho de dissertação empregamos o modelo de duas e três sub- redes para obter as equações de estado nos seguintes sistemas: troca tipo RKKY em RNi2B2C, supertroca em (Y3-zRz)(T1xFe1-x)(T2yFe3-y)O12, LixFe3-xO4 e (NixMn1- x)1.5[Cr(CN)6]. Nestas fórmulas, R representa um íon de terra rara, T1 e T2 representam íons não magnéticos. Alguns casos representativos são apresentados para exemplificar as diferentes equações de estado e comportamento das sub- redes, metamagnetismo, temperatura de compensação etc. Em princípio, a extensão para outros sistemas semelhantes, usando os modelos apresentados aqui, pode ser direta ou precisar incorporar parâmetros fenomenológicos adicionais.
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Krupskaya, Yulia. "Magnetic Properties of Molecular and Nanoscale Magnets." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-73289.
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The idea of miniaturizing devices down to the nanoscale where quantum ffeffects become relevant demands a detailed understanding of the interplay between classical and quantum properties. Therefore, characterization of newly produced nanoscale materials is a very important part of the research in this fifield. Studying structural and magnetic properties of nano- and molecular magnets and the interplay between these properties reveals new interesting effects and suggests ways to control and optimize the respective material. The main task of this thesis is investigating the magnetic properties of molecular magnetic clusters and magnetic nanoparticles recently synthesized by several collaborating groups. This thesis contains two main parts focusing on each of these two topics.
In the first part the fundamental studies on novel metal-organic molecular complexes is presented. Several newly synthesized magnetic complexes were investigated by means of different experimental techniques, in particular, by electron spin resonance spectroscopy. Chapter 1 in this part provides the theoretical background which is necessary for the interpretation of the effects observed in single molecular magnetic clusters. Chapter 2 introduces the experimental techniques applied in the studies. Chapter 3 contains the experimental results and their discussion. Firstly, the magnetic properties of two Ni-based complexes are presented. The complexes possess different ligand structures and arrangements of the Ni-ions in the metal cores. This difffference dramatically affffects the magnetic properties of the molecules such as the ground state and the magnetic anisotropy. Secondly, a detailed study of the Mn2Ni3 single molecular magnet is described. The complex has a bistable magnetic ground state with a high spin value of S = 7 and shows slow relaxation and quantum tunnelling of the magnetization. The third section concentrates on a Mn(III)-based single chain magnet showing ferromagnetic ordering of the Mn-spins and a strong magnetic anisotropy which leads to a hysteretic behavior of the magnetization. The last section describes a detailed study of the static and dynamic magnetic properties of three Mn-dimer molecular complexes by means of static magnetization, continuous wave and pulse electron spin resonance measurements. The results indicate a systematic dependence of the magnetic properties on the nearest ligands surrounding of the Mn ions.
The second part of the thesis addresses magnetic properties of nano-scaled magnets such as carbon nanotubes fifilled with magnetic materials and carbon-coated magnetic nanoparticles. These studies are eventually aiming at the possible application of these particles as agents for magnetic hyperthermia. In this respect, their behavior in static and alternating magnetic fifields is investigated and discussed. Moreover, two possible hyperthermia applications of the studied magnetic nanoparticles are presented, which are the combination of a hyperthermia agents with an anticancer drug and the possibility to spatially localize the hyperthermia effffect by applying specially designed static magnetic fifields.
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Henderson, John. "SPIN QUANTUM DYNAMICS IN MOLECULAR MAGNETS." Doctoral diss., University of Central Florida, 2009. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3535.
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Molecular magnets are ideal systems to probe the realm that borders quantum and classical physics, as well as to study decoherence phenomena in nanoscale systems. The control of the quantum behavior of these materials and their structural characteristics requires synthesis of new complexes with desirable properties which will allow probing of the fundamental aspects of nanoscale physics and quantum information processing. Of particular interest among the magnetic molecular materials are single-molecule magnets (SMMs) and antiferromagnetic (AFM) molecular wheels in which the spin state of the molecule is known to behave quantum mechanically at low temperatures. In previous experiments the dynamics of the magnetic moment of the molecules is governed by incoherent quantum tunneling. Short decoherence times are mainly due to interactions between molecular magnets within the crystal and interactions of the electronic spin with the nuclear spin of neighboring ions within the molecule. This decoherence problem has imposed a limit to the understanding of the molecular spin dynamics and the sources of decoherence in condensed matter systems. Particularly, intermolecular dipolar interactions within the crystal, which shorten the coherence times in concentrated samples, have stymied progress in this direction. Several recent works have reported a direct measurement of the decoherence time in molecular magnets. This has been done by eliminating the dephasing created by dipolar interactions between neighboring molecules. This has been achieved by a) a dilution of the molecules in a liquid solution to decrease the dipolar interaction by separating the molecules, and b) by polarizing the spin bath by applying a high magnetic field at low temperatures. Unfortunately, both approaches restrict the experimental studies of quantum dynamics. For example, the dilution of molecular magnets in liquid solution causes a dispersion of the molecular spin orientation and anisotropy axes, while the large fields required to polarize the spin bath overcome the anisotropy of the molecular spin. In this thesis I have explored two methods to overcome dipolar interactions in molecular magnets: a) studying the dynamics of molecular magnets in single crystals where the separation between magnetic molecules is obtained by chemical doping or where the high crystalline quality allows observations intrinsic to the quantum mechanical nature of the tunneling of the spin, and b) studying the electronic transport through an individual magnetic molecule which has been carefully placed in a single-electron transistor device. I have used EPR microstrip resonators to measure Fe17Ga molecular wheels within single crystals of Fe18 AFM wheels, as well as demonstrating, for the first time in a Single Molecule Magnet, the complete suppression of a Quantum Tunneling of the Magnetization transition forbidden by molecular symmetry.Ph.D.
Department of Physics
Sciences
Physics PhD
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Aliabadi, Azar. "ESR and Magnetization Studies of Transition Metal Molecular Compounds." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-195440.
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Molecule-based magnets (molecular magnets) have attracted much interest in recent decades both from an experimental and from a theoretical point of view, not only because of their interesting physical effects, but also because of their potential applications: e.g., molecular spintronics, quantum computing, high density information storage, and nanomedicine. Molecular magnets are at the very bottom of the possible size of nanomagnets. On reducing the size of objects down to the nanoscale, the coexistence of classical properties and quantum properties in these systems may be observed. In additional, molecular magnets exist with structural variability and permit selective substitution of the ligands in order to alter their magnetic properties. Therefore, these characteristics make such molecules suitable candidates for studying molecular magnetism. They can be used as model systems for a detailed understanding of interplay between structural and magnetic properties of them in order to optimize desired magnetic properties.
This thesis considers the investigation of magnetic properties of several new transition metal molecular compounds via different experimental techniques (continuous wave electron spin resonance (CW ESR), pulse ESR, high-field/high-frequency ESR (HF-ESR) and static magnetization techniques).
The first studied compounds were mono- and trinuclear Cu(II)-(oxamato, oxamidato)/bis(oxamidato) type compounds. First, all components of the g-tensor and the tensors of onsite ACu and transferred AN HF interactions of mononuclear Cu(II)- bis(oxamidato) compounds have been determined from CW ESR measurements at 10 GHz and at room temperature and pulse ELDOR detected NMR measurements at 35 GHz and at 20 K. The spin density distributions of the mononuclear compounds have been calculated from the experimentally obtained HF tensors. The magnetic exchange constants J of their corresponding trinuclear compounds were determined from susceptibility measurements versus temperature. Our discussion of the spin density distribution of the mononuclear compounds together with the results of the magnetic characterization of their corresponding trinuclear compounds show that the spin population of the mononuclear compounds is in interplay with the J values of their corresponding trinuclear compounds.
The second studied compounds were polynuclear Cu(II)-(bis)oxamato compounds with ferrocene and ferrocenium ligands. The magnetic properties of these compounds were studied by susceptibility measurements versus temperature to determine J values. In addition, the ESR technique is used to investigate the magnetic properties of the studied compounds because they contain two different magnetic ions and because only the ESR technique can selectively excite different electron spin species. These studies together with geometries of the ferrocenium ligands determined by crystallographic studies indicate that the magnetic interaction between a central Cu(II) and a Fe(III) ions changed from the antiferromagnetic coupling to the ferromagnetic coupling when a stronger distortion of the axial symmetry in the feroccenium cation exists. Therefore, the degree of the distortion of the feroccenium cation is a control parameter for the sign of the interaction between the central Cu(II) ion and the Fe(III) spins of the studied compounds.
The last two studied molecular magnets were a binuclear Ni(II) compound (Ni(II)-dimer) and a cube-like tetranuclear compound with a [Fe4O4]-cube core (Fe4-cube). HF-ESR measurements enabled us to determine the g-factor, the sign, and the absolute value of the magnetic anisotropy parameters. Using this information together with static magnetization measurements, the J value and the magnetic ground state of the studied compounds have been determined. In Ni(II)-dimer, two Ni(II) ions, each having a spin S = 1, are coupled antiferromagnetically that leads to a ground state with total spin Stot = 0. An easy plane magnetic anisotropy with a preferable direction for each Ni(II) ion is found. For Fe4-cube, a ground state with total spin Stot = 8 has been determined. The analysis of the frequency dependence and temperature dependence of HF-ESR lines reveals an easy axis magnetic anisotropy (Dcube = -22 GHz (-1 K)) corresponding to an energy barrier of U = 64 K for the thermal relaxation of the magnetization. These results indicate that Fe4-cube is favorable to show single molecular magnet (SMM) behavior.
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Inglis, Ross. "Oxime based manganese molecular magnets." Thesis, University of Edinburgh, 2010. http://hdl.handle.net/1842/4755.
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The synthesis and characterisation of a large family of hexametallic [MnIII 6] Single-Molecule Magnets with general formula [MnIII 6O2(R-sao)6(X)2(L)4-6] (where sao2- = dianion of salicylaldoxime; R = H, Me, Et, Ph; X = O2CR' (R' = H, Me, Ph etc), Hal , O2PHPh or O2P(Ph)2; L = solvent) are presented. Deliberate structural distortions of the [Mn3O] trinuclear moieties within the complexes are used to tune the observed magnetic properties. These findings highlight a qualitative magnetostructural correlation whereby the type (anti- or ferromagentic) of each Mn2 pairwise magnetic exchange is dominated by the magnitude of each individual Mn-N-O-Mn torsion angle. To shed further light on this intriguing family of nanomagnets, a large family of the analogous “half” molecules has been synthesised and fully characterised. These trimetallic [MnIII 3] complexes can be divided into three categories with general formulae (type 1) [MnIII 3O(R-sao)3(X)(sol)3-4] (where R = H, Me, tBu; X = O2CR (R = H, Me, Ph etc); sol = py and / or H2O), (type 2) [MnIII 3O(R-sao)3(X)(sol)3-5] (where R = Me, Et, Ph, tBu; X = O2CR (R = H, Me, Ph etc); sol = MeOH, EtOH and / or H2O), and (type 3) [MnIII 3O(R-sao)3(sol)3](XO4) (where R = H, Et, Ph, Naphth; sol = py, MeOH, -pic, Et-py, tBu-py; X = Cl, Re). In the crystals the ferromagnetic triangles are involved in extensive inter-molecular H-bonding which is clearly manifested in the magnetic behaviour, producing exchange-biased SMMs. These interactions can be removed by ligand replacement to give “simpler” SMMs. The [MnIII 6] and [MnIII 3] molecular nanomagnets are then exploited as building blocks to construct supramolecular architectures by means of host-guest interactions and coordination driven self-assembly. A number of discrete and infinite architectures based on the molecular triangle [Mn3] and various pyridyl-type ligands were obtained and structurally and magnetically characterised.
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Tancharakorn, Somchai. "Instrumentation development for studies of magnetic and structural properties of molecular magnets." Thesis, University of Edinburgh, 2008. http://hdl.handle.net/1842/9781.
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Tetramethyl ammonium manganese trichloride ([CH3)4N][Mn(II)Cl3]) known as TMMC, has been one of the most interesting systems in experimental magnetism due to its highly one-dimensional magnetic Heisenberg behaviour. The focus of this research programme was to study its magnetic and structural properties as a function of pressure. TMMC crystals were prepared by slow evaporation technique at room temperature; however it quickly became apparent that the material is only weakly magnetic and requires a pressure cell with a very low background. This discovery lead to the programme of instrumentation development for studies of weakly magnetic materials and gave a dualistic nature to the project. The first pressure cell developed was a piston-cylinder type cell for magnetic susceptibility measurements in a Magnetic Properties Measurement System (MPMS®) based on Superconducting Quantum Interference Device (SQUID) technology from Quantum Design, USA. It has been carefully designed in order to reduce the magnetic background. One way in which this has been achieved was through making the pressure cell symmetric with respect to the sample in order to provide an integrable response in the SQUID magnetometer. The cell was made of beryllium copper alloy which has a low background even at low temperature. The use of a multi-layered cylinder with the interference fit method has resulted in the increased strength of the cell and allowed larger sample volume. The use of Lamé equation and finite element method to calculate the change of the cell diameter or cell length as a function of internal pressure enables us to eradicate the need of superconductive manometer. The cell has been successfully tested up to a maximum pressure of 10 kbar. Further development of the cell has resulted in development of an electrical plug for in situ pressure measurement inside the pressure cell. This has been achieved by means of a manganin pressure sensor calibrated to provide pressure reading at any given temperature. For structural studies, a diamond anvil cell (DAC) was designed to conduct singlecrystal X-ray diffraction measurements at low temperature. The design was based on the well-known Merrill-Bassett DAC and on the design of the miniature DAC which has been developed for use within He-3 system in the Physical Properties Measurement System (PPMS®), Quantum Design. The cell has been tested down to liquid nitrogen temperatures with a cryostream cooling system and has shown a significant improvement compared to the standard pressure cells. The frost formation on the surface of the cell has slowed down significantly compared to the tests on the Merrill-Bassett cell, which led to a better quality diffraction pattern from the sample inside the cell. This result has been achieved due to the high thermal conductivity of the materials used in the construction and the minimisation of the DAC, which was effectively built around the Boehler-Almax diamond anvils. With the help of some of the high-pressure instruments mentioned above, highpressure properties of TMMC have been studied in this project. The structuremagnetism relationship was established from the results of magnetic and structural measurements under pressure. The magnetic susceptibility data helped to establish the change of the intrachain antiferromagnetic coupling constant as a function of pressure, while X-ray structures of TMMC were refined from ambient pressure to 17 kbar using a synchrotron X-ray diffraction technique. The structure of TMMC at room temperature was confirmed to be hexagonal. However, indirect evidence of the hexagonal-monoclinic structural phase transition was observed at above 17 kbar and room temperature. The combination of the magnetic and structural data has helped to establish that the interaction between high spin d5 metal orbitals (Mn(II)) in facesharing octahedral has a contribution from both direct exchange and superexchange interactions. The power-law relationship developed by Bloch was also observed in this system.
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Liu, Mingde. "Magnetization-steps spectroscopy in dilute magnetic semiconductors and in molecular magnetism /." Thesis, Connect to Dissertations & Theses @ Tufts University, 1998.
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Thesis (Ph.D.)--Tufts University, 1998 .Adviser: Yaacov Shapira. Submitted to the Dept. of Physics. Includes bibliographical references. Access restricted to members of the Tufts University community. Also available via the World Wide Web;
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Deumal, i. Solé Mercè. "Estudi teòric del magnetisme en cristalls moleculars: mecanismes d'interacció i empaquetament." Doctoral thesis, Universitat de Barcelona, 1998. http://hdl.handle.net/10803/667548.
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Els materials magnètics són molt importants en el desenvolupament de la tecnologia actual. Durant molts anys, la recerca científica en magnetisme ha estat dirigida a l'estudi de les propietats magnètiques dels elements i dels seus aliatges en funció de la temperatura, amb l'objectiu d'optimitzar el cost i la densitat magnètica utilitzable tecnològicament. El progrés en aquest camp ha estat obstaculitzat pel fet que el magnetisme a nivell molecular no s'entén d'un manera adequada. Aquest és el motiu per al qual, avui en dia, hi ha una clara tendència a estudiar els fenòmens magnètics a nivell microscòpic. El perquè d'aquesta tendència és molt senzill: si es controlessin els principis moleculars que regeixen el magnetisme, seria factible el disseny i la síntesi de nous materials magnètics.
Ara bé, d'entre tots els materials magnètics, el camp dels materials magnètics purament orgànics de base molecular és el més interessant des del punt de vista teòric, atès el grau de control que s'ha assolit en la seva síntesi i a la dificultat per entendre el mecanisme de la seva interacció magnètica. Aquest tipus de materials combinaran les propietats inherents dels compostos orgànics amb propietats magnètiques; per tant es podran usar en aparells òptics per manipular la llum polaritzada, dissenyar magnets que no siguin conductors elèctrics o crear possibles aplicacions biomèdiques, com per exemple agents de contrast selectiu en RMN o per dirigir l’aplicació de determinats medicaments.
Totes aquestes propietats i possibles aplicacions van motivar l’estudi d’una de les famílies de compostos més interessants i millor documentades en el camp del ferromagnetisme orgànic: la dels derivats alfa-nitronil-nitròxid. Totes aquestes molècules tenen electrons desaparellats (en general, son doblets), fet que permetrà que, en agrupar-se formant un sòlid, aquest solid pugui presentar magnetisme. Depenent de com s'agrupin, l'estat electrònic resultant serà d'alt spin (ferromagnètic) o de baix spin (anti-ferromagnètic). Així doncs, l'empaquetament cristal·lí d'aquests radicals lliures persistents decidirà si les interaccions ferromagnètiques intermoleculars es poden propagar arreu del cristall. En conseqüència, el primer objectiu d'aquest treball ha estat posar a punt un procediment sistemàtic per racionalitzar l'estructura de qualsevol cristall molecular. Aquesta racionalització implica, alhora, un estudi adequat a nivell MP2 de l'energia d'interacció dels contactes intermoleculars existents dins dels cristalls.
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Prescimone, Alessandro. "Effect of high-pressure on molecular magnetism." Thesis, University of Edinburgh, 2010. http://hdl.handle.net/1842/4619.
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The effect of pressure on a number of magnetically interesting compounds such as single-molecule magnets and dimeric copper and manganese molecules has been investigated to probe the validity of ambient magneto-structural correlations. The first chapter is an introduction to the equipment and methodologies that have been adopted to carry out the experimental high-pressure work. The second chapter reports the first combined high-pressure single crystal X-ray diffraction and high pressure magnetism study of four single-molecule magnets (SMMs). At 1.5 GPa the structures [Mn6O2(Et-sao)6(O2CPh(Me)2)2(EtOH)6] (1) – an SMM with a record effective anisotropy barrier of ~86 K – and [Mn6O2(Etsao) 6(O2C-naphth)2(EtOH)4(H2O)2] (2) both undergo significant structural distortions of their metallic skeletons which has a direct effect upon the observed magnetic response. Up to 1.5 GPa pressure the effect is to flatten the Mn-N-O-Mn torsion angles weakening the magnetic exchange between the metal centres. In both compounds one pairwise interaction switches from ferro- to antiferromagnetic, with the Jahn-Teller (JT) axes compressing (on average) and re-aligning differently with respect to the plane of the three metal centres. High pressure dc χMT plots display a gradual decrease in the low temperature peak value and slope, simulations showing a decrease in |J| with increasing pressure with a second antiferromagnetic J value required to simulate the data. The “ground states” change from S = 12 to S = 11 for 1 and to S = 10 for 2. Magnetisation data for both 1 and 2 suggest a small decrease in |D|, while out-of-phase (χM //) ac data show a large decrease in the effective energy barrier for magnetisation reversal. The third SMM is the complex [Mn3(Hcht)2(bpy)4](ClO4)3·Et2O·2MeCN (3·Et2O·2MeCN) that at 0.16 GPa loses all associated solvent in the crystal lattice, becoming 3. At higher pressures structural distortions occur changing the distances between the metal centres and the bridging oxygen atoms making |J| between the manganese ions weaker. No significant variations are observed in the JT axis of the only MnIII present in the structure. Highpressure dc χMT plots display a gradual decrease in the low temperature peak value and slope. Simulations show a decrease in J with increasing pressure although the ground state is preserved. Magnetisation data do not show any change in |D|. The fourth SMM, [(tacn)6Fe8O2(OH)12](ClO4)3.9Br4.1⋅6H2O, (4) is the largest inorganic compound ever studied at high-pressure. Up to 2.0 GPa the conformation of the complex remains largely unaffected, with the counter ions and water molecules moving around to accommodate a compression of the unit cell volume. High pressure magnetic susceptibility data collected up to 0.93 GPa confirm minimal changes in the intra-molecular exchange interactions. The third chapter focuses on three hydroxo-bridged CuII dimers: [Cu2(OH)2(H2O)2(tmen)2](ClO4)2 (5), [Cu2(OH)2(tben)2](ClO4)2 (6) and [Cu2(OH)2(bpy)2](BF4)2 (7) have been structurally determined up to 2.5, 0.9 and 4.7 GPa, respectively. 6 and 7 have never been reported before. Pressure imposes important distortions in the structures of all three complexes, particularly on the bond distances and angles between the metal centres and the bridging hydroxo groups. 5 undergoes a phase transition between 1.2 and 2.5 GPa caused by the loss of a coordinated water molecule. This leads to a loss of symmetry and dramatic changes in the molecular structure of the complex. The structural changes are manifested in different magnetic behaviours of the complexes as seen in dc susceptibility measurements up to ~0.9 GPa: J becomes less antiferromagnetic in 5 and 6 and more ferromagnetic in 7. The fourth chapter shows the compression of two oxo-bridged MnII/MnIII mixed valence dimers: [Mn2O2(bpy)4](ClO4)3⋅3CH3CN, (8) has been squeezed up to 2.0 GPa whilst [Mn2O2(bpy)4](PF6)3⋅2CH3CN⋅1H2O, (9) could be measured crystallographically up to 4.55 GPa. 9 has never been reported before, while 8 has been reported in a different crystallographic space group. The application of pressure imposes significant alterations in the structures of both complexes. In particular, in 8 the Mn-Mn separation is reduced by the contraction of some of the Mn-O bond distances, 9 shows essentially analogous behaviour: the Mn-Mn distance and nearly all the Mn-N bonds shrink significantly. The magnetic behaviour of the complexes has been measured up to 0.87 GPa for 8 and 0.84 GPa for 9, but neither display any significant differences with respect to their ambient data.
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Boeer, Angelika. "Anisotropy in Molecular Magnetism- Magnetic Exchange Coupling of Octahedral Cobalt (II) Ions." Thesis, University of Manchester, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.503050.
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Ghannadzadeh, Saman. "Investigating magnetism and superconductivity using high magnetic fields." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:4b78618e-89a3-424e-a673-59d363a2605d.
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This thesis investigates a number of transition-metal coordination polymers and iron-pnictide superconductors through the use of high magnetic fields, low temperatures, and on occasion, high pressures. The thesis will begin by describing my development of the proximity detector dynamic susceptometer, a novel technique that can be used for magnetometery and transport measurements in high magnetic fields. This technique is highly compact and has no moving parts, making it suitable for use in pressure cells, hence opening the way for a variety of new experiments. Through high-field magnetometery and other measurements, I will demonstrate that the pressure can be used to directly control the magnetic properties of the polymeric magnet CuF2(H2O)2(pyrazine). In particular, I observe a transition from quasi-two-dimensional to quasi-one-dimensional antiferromagnetism at 9~kbar, driven by the rotation of the Jahn-Teller axis. I will then present a series of measurements on two coordination polymers, showing how a small chemical difference can lead to drastically different magnetic properties. I show that [Cu(pyrazine)H2O(glycine)2]ClO4 is an excellent spin-chain, while the sister compound [Cu(pyrazine)(glycine)]ClO4 is a dimerised material that shows a spin-gap and is disordered down to very low temperatures, but then undergoes a field-induced phase transition to an ordered phase. I will also describe a series of pulsed-field measurements of the upper critical field of the iron-based superconductors NaFe1-xCoxAs across the whole of the doping phase diagram. It is shown that paramagnetic pair-breaking effects dominate the critical field when the field is parallel to the crystal planes. In the parent compound the paramagnetic limit is equal to that expected from BCS theory, but becomes significantly enhanced above the BCS limit upon doping. It is shown that the multi-band nature of the superconductivity leads to a convex curvature in the evolution of the critical field as the temperature is reduced.
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Krupskaya, Yulia [Verfasser], Bernd [Akademischer Betreuer] Büchner, and Rüdiger [Akademischer Betreuer] Klingeler. "Magnetic properties of molecular and nanoscale magnets / Yulia Krupskaya. Gutachter: Rüdiger Klingeler. Betreuer: Bernd Büchner." Dresden : Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://d-nb.info/1028864981/34.
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Morin, Brian Gerald. "Disorder in molecular magnets." The Ohio State University, 1994. http://rave.ohiolink.edu/etdc/view?acc_num=osu1343230971.
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Walsh, James Paul Slater. "Anisotropy in molecular magnetism." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/anisotropy-in-molecular-magnetism(11474b91-0d3d-4b0a-97cd-214d1713674e).html.
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A collection of studies are reported that focus on the examination of exchange interactions in complexes containing paramagnetic ions with a large magnetic anisotropy. A number of complementary techniques are used to analyse the complicated systems that arise, including high-field high-frequency electron paramagnetic resonance, inelastic neutron scattering, SQUID magnetometry, and ab initio calculations. The nuclearity of the complexes ranges from dimetallic, to trimetallic, to octametallic. A family of five water- and carboxylate-bridged nickel(II) dimetallics are the focus of a magneto-structural correlation study that succeeds in measuring the magnitude of the exchange interaction despite dominating effects from large zero-field splitting effects. Similar work is reported for four cobalt(II) analogues of these compounds, with the relationship between exchange interactions and geometry also being probed by pressure INS. Charge density studies that combine high resolution X-ray and neutron diffraction studies are reported on cobalt and nickel analogues from the same family of dimetallics, revealing strong evidence for non-direct exchange. A family of four trimetallic triangle complexes containing two nickel(II) ions and one chromium(III) ion bridged by a central fluoride and a total of six carboxylates are reported, and the exchange interactions are elucidated from a global model that accounts for the low-field magnetic, heat capacity, and EPR data. Two new octametallic vanadium(III) wheels—where each pair of adjacent metals are bridged by a fluoride and two carboxylates—are reported along with preliminary results from magnetic measurements and solid state proton NMR spectra, which reveal significant field-dependent effects arising from level crossings at high fields.
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Vela, Llausí Sergi. "Computational Modeling of Molecular Magnetic Materials." Doctoral thesis, Universitat de Barcelona, 2014. http://hdl.handle.net/10803/285357.
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Molecular materials have raised much interest in the last decades in the quest for new multifunctional devices. Among the multiple properties that those materials may present, one of the most typical is magnetism, which arises from the presence of unpaired electrons in the molecules that constitute the three-dimensional crystal. Magnetism has a macroscopic observable, the magnetic susceptibility (Ji), which is usually rationalized in terms of a set of JAB magnetic interactions between pairs of molecules. However, any experimental technique allows for such direct correspondence and, thus, the experimental interpretation of the magnetic properties usually requires further analysis from the point of view of computational chemistry.
Consequently, the present PhD thesis is a contribution to the computational modeling of molecule-based magnetic materials. Specifically, we describe how the tools of computational chemistry may be used in order to study those materials from different perspectives. With this aim in mind, we have applied computational chemistry techniques to rationalize the magnetic properties of several systems of interest, ranging from metal-organic compounds, based on Cu(II), to pure organic radicals based on the DTA and Benzotriazinyl building blocks, and including compounds based on the metal-radical synthetic approach, and also spin crossover materials based on Fe(II).
Along the thesis we have demonstrated that computational chemistry is a helpful discipline, capable to aid in the interpretation of experimental results and in the prediction of interesting properties, especially when working in close collaboration with experimentalists. In particular, the First Principles Bottom-Up (FPBU) procedure, extensively developed in our group, is a useful tool to rationalize the magnetic properties of any molecular magnetic material. To this purpose, the magnetic topology (i.e. the network of JAB within the crystal) is the key element. Regarding the magnetic topology, we have also demonstrated that it can be more intricate and complex than expected, and that it cannot be directly inferred from the coordination pattern of the molecule-based material. Therefore, the experimental assignation of the magnetic topology, by means of a fitting procedure, must be taken with caution.
About the JAB values, we have proved that they depend on temperature, and that this dependence may be especially important when working with organic radicals. On this class of materials, we have analyzed how the JAB values evolve with time, and seen that this evolution may involve huge fluctuations of their magnitude as a consequence of the thermal motion at finite temperatures. Interestingly, we demonstrate herein that, when the JAB values depend non-linearly with the thermal vibrations of a material, the standard static perspective of magnetism is not valid to fully understand their magnetic properties, and that it is then required to adopt a dynamic perspective.
Regarding the computational modeling of JAB values, we have seen that the combination of UB3LYP and the Broken-Symmetry approach yields JAB values, when transformed into the macroscopic observables, are in good agreement with experiment. In fact, we have demonstrated that, in order to predict the strength of a given JAB value, small distortions in the crystal structure can induce large variations, which may be much more important than the intrinsic error associated with the theoretical method employed. We have also observed that the counterions and diamagnetic ligands may have an important effect in defining the magnetic properties of a system. Overall, we have demonstrated that the magnetic topology and, thus, the macroscopic magnetic properties of a given material, cannot be understood without the proper knowledge of their crystal structure.Els materials moleculars han despertat molt d'interès en les últimes dècades degut a la seva possible aplicació en nous dispositius multifuncionals. Entre les diferents propietats que aquests materials poden presentar, una de les més típiques és el magnetisme, el qual sorgeix de la presència d’electrons desaparellats en les molècules que constitueixen el cristall tridimensional. El magnetisme té un observable macroscòpic, la susceptibilitat magnètica (Ji), que sol ser racionalitzada en termes microscòpics mitjançant el conjunt d'interaccions magnètiques JAB entre determinats parells de molècules. No obstant això, cap tècnica experimental permet aquesta correspondència directa i, per tant, la interpretació experimental de les propietats magnètiques sol requerir d’un posterior anàlisi des del punt de vista de la química computacional. La present tesi doctoral pretén doncs contribuir en el camp del magnetisme molecular i, més concretament, en com es poden utilitzar les eines de la química computacional per a modelitzar materials magnètics moleculars des de diferents perspectives. Amb aquest objectiu en ment, s’han racionalitzat les propietats magnètiques de diversos sistemes d'interès, que van des de compostos metal•lorgànics basats en ions de Cu(II) o de Co(II), radicals orgànics purs, compostos basats en l’estratègia sintètica de “metall-radical”, i finalment també materials de spin crossover basats en Fe(II). Al llarg de la tesi s'ha demostrat que la química computacional és una disciplina útil, capaç d'ajudar a la interpretació dels resultats experimentals i en la predicció de propietats interessants, especialment quan es treballa en estreta col•laboració amb els experimentadors. En particular, el procediment de primers principis Bottom-Up (FPBU, per les seves sigles en anglès), desenvolupat àmpliament en el nostre grup, és una eina útil per racionalitzar les propietats magnètiques de qualsevol material magnètic molecular. Per a aquest propòsit, la topologia magnètica (és a dir, la xarxa de JAB dins del cristall) és l'element clau. A més, hem analitzat diversos factors que afecten aquesta topologia magnètica, com els contraions, els radicals diamagnètics o l’efecte de la temperatura, mitjançant el la seva manifestació en les vibracions del cristall i en la contracció (expansió) que pateix al refredar-se (escalfar-se).
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Haque, Md Firoze. "CONTROLLED DEPOSITION OF MAGNETIC MOLECULES AND NANOPARTICLES ON ATOMICALLY FLAT GOLD SURFACES." Master's thesis, University of Central Florida, 2008. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/2109.
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In this thesis I am presenting a detailed study to optimize the deposition of magnetic molecules and gold nanoparticles in atomically flat surfaces by self-assembling them from solution. Epitaxially grown and atomically flat gold surface on mica is used as substrate for this study. These surfaces have roughness of the order one tenth of a nanometer and are perfect to image molecules and nanoparticles in the 1-10 nanometers range. The purpose of these studies is to find the suitable parameters and conditions necessary to deposit a monolayer of nano-substance on chips containing gold nanowires which will eventually be used to form single electron transistors by electromigration breaking of the nanowire. Maximization of the covered surface area is crucial to optimize the yield of finding a molecule/nanoparticle near the gap formed in the nanowire after electromigration breaking. Coverage of the surface by molecules/nanoparticles mainly depends on the deposition time and concentration of the solution used for the self-assembly. Deposition of the samples under study was done for different solution concentrations and deposition times until a self-assembly monolayer covering most of the surface area is obtained. Imaging of the surfaces after deposition was done by tapping-mode AFM. Analysis of the AFM images was performed and deposition parameters (i.e. coverage or molecule/particle size distribution) were obtained. The subjects of this investigation were a molecular polyoxometalate, a single-molecule magnet and functionalized gold nanoparticles. The obtained results agree with the structure of each of the studied systems. Using the optimized deposition parameters found in this investigation, single-electron transport measurements have been carried out. Preliminary results indicate the right choice of the deposition parameters.M.S.
Department of Physics
Sciences
Physics MS
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Moreno, Pineda Eufemio. "New f-block and mixed d,f-block molecular nanomagnets." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/new-fblock-and-mixed-dfblock-molecular-nanomagnets(2f53085a-081b-4b27-a866-28f37f1fd633).html.
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Molecular Nanomagnets have been proposed as plausible candidates in a variety of futuristic applications. Thorough understanding of the magnetic properties of these systems is therefore necessary to develop devices that include such units. The aim of this thesis is to synthesise and structurally and magnetically characterise a range of systems that could be used as elementary units in three proposed applications such as: data storage devices, magnetic refrigerants and qubits for quantum computing. A series of mixed 3d/4f metal complexes were synthesised through solvothermal reactions and characterised by X-ray single crystal diffraction and SQUID magnetometry. Through indirect methods it was possible to obtain high magnetic entropy change for some systems. It was also possible to obtain some insight into the magnetic interactions within the systems through modelling the magnetic data. The role of the 4f-4f and 3d-4f interactions in two sets of molecules is also described. The first study is in an asymmetric dysprosium dimer, where through a range of experimental techniques and advanced theoretical methods, such ab-initio calculations we are able to explain the role of the intramolecular interactions and their effect on the SMM properties of this system. Similarly, insight into the role of the 3d-4f interactions is achieved through the observation of the magnetic behaviour of a family of 27 tetranuclear systems, though SQUID data and ab-initio calculations. Finally, chemical functionalization of a well-proposed qubits, namely {Cr7Ni} and subsequent reaction with a redox active metal ion, CoII/III, two {Cr7Ni} systems are linked. The magnitude of the exchange interaction between the {Cr7Ni}-CoII-{Cr7Ni} was determined through Electron Paramagnetic Resonance. Furthermore, by chemical oxidation/reduction of the cobalt between paramagnetic and diamagneticstates, i.e. CoII and CoIII respectively, we demonstrate that the interaction can be switched ON/OFF. This characteristic makes of these systems candidates to function as a SWAP gate.
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Pontillon, Yves. "Étude de la densité de spin de composés magnétiques moléculaires par diffraction de neutrons polarisés." Université Joseph Fourier (Grenoble ; 1971-2015), 1997. http://www.theses.fr/1997GRE10220.
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Au cours de ce travail, nous avons applique la technique de la diffraction de neutrons polarises a l'etude de la densite de spin dans des composes magnetiques moleculaires. Nous avons tout d'abord etudie deux nitronyl nitroxydes presentant une structure en chaine via des liaisons hydrogenes : la densite de spin est majoritairement localisee dans une orbitale antiliante * construite sur les fonctions 2#p des atomes d'azote et d'oxygene des groupes nitroxydes. Par ailleurs, nous avons mis en avant le role primordial des liaisons hydrogenes dans le couplage ferromagnetique intermoleculaire, role confirme par des calculs theoriques ab initio (methode de la fonctionnelle de la densite). Nous nous sommes ensuite interesses a un complexe cuivrique d'une enaminocetone nitroxyde. Nous avons pu determiner qu'une dimerisation quasi totale avait lieu a basse temperature entre deux fonctions nitroxydes, de deux molecules differentes, se faisant face et distantes de 3. 40a. Puis, nous presentons l'etude d'un ferroaimant moleculaire (radical nitronyl nitroxyde). La densite de spin est en grande partie concentree sur le fragment oncno du cycle nit dans une orbitale magnetique moleculaire antiliante *, le couplage ferromagnetique intermoleculaire modifiant l'orientation relative des orbitales magnetiques des deux atomes d'oxygene des fonctions -no, rotation s'accompagnant d'une hybridation sp. La derniere etude presentee dans ce manuscrit a trait a un alkyl nitroxyde presentant un ordre ferromagnetique. Nous avons verifie la localisation majoritaire de l'electron non apparie sur le groupe -no et la nature antiliante * de l'orbitale moleculaire magnetique. Les populations individuelles de spin obtenues sur l'ensemble de la molecule, nous ont permis de proposer un mecanisme expliquant les couplages ferromagnetiques intermoleculaires.
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Tyagi, Pawan. "FABRICATION AND CHARACTERIZATION OF MOLECULAR SPINTRONICS DEVICES." UKnowledge, 2008. http://uknowledge.uky.edu/gradschool_diss/614.
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Fabrication of molecular spin devices with ferromagnetic electrodes coupled with a high spin molecule is an important challenge. This doctoral study concentrated on realizing a novel molecular spin device by the bridging of magnetic molecules between two ferromagnetic metal layers of a ferromagnetic-insulator-ferromagnetic tunnel junction on its exposed pattern edges. At the exposed sides, distance between the two metal electrodes is equal to the insulator film thickness; insulator film thickness can be precisely controlled to match the length of a target molecule. Photolithography and thin-film deposition were utilized to produce a series of tunnel junctions based on molecular electrodes of multilayer edge molecular electrodes (MEME) for the first time. In order to make a microscopic tunnel junction with low leakage current to observe the effect of ~10,000 molecules bridged on the exposed edge of a MEME tunnel barrier, growth conditions were optimized; stability of a ~2nm alumina insulator depended on its ability to withstand process-induced mechanical stresses. The conduction mechanism was primarily 1) tunneling from metal electrode to oranometalic core by tunneling through alkane tether that acts as a tunnel barrier 2) rapid electron transfer within the oranometalic Ni-CN-Fe cube and 3) tunneling through alkane tether to the other electrode. Well defined spin-states in the oranometalic Ni-CN-Fe cube would determine electron spin-conduction and possibly provide a mechanism for coupling.
MEME with Co/NiFe/AlOx/NiFe configurations exhibited dramatic changes in the transport and magnetic properties after the bridging of oranometalic molecular clusters with S=6 spin state. The molecular cluster produced a strong antiferromagnetic coupling between two ferromagnetic electrodes to the extent, with a lower bound of 20 erg/cm,2 that properties of individual magnetic layers changed significantly at RT. Magnetization, ferromagnetic resonance and magnetic force microscopy studies were performed. Transport studies of this configuration of MEME exhibited molecule-induced current suppression by ~6 orders by blocking both molecular channels and tunneling between metal leads in the planar 25μm2 tunnel junction area. A variety of control experiments were performed to validate the current suppression observation, especially critical due to observed corrosion in electrochemical functionalization step. The spin devices were found to be sensitive to light radiation, temperature and magnetic fields.
Along with the study of molecular spin devices, several interesting ideas such as ~9% energy efficient ultrathin TaOx based photocell, simplified version of MEME fabrication, and chemical switching were realized. This doctoral study heralds a novel molecular spin device fabrication scheme; these molecular electrodes allow the reliable study of molecular components in molecular transport.
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Fraser, Hector William Lucas. "Magneto-structural studies of paramagnetic metal cages." Thesis, University of Edinburgh, 2018. http://hdl.handle.net/1842/33166.
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A central concern within the field of molecular magnetism has been the elucidation of magneto-structural correlations. This thesis describes a variety of systems and endeavours to study the relationship between structure and magnetic properties in these systems. The first body of work (chapters 2 and 3) studies CrIII dimers, with the metal centres displaying a dialkoxo bridging moiety and latterly an additional carboxylate bridge to direct the synthesis of ferromagnetic analogues. The second section of work (chapters 4‐6) moves forward to the study of larger, heterometallic 3d‐3d compounds, through the synthesis of a large family of Anderson type MIII 2MII 5 wheels and a subsequent family of (VIVO)2MII 5 wheels. Chapter 2 describes a series of di‐alkoxo bridged Cr(III) dimers, synthesised using the pyridine alcohol ligands 2‐pyridinemethanol (hmpH) and 2‐pyridineethanol (hepH) as well as 2‐picolinic acid (picH). The structures fall into four general categories and are of formula: [Cr2(OMe)2(pic)4], [Cr2(hmp)2(pic)2X2] (where X = Cl, Br), [Cr2(L)2Cl4(A)2] (where L = hmp, A = H2O; L = hmp, A = pyridine; L = hmp, A = 4‐picoline; L = hep, A = H2O), and [Cr(hmp)(hmpH)Cl2. Magnetic studies show relatively weak antiferromagnetic exchange interactions between the Cr(III) centres and DFT calculations are used to develop magneto‐structural correlations, showing that the magnitude and sign of the J value is strongly dependent upon the orientation of the dihedral angle formed between the bridging Cr2O2 plane and the O-R vector of the bridging group, and the Cr-O-Cr-O dihedral angle. Chapter 3 builds on the work from the previous chapter with discussion of a large family of chromium(III) dimers, synthesised using a combination of carboxylate and diethanolamine type ligands. The compounds have the general formula [Cr2(R1‐deaH)2(O2CR2)Cl2]Cl where R1 = Me and R2 = H, Me, CMe3, Ph, 3,5‐(Cl)2Ph, (Me)5Ph, R1 = Et and R2 = H, Ph. The compound [Cr2(Me‐deaH)2Cl4] was also synthesised in order to study the effect of removing/adding the carboxylate bridge to the observed magnetic behaviour. Magnetic studies reveal ferromagnetic exchange interactions between the Cr(III) centres in the carboxylate bridged family with coupling constants in the range +0.37 < J < +8.02 cm‐1. Removal of the carboxylate to produce the dialkoxide‐bridged compound results in antiferromagnetic exchange between the Cr(III) ions. DFT calculations to further develop the magneto-structural correlations reveal the ferromagnetic exchange is the result of an orbital counter-complementarity effect occurring upon introduction of the bridging carboxylate. Chapter 4 reports a family of heterometallic Anderson‐type 'wheels' of general formula [MIII 2MII 5(hmp)12](ClO4)4 (where MIII = Cr or Al and MII = Ni or Zn giving [Cr2Ni5], [Cr2Zn5], [Al2Ni5] and [Al2Zn5]; hmpH = 2‐pyridinemethanol) synthesised solvothermally. The metallic skeleton describes a centred hexagon with the MIII sites disordered around the outer wheel. The structural disorder is characterised via single crystal X‐ray crystallography, 1‐3D 1H and 13C solution‐state NMR spectroscopy of the diamagnetic analogue, and solid‐state 27Al MAS NMR spectroscopy of the Al containing analogues. Alongside ESI mass spectrometry, these techniques show that structure is retained in solution, and that the disorder is present in both the solution and solid‐state. Solid‐state dc susceptibility and magnetisation measurements on [Cr2Zn5] and [Al2Ni5] reveal the Cr‐Cr and Ni‐Ni exchange interactions to be JCr‐Cr = ‐1 cm‐1 and JNi‐Ni,r = ‐5 cm‐1, JNi‐Ni,c = 10 cm‐1. Fixing these values allows us to extract JCr‐Ni,r = ‐1.2 cm‐1, JCr‐Ni,c = 2.6 cm‐1, the exchange between adjacent Ni and Cr ions on the ring is antiferromagnetic and between Cr ions on the ring and the central Ni ion is ferromagnetic. Chapter 5 focusses on planar molecules, espanding the family of heterometallic Anderson‐type 'wheels' discussed in chapter 4 to include MIII = Cr, Al and MII = Co, Fe, Mn, Cu, affording five new species of formulae [Cr2Co5(hmp)12](ClO4)4, [Cr2Fe5(hmp)12](ClO4)4, [Cr2Mn5(hmp)12](ClO4)4, [Cr2Cu5(hmp)12](ClO4)2(NO3)2 and [Al2Co5(hmp)12](ClO4)4. As per previous family members, the two MIII sites are disordered around the outer wheel, with the exception of [Cr2Cu5] where the the CuII sites are localised. A structurally related, but enlarged planar disc possessing a [MIII 6MII] hexagon capped on each edge by a CuII ion is also reported, which is formed only when MIII = Al and MII = Cu. In [AlIII 6CuII 7(OH)12(hmp)12](ClO4)6(NO3)2 the Anderson moiety contains a central, (symmetry‐imposed) octahedral CuII ion surrounded by a wheel of AlIII ions. Solid‐state dc susceptibility and magnetisation measurements reveal the presence of competing exchange interactions in the Anderson wheels family, and weak antiferromagnetic exchange between the CuII ions in [Al6Cu7]. Chapter 6 describes two heterometallic wheels of formula [(VIVO)2MII 5(hmp)10Cl2](ClO4)2∙2MeOH (where MII = Ni or Co) displaying the same Anderson‐type structure as seen in chapters 4 and 5, however the use of the vanadyl moiety has the effect of removing the disorder, with the two vanadyl ions sitting on opposing sides of the ring. The magnetic properties of both show competing antiferroand ferromagnetic interactions.
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Brunet, Gabriel. "Molecular Engineering of Metal-Organic Assemblies: Advances Toward Next Generation Porous and Magnetic Materials." Thesis, Université d'Ottawa / University of Ottawa, 2020. http://hdl.handle.net/10393/40385.
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The controlled assembly of molecular building blocks is an emerging strategy that allows for the preparation of materials with tailor-made properties. This involves the precise combination of molecular subunits that interact with one another via specifically designed reactive sites. Such a strategy has produced materials exhibiting remarkable properties, including those based on metal-organic frameworks and single-molecule magnets. The present Thesis aims to highlight how such metal-organic assemblies can be engineered at the molecular level to promote certain desired functionalities. Specifically, Chapter 2 will focus on the confinement effects of a crystalline sponge on a ferrocene-based guest molecule that is nanostructured within the porous cavities of a host material. In doing so, we evaluate how one can exert some level of control over the binding sites of the guest molecule, through the addition of electron-withdrawing groups, as well as tuning the physical properties of the guest itself through molecular encapsulation. Notably, we demonstrate a distinct change in the dynamic rotational motion of the ferrocene molecules once confined within the crystalline sponge. In Chapter 3, we investigate the generation of slow relaxation of the magnetization from a Co(II)-based metal-organic framework. We compare this to a closely related 2D Co(II) sheet network, and how slight changes in the crystal field, probed through computational methods, can impact the magnetic behaviour. This type of study may be particularly beneficial in the optimization of single-ion magnets, by sequestering metal centres in select chemical environments, and minimizing molecular vibrations that may offer alternative magnetic relaxation pathways.
We extend these principles in Chapter 4, through the use of a nitrogen-rich ligand that acts as a scaffold for Ln(III) ions, thereby yielding 0D and 1D architectures. The coordination chemistry of Ln(III) ions with N-donor ligands remains scarce, especially when evaluated from a magnetic perspective, and therefore, we sought to determine the magnetic behaviour of such compounds. The monomeric unit displays clear single-molecule magnet behaviour with an energetic barrier for the reversal of the magnetization, while the 1D chain displays weaker magnetic characteristics. Nevertheless, such compounds incorporating nitrogen-rich ligands offer much promise in the design of environmentally-friendly energetic materials. In Chapter 5, we take a look at different two different systems that involve the formation of radical species. On one hand, we can promote enhanced magnetic communication between Ln(III) ions, which is typically quite challenging to achieve given the buried nature of the 4f orbitals, and on the other hand, we rely on a redox-active ligand to design stimuli-responsive metal-organic assemblies. The latter case provides access to “smart” molecular materials that can respond to changes in their environment. Here, a multi-stimuli responsive nanobarrel was studied, which displayed sensitivity to ultraviolet radiation, heat and chemical reduction.
Lastly, Chapter 6 provides a new method for the systematic generation of cationic frameworks, termed Asymmetric Ligand Exchange (ALE). This strategy focuses on the replacement of linear dicarboxylates with asymmetric linkers that features one less negative charge, in order to tune the ionicity of porous frameworks. This allows for the retention of the structural topology and chemical reactivity of the original framework, representing distinct advantages over other similar strategies. Methods to retain permanent porosity in such cationic frameworks are also proposed. Altogether, these studies highlight how the directed assembly of ordered networks can generate varied properties of high scientific interest.
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Hu, Jianming, and 胡建明. "Macroscopic quantum phenomenon in molecular magnets." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2003. http://hub.hku.hk/bib/B26724674.
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Amjad, Asma. "Exchange coupling in molecular magnets: Zero, one and three dimensions." Doctoral diss., University of Central Florida, 2013. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/5755.
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Molecular magnets with different dimensionality, whether they are zero-dimensional single-molecule magnets (SMM) or one-dimensional single-chain magnets (SCM) are very interesting, since they allow probing the fundamental aspects bordering quantum and classical physics at the nanoscale level. This dissertation covers experimental studies of two Mn-based exchange-coupled molecule-based magnets and two Co-based single-chain magnets, using both dc Hall-effect magnetometry and electron paramagnet resonance (EPR) techniques. In these multi-dimensional systems, the spin of the molecule exhibits quantum mechanical behavior at low temperature. It is quite interesting to observe the effect of magnetic exchange interactions on the magnetic properties of various complexes; hence they strongly affect the magnetic behavior. In this dissertation, the research is initiated with the study of low-magnetic-nuclearity molecules, starting with a spectroscopic study of a significantly anisotropic Mn(IV) monomer. At low temperature the molecule possesses easy-plane type anisotropy of a remarkable magnitude. Although the molecule is not a single-molecule magnet, the remarkable anisotropy can initiate synthesis of newer and better molecular magnets with Mn(IV) as the main building block. Furthermore, the interplay between the magnetic anisotropy and the inter-ion exchange interactions (J) within the molecule are probed for a dimer and a trimer where the magnetic core is comprised of two and three ions respectively. In the Mn-based case of the dimer, the low coupling between the atoms leads to significant state mixing, thus making it impossible to assign the individual spin states to the dimer or to the respective individual Mn(II) ions. In the case of the trimer, lowering of the symmetry achieved by fine tuning of the inter-ion exchange interactions leads to relieving of frustration in the antiferromagnetic (AF) triangular Mn(III) system, resulting in a well defined ground state and significant zero field splitting. Also a clear hysteretic behavior observed in this system demonstrates its SMM nature at low temperature. Finally, high-field high-frequency magnetic and spectroscopic studies performed on two cobalt-based SCMs reveal that formation of magnetic domains by exchange interactions within the chain are strongly influenced by thermal fluctuations. The chain possesses a uniaxial anisotropy with the quantization axis lying along the length of the chain. Moreover it is shown that modulation of the magnitude of inter- and intra-chain interactions results in a three-dimensional dynamics in one of the samples. Interestingly, detailed dc magnetic studies show a tunable crossover between one- and three-dimensional magnetic dynamics as a function of temperature and/or magnetic field sweep rate. Our voyage through several molecular systems of different dimensionality have allowed us to expand our understanding of the role of exchange interactions on the magnetic behavior in molecular magnetism.M.A.
Masters
Physics
Sciences
Physics
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Zagaynova, Valeria. "Carbon-based magnetic nanomaterials." Doctoral thesis, Umeå universitet, Institutionen för fysik, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-53568.
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Magnetism of carbon-based materials is a challenging area for both fundamental research and possible applications. We present studies of low-dimensional carbon-based magnetic systems (fullerene-diluted molecular magnets, carbon nanotubes, graphite fluoride, and nanoporous carbon) by means of SQUID magnetometer, X-ray diffraction and vibrational spectroscopy, the latter techniques used as complementary instruments to find a correlation between the magnetic behaviour and the structure of the samples.In the first part of the thesis, characteristic features of the magnetization process in aligned films of carbon nanotubes with low concentration of iron are discussed. It is shown that the magnetism of such structures is influenced by quantum effects, and the anisotropy behaviour is opposite to what is observed in heavily doped nanotubes.In the second part, Mn12-based single molecular magnets with various carboxylic ligands and their 1:1 fullerene-diluted complexes are studied. We prove that magnetic properties of such systems strongly depend on the environment, and, in principle, it is possible to design a magnet with desirable properties. One of the studied compounds demonstrated a record blocking temperature for a single molecular magnet. Both fullerene-diluted complexes demonstrated “magnetization training” effect in alternating magnetic fields and the ability to preserve magnetic moment.The third and the fourth parts of the thesis are dedicated to the analysis of various contributions to the magnetic susceptibility of metal-free carbon-based systems – intercalated compounds of graphite fluorides and nanoporous oxygen-eroded graphite. The magnetic properties of these systems are strongly dependent on structure, and can be delicately tuned by altering the π-electron system of graphite, i. e. by degree of fluorination of intercalated compounds and by introduction of boron impurity to the host matrix of nanoporous graphite.Magnetism av kolbaserade material är ett utmanande område för både grundforskning och möjliga tillämpningar. Vi presenterar studier med låg-dimensionella kolbaserade magnetiska system (fulleren-utspädda molekylära magneter, kolnanorör, grafit fluorid och nanoporösa kol) med hjälp av SQUID magnetometer, röntgendiffraktion och vibrerande spektroskopi, de senare tekniker som används som komplement instrument för att finna sambandet mellan den magnetiska uppträdande och strukturen hos proven. I den första delen av avhandlingen är egenheter från magnetisering processen i linje filmer av kolnanorör med låg koncentration av järn diskuteras. Det visas att magnetism av sådana strukturer påverkas av kvantmekaniska effekter och anisotropin beteende är motsatsen till vad som observerats i kraftigt dopade nanorör. I den tvåa delen är Mn12-baserade enda-molekyl magneter med olika karboxylsyror ligander och deras 1:1 fulleren-utspädda komplex studeras. Vi visar att magnetiska egenskaperna hos sådana system beror i hög grad på miljön, och i princip är det möjligt att utforma en magnet med önskvärda egenskaper. En av de studerade föreningarna visade en post blockeringstemperaturen för en enda molekylär magnet. Både fulleren-utspädda komplex visade "magnetisering utbildning" effekt i alternerande magnetfält och möjligheten att bevara magnetiskt moment. Den tredje och fjärde delarna av avhandlingen är avsedda för inneboende magnetism av analys av olika bidrag till magnetisk susceptibilitet av metall-fritt kol-baserade system -inskjutna föreningar grafit fluorider och nanoporösa O2-eroderade grafit. Magnetiska egenskaperna hos dessa system är starkt beroende av strukturen, och kan fint avstämmas genom att man ändrar π-elektronsystem av grafit, i. e. med graden av fluorering av inskjutna föreningar och genom införandet av bor föroreningar till värd matris av nanoporösa grafit.
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Alves, Lemes Maykon. "Magnetic Exchange Coupling in 3d and 4f Complexes Using Radical Tetrazine-based Frameworks." Thesis, Université d'Ottawa / University of Ottawa, 2019. http://hdl.handle.net/10393/39598.
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High symmetry and low coordinated single-ion lanthanides have been a successful recipe to design high performance single molecule magnets (SMMs). However, enhancement of the magnetic properties of polymetallic SMMs is an important challenge. Therefore, this thesis describes the use of redox non-innocent tetrazine-based ligands and d- and f- elements in order to fine-tune the electronic structure of the resulting compounds to promote strong exchange interactions between the spin carriers. As reported in the literature, radical-bridged ligands represent a potential strategy to improve the magnetic properties of polymetallic SMMs. Thus, chapter one introduces principal concepts that govern the physical properties of metal complexes containing radical-bridged ligands.
Chapter two describes the magnetic properties of a unique air-stable tetratopic radical- bridged bpymtz•− (3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine) templating four Ni(II) metal ions. The dc magnetic studies along with DFT calculations reveal strong ferromagnetic exchange coupling between the Ni− bpymtz•− of J = 98 cm-1 with a spin ground state of S = 9/2.
Chapter three describes the application of another tetrazine-based ligand, bpytz (3,6- bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine) to probe magnetic exchange interactions in a {CoII4} supramolecular square. The modelling of dc susceptibility data shows significant Co(II) - bpytz•− magnetic coupling of J = - 118 cm-1 for a spin ground state ST = 4. While the non-reduced analog displays weak Co(II)-Co(II) exchange of J = - 0.64 cm-1 (S = 0 ground state). Additionally, the radical-radical magnetic exchange contribution was probed with an analogous {ZnII4} square, where a J = -15.9 cm-1 was found.
Chapter four extends the application of reduced tetrazine ligands to lanthanide systems. Here we demonstrate that the systematic reduction of the ligand with cobaltacene (CoCp2) led to the formation of a strongly coupled bpytz•−−bpytz•− bridging ligand. Magnetic measurements combined with ab initio calculations confirm unprecedented intramolecular pi-dimerization preventing strong magnetic Dy(III)−bpytz•− communication.
Chapter five describes the synthesis and characterization of {LnIII4} (Ln = Dy, Gd and Lu) where the Ln(III) ions are bridged by peripheral bpytz•−. The oxophilicity and high coordination numbers preferred by lanthanides ions lead to the formation of a cubane core made up of metal ions bridged by hydroxy ligands (M3-OH−). Experimental and computational studies were applied to verify the nature and strength of the magnetic interactions between the spin carriers.
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Allan, Margaret Lucy. "Magnetic interactions in molecular materials." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.387026.
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Staniland, Sarah S. "Sulphur-rich molecular magnetic materials." Thesis, University of Edinburgh, 2005. http://hdl.handle.net/1842/12990.
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The work within this thesis is concerned with several different magnetic systems all involving materials that are rich in sulphur and are thus predicted to have strong magnetic coupling through these sulphur atoms. Firstly, a range of molecular salt materials of [dithiazolyl]x[M(dithiolene)2] were synthesised and their magnetic behaviour analysed. Dithiolene complex salts of [BDTA]+ [2]-[6] were found to have a mainly mixed anion-cation stacked structural motif and show a number of interesting magnetic properties, such as an almost perfect one dimensional antiferromagnetic chain system ([BDTA]2[Cu(mnt)2 [2], as well as the conducting charge transfer salt [BDTA][Ni(dmit)2]2 [3] that is the first example of the [BDTA]+ cation behaving as an open shell cation. The [BDTA]2[Fe(tdas)2Cl] salt [5] has the first mixed stacking structural motif of this anion (as it usually dimerises) and this material was found to be paramagnetic. The paramagnetic radical dithiazolyl cation [BBDTA]+ was also co-precipitated with dithiolene complexes to create magnetic salts [11]-[15]. These all showed mixed magnetic effects with some unusual magnetic interactions occurring. [BDTA]+ was also co-crystallised with MC142- anions to form molecular materials containing two dimensional magnetic structure with a square lattice magnetic pathway. This magnetic mechanism provides ideal materials for the study of ground state quantum magnetic effects that are thought to be related to the fundamental mechanism of superconductivity. This work also contains the study of the non interacting magnetic molecular thiophosphine complexes of the form [M((SPR1R2)2N)2] (M = Mn, Co, Ni, R1 = R2 = Ph, iPr and R1=Ph and R2=iPr) which behave as simple paramagnets ([16]-[24]). All M = Mn and Co complexes were found to be simple paramagnetic materials with a strong ligand field splitting.
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Filipovic, Milena [Verfasser]. "Quantum Transport Through Molecular Magnets / Milena Filipovic." Konstanz : Bibliothek der Universität Konstanz, 2015. http://d-nb.info/1098136519/34.
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Habib, Fatemah. "Investigating and Enhancing Spin Reversal Barriers in Dinuclear 4f Single-Molecule Magnets and the Ultimate Shift to Mononuclear 3d Complexes." Thesis, Université d'Ottawa / University of Ottawa, 2015. http://hdl.handle.net/10393/32174.
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In order for molecular magnetic materials to become applicable, they must retain their magnetisation at reasonable temperatures, which can be achieved with high energy barriers for spin reversal and high blocking temperatures. In the field of Single-Molecule Magnets (SMMs), over the last decade, the main focus has shifted from large spin complexes to highly anisotropic systems which have displayed record energy barriers. There are two main methods of increasing magnetic anisotropy in a complex: i) Choosing a metal ion that boasts high magnetic anisotropy then coupling two such ions through magnetic interactions to induce large global anisotropy, and ii) maintain a low spin or use a mononuclear complex while minimising quantum tunnelling of the magnetisation by controlling the geometric features of the metal ion. Both strategies are equally valid and have been explored in this thesis using dinuclear lanthanide as well as mononuclear 3d complexes.
In the pursuit of high-barrier SMMs via alignment of anisotropy axes, two dinuclear, quadruple-stranded helicates and one mesocate were isolated and are described in detail herein, both structurally and magnetically. Furthermore, theoretical calculations have been performed to determine the energies of Kramers doublets on each DyIII centre to derive magneto-structural correlations. To induce magnetic interactions between DyIII ions, a centrosymmetric dinuclear SMM was synthesised. Investigation of the crucial DyIII…DyIII interaction as well as its effect on the quantum tunnelling of the magnetisation has been carried out using ab initio calculations and magnetic dilution studies. Using the same system, a method of greatly enhancing the energy barriers in SMMs has been developed. It involves modifying the coordinating ligands to include electron withdrawing groups in order to yield more anisotropic metal ions. The energy barrier for spin reversal has been increased 7-fold in one case. While lanthanide chemistry has proven to be quite versatile and promising, a new branch of nanomagnets is currently being pursued: mononuclear 3d complexes as SMMs. The advantages of 3d metals include high anisotropy per ion, low spin (as anisotropy decreases with increasing spin), well-understood electronic structures and clear correlations between geometry and magnetic anisotropy. The structural and magnetic properties of three complexes based on CoII and terpyridine ligands as well as a seven-coordinate CoII complex with positive anisotropy are discussed at length. The unique slow relaxation dynamics and spin crossover behaviour has been followed using DFT and ab initio calculations, as well as EPR and magnetic dilution studies.
Overall, this thesis describes the efforts taken to synthesise high-barrier nanomagnets through understanding the origins and mechanisms of slow magnetic relaxation in both lanthanide and 3d metal complexes.
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Xu, Xiaoshan. "The magnetism of free cobalt clusters measured in molecular beams." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/14649.
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Magnetic properties of cobalt clusters (20 N 200) were studied in molecular beams. The magnetization of cobalt clusters is studied at a broad range of temperatures, magnetic fields and clusters sizes. It is shown that the agnetization of ferromagnetic clusters in a cluster beam can be understood as an adiabatic process using the avoided crossing theory. Besides the ground state that bears magnetic moment of about 2 Bohr magneton per atom, an excited state that has 1 Bohr magneton per atom was discovered for every cobalt cluster observed. The energy separations between the two states was investigated by photo-ionization experiments. The ionization threshold shows that the energy gap between the two states is on the order of 0.1 eV for small clusters (N 100) and vanishes for larger clusters. Experiments also show that the polarizability of the excited state is lower than that of the ground state, which indicates a significant electronic tructure difference between the two states. Two states are also found for iron clusters (20 N 200) for which the magnetic moments per atom are about 3 Bohr magneton for the ground state and 1 Bohr magneton for the excited states. This explains the fractional magnetic moments as well as the local magnetic order observed above the Curie temperatures for iron group ferromagnets. Further experiments show two states for manganese clusters for which the ground state has magnetic moment of 1 Bohr magneton per atom in about the same size range. This suggests that the two states are a universal phenomenon of 3d transition metal clusters, which originate from the interaction between 3d and 4s electrons.
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Tyree, William Stuart. "Correlation of Structure and Magnetic Properties in Charge-Transfer Salt Molecular Magnets Composed of Decamethylmetallocene Electron Donors and Organic Electron Acceptors." Thesis, Virginia Tech, 2005. http://hdl.handle.net/10919/34436.
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Di-n-propyl dicyanofumarate (DnPrDCF) and di-isopropyl dicyanofumarate (DiPrDCF) have been used as one-electron acceptors in the synthesis of charge-transfer salt magnets with decamethylmetallocenes, MCp*2 (M = Mn, Cr). Salts of each acceptor with each metallocene have been characterized and the structures of the chromium analogues have been solved. The two acceptors are structurally similar to dimethyl dicyanofumarate (DMeDCF) and diethyl dicyanofumarate (DEtDCF), which have been previously studied and found to form charge-transfer salt magnets with the aforementioned decamethylmetallocenes. A typical structural motif is present in these types of charge-transfer salts which allows for the comparison of magnetic properties based on the length or size of the alkyl group of the dialkyl dicyanofumarate. Some trends were established based on the magnetic properties of the homologous series including ordering temperature/bulkiness of the alkyl group and intrastack distances/theta values. Correlation of magnetic and structural properties may give some insight into "through-space" magnetic coupling, of which little is understood.Master of Science
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Subedi, Pradeep. "Collective effects in Single Molecule Magnets." Thesis, New York University, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=3602734.
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Single molecule magnets (SMMs), such as Mn12-acetate, are composed of transition metal ions and consists of identical molecules with large ground-state spin (S = 10) and a strong uniaxial anisotropy (65 K). Below about 3 K, Mn12-acetate exhibits magnetic hysteresis with steps at specific values of longitudinal magnetic field due to resonant quantum tunneling between spin up and down projections along the easy axis. The intermolecular exchange interactions between spins on molecules are quite small and spins are considered to be independent and non-interacting.
However, the molecules do interact with each other both through magnetic dipolar interactions and through the lattice (e.g. phonons). I have investigated collective effects in SMMs due to these intermolecular interactions. In the thesis I will present experiments that explored magnetic ordering due to magnetic dipole interactions in Mn12-acetate and Mn12-acetate-MeOH. I will also present exper- iments on the onset of magnetic de agration in Mn12-acetate due to a thermal instability.
The magnetic ordering studies involved investigating the effect of transverse fields on the susceptibility of single crystals of Mn12-acetate and Mn12-acetate- MeOH. Transverse fields increase quantum spin uctuations that suppress long- range order. However, the suppression of the Curie temperature by transverse fields in Mn12-acetate is far more rapid than predicted by the Transverse-Field Ising Ferromagnetic Model (TFIFM) and instead agrees with the predictions of the Random-Field Ising Ferromagnet Model. It appears that solvent disorder in Mn12-acetate gives rise to a distribution of random-fields that further suppress long-range order. Subsequent studies on Mn12-acetate-MeOH, with the same spin and similar lattice constants but without solvent disorder as Mn12-acetate, agrees with the TFIFM.
The magnetic de agration studies involved studying the instability that leads to the ignition of magnetic deflagration in a thermally driven Mn 12-acetate crystal. When spins prepared in a metastable state reverse, Zeeman energy is released that diffuses away. In some circumstances, the heat released cannot be compensated by thermal diffusion, resulting in an instability that gives rise to a front of rapidly reversing spins traveling through the crystal. We observed a sharp crossover from relaxation driven by heat diffusion to a self-sustained reversal front that propagates at a constant subsonic speed.
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Jones, Nicholas. "Quantum molecular dynamics and molecular interactions studied by NMR and INS." Thesis, University of Nottingham, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.251791.
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Fegy, Karine. "Ingénierie de matériaux moléculaires magnétiques : complexes de radicaux libres nitronyl nitroxydes bis-bidentes ; synthèse, structure et magnétisme." Université Joseph Fourier (Grenoble ; 1971-2015), 1997. http://www.theses.fr/1997GRE10197.
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La chimie de coordination des radicaux libres nitronyl nitroxydes est propice aux composes moleculaires magnetiques (aimants moleculaires ou transitions de spin). Cependant le caractere base de lewis faible du groupe nitroxyde >n-o limite dans de nombreux cas leur utilisation aux ions metalliques porteurs de ligands attracteurs. Pour eviter ce probleme, nous avons choisi de substituer les radicaux par des groupes donneurs. Ce travail concerne la synthese, la caracterisation structurale et l'etude des proprietes magnetiques de complexes d'ions metalliques avec les radicaux nitronyl nitroxydes porteurs des groupes imidazolyle (nitimh) ou benzimidazolyle (nitbzimh). La premiere partie est consacree a la complexation des radicaux non deprotones. Avec les ions metalliques de transition mn#2#+, ni#2#+, zn#2#+ et de terres rares gd#3#+, dy#3#+, plusieurs molecules mononucleaires qui sont des entites discretes, ont ete isolees. Leurs structures determinees par diffraction des rayons x sur monocristal montrent pour la premiere fois des centres metalliques entoures exclusivement de radicaux nitroxydes ; m(l)#3(mn#2#+, ni#2#+, zn#2#+) ou m(l)#4 (gd#3#+, dy#3#+)(l = nitimh ou nitbzimh). L'etude de leurs proprietes magnetiques revele des comportements varies. La seconde partie du travail decrit une serie de composes etendus du manganese(ii), obtenus lorsque les radicaux sont deprotones ; nitim#- ou nitbzim#-. La dimension des composes depend alors des conditions experimentales de deprotonation et du type de contre anions utilises. Quatre chaines ferrimagnetiques dans lesquelles alternent les ions manganese(ii) et les radicaux ont ete isolees, et leurs structures determinees. L'etude de leurs proprietes magnetiques met en evidence une aimantation spontanee au-dessous de temperature comprises entre 2 k et 4,5 k. Selon des conditions experimentales differentes, une serie de composes bidimensionnels a egalement ete synthetisee. La determination structurale de l'un d'entre eux montre qu'il s'agit de composes lamellaires constitues de feuillets mn#2(l)#3#+, separes par les contre anions. Ces composes sont des aimants en dessous de temperatures allant de 1,4 k a 40 k. Cette strategie est donc prometteuse et devrait ouvrir de nouvelles perspectives pour la construction d'edifices tridimensionnels.
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Goodwin, Jeremy C. "An investigation into the synthesis characterisation and magnetic properties of high nuclearity transition metal arrays." Thesis, University of Sheffield, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.369849.
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Frost, Jamie Michael. "Ligand design strategies for molecular nanomagnets." Thesis, University of Edinburgh, 2015. http://hdl.handle.net/1842/17990.
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This thesis describes the synthesis and magnetic characterisation of a series of polynuclear 3d and 3d/4f complexes built using phenolic oxime type ligands. Chapter two describes the reaction of salicylaldoxime and its derivatised analogues (R-saoH2) with the alkoxide containing co-ligands triethanolamine (TeaH3) and 2-(hydroxymethyl)pyridine (Hhmp), in the presence of Mn(II)/Ln(III) salts. This results in the formation of a family of sixteen [MnIII4LnIII2] clusters, which are structurally related to a previously studied [MnIII6] family of Single-Molecule Magnets (SMMs). The magnetic properties of the Ln = Y, Gd and Lu analogues can be qualitatively rationalised on the basis of a magneto-structural correlation (MSC), previously developed for MnIII alkoxide/oxime bridged dimers. Chapter three describes how the combination of two complimentary ligands, the phenolic oximes (R-SaoH2) and the diethanolamines (DeaH3), into one organic framework, creates new ligand types (H3L1 and H4L2) which can be used to construct a hexametallic MnIII wheel; [MnIII6Na(L1)6]Cl, the first example of a ferromagnetically coupled dodecametallic MnIII wheel;[MnIII 12(OMe)16(L2)4(O2CCMe3)4(MeOH)4], and the first example of a dodecametallic MnIII truncated tetrahedron; [MnIII12O4(H3L2)8(H2L2)4(TMA)4 (TMA = trianion of trimesic acid). Single crystal hysteresis measurements reveal both 3.2 and 3.3 to be SMMs at low temperature. Chapter four deals with the use of H4L2 in Cu coordination chemistry. Phenolic oximes are known to form monometallic complexes with CuII ions, as are the diethanolamines. However, the deliberate incorporation of one ligand onto the organic framework of the other permits the preparation of a family of [CuIIn] wheels (n = 4, 6, 8). In each case nearest neighbour interactions between CuII ions are shown to be strongly antiferromagnetic. DFT calculations suggest the origin of this interaction is related to the Cu-O-N-Cu dihedral angle, an observation which allows for the development of a theoretical MSC, that suggests a switch from antiferro- to ferro-magnetic exchange is possible at Cu-O-N-Cu angles > 60o.
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Kirchner, Nadejda. "Analysis of magnetic excitations in molecular nanomagnets." [S.l. : s.n.], 2006. http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-26254.
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Zhu, Bo Ph D. Massachusetts Institute of Technology. "Acoustical-molecular techniques for magnetic resonance imaging." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/103499.
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Thesis: Ph. D. in Biomedical Engineering, Harvard-MIT Program in Health Sciences and Technology, 2016.Cataloged from PDF version of thesis.
Includes bibliographical references.
Magnetic resonance imaging (MRI) is a remarkably flexible diagnostic platform due to the variety of clinically relevant physical, chemical, and biological phenomena it can detect. In addition to the host of endogenous contrast mechanisms available, MRI functionality can be further extended by incorporating exogenous factors to attain sensitivity to new classes of indicators. Molecular imaging with targeted injectable contrast agents and MR elastography with externally delivered acoustic vibrations are two such advancements with increasing clinical significance. Conventionally employed separately, this work explores how exogenous components can interact cooperatively in imaging disease and may be combined to more accurately stage disease progression and generate novel mechanisms of MR contrast, using contrast agents and acoustic stimulation as model systems. We imaged hepatic fibrosis in a rat model and found that collagen-binding paramagnetic contrast agents and shear wave MR elastography had partially uncorrelated staging abilities, due to the disease condition's differential timing of collagen production and its stiff cross-linking. This complementary feature enabled us to form a composite multivariate model incorporating both methods which exhibited superior diagnostic staging over all stages of fibrosis progression. We then integrated acoustics and molecular-targeting agents at a deeper level in the form of a novel contrast mechanism, Acoustically Induced Rotary Saturation (AIRS), which switches "on" and "off" the image contrast due to the agents by adjusting the resonance of the spin-lock condition. This contrast modulation ability provides unprecedented clarity in identifying contrast agent presence as well as sensitive and quantitative statistical measurements via rapidly modulated block design experiments. Finally, we extend the AIRS method and show preliminary results for Saturation Harmonic Induced Rotary Saturation (SHIRS), which detects the second harmonic time-oscillation of iron oxide nanoparticles' magnetization in response to an oscillating applied field around B0. We also illustrate an exploratory method of selectively imaging iron oxide agents by diffusion kurtosis measures of freely diffusing water in solutions of magnetic nanoparticles.
by Bo Zhu.
Ph. D. in Biomedical Engineering
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Önal, Emel. "Molecular magnetic materials based on porphyrin macrocyles." Thesis, Lyon 1, 2014. http://www.theses.fr/2014LYO10103/document.
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La construction de nouvelles architectures d'aimant moléculaire basé sur l'approche métal-radical repose sur la conception de nouvelles molécules radicalaires. Dans cette optique, notre stratégie s'est concentrée sur la synthèse de porphyrins incorporant des composés radicaux libres. En effet, les porphyrines sont des composés π conjugués qui devraient favoriser la délocalisation de spin et la transmission des interactions magnétiques sur l'entier macrocycle et de plus sur l'ensemble du composé obtenu. A cause de leur excellente stabilité dans une grande diversité d'environnements chimiques et leur capacité à coordonner avec des métaux de transitions, notre choix s'est porté sur les radicaux nitroxides. Dans cette thèse, une série de porphyrines contenant des tBUNO, nitronyl and imino nitroxide directement liés sur le squelette de la molécule ont été synthétisées. Les macrocycles obtenus ont été caractérisés par UV-Vis, Masse et RPE spectroscopie. De plus, durant ce travail, des intermédiaires réactionnaires intéressant ont été obtenus et caractérisés pour la première fois. Ce fut le cas pour la meso-tetrakis(4-formylphenyl)porphyrin et ses équivalents métallés pa le Cu(II) et le Mn(II). Ainsi que pour quelques précurseurs prometteurs de macrocycle tetrapyrroliques comme 2-(3,4-dicyanophenyl)-4,4,5,5-tetramethylimidazoline-3-oxide-1oxyl, 2-(3,4-dicyanophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl and 2-(4-benzaldehyde)-4,4,5,5-tetramethylimidazoline-1-oxyl. D'après ce que nous savons, ces composés représentent le premier exemple de porphyrines substituées par des nitronyl et imino nitroxide radical, ayant été caractérisées sans ambiguïté par des méthodes spectroscopiquesThe preparation of Molecule-Based Magnets is based on the assembling carriers of magnetic moment. These may be the metal ions only with diamagnetic linkers or the metal ions connected through open-shell organic molecule. The building of novel Molecule-Based Magnets architectures following the metal-radical approach relies on the design of innovative open-shell organic molecular blocks. In this regard, we focus our strategy on the synthesis of porphyrins incorporating free radicals. Indeed, porphyrins compounds are π-conjugated systems which should favor spin delocalization and the transmission of the magnetic interactions on the overall macrocycle and further over the all architecture. Due to their excellent stability in a wide variety of chemical environments, and their abilities to coordinate with transition metal we focus our attention on nitroxide radicals. In this dissertation a series of porpyrin macrocycles were synthesized, bearing tBuNO, nitronyl and imino nitroxide covalently linked to the skeleton. Characterization was done by UV-Vis, Mass and EPR spectroscopy. Moreover during this work some interesting synthetic intermediates were obtained with good yield and characterize for the first time. This was the case for meso-tetrakis(4-formylphenyl)porphyrin and its corresponding metallated derivatives by Cu(II) and Mn(II). Some novel promising tetrapyrrolic macrocycle precursors bearing nitronyl and imino nitroxides free radicals as 2-(3,4- dicyanophenyl)-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl, 2-(3,4- dicyanophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl and 2-(4-benzaldehyde)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl. To the best of our knowledge, these compounds represent the first example of nitronyl and imino nitroxide substituted porphyrin derivatives that have been unambiguously characterized by spectroscopic techniques
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White, Christopher A. "Magneto-structural correlations of novel copper(II) molecular materials." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape7/PQDD_0002/NQ42812.pdf.
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White, Christopher A. (Christopher Andrew) Carleton University Dissertation Chemistry. "Magneto-structural correlations of novel copper(II) molecular materials." Ottawa, 1999.
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Heß, Volkmar [Verfasser], Daniel [Akademischer Betreuer] Bürgler, and Thomas [Akademischer Betreuer] Michely. "Scanning tunneling microscopy of single-molecule magnets and hybrid-molecular magnets: Two approaches to molecular spintronics / Volkmar Heß. Gutachter: Daniel Bürgler ; Thomas Michely." Köln : Universitäts- und Stadtbibliothek Köln, 2016. http://d-nb.info/1099592941/34.
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Berlinguette, Curtis Paul. "Nanomagnetic molecular materials based on the hexacyanometallate building block: the preparation and characterization of high-spin cluster and chain compounds." Diss., Texas A&M University, 2003. http://hdl.handle.net/1969.1/2196.
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The work presented herein describes efforts to synthesize and characterize cyanide-bridged molecular compounds with high-spin ground states. This investigation focused primarily on the assembly of hexacyanometallate units with convergent cationic metal complexes that are coordinated to capping ligands. In this manner, a family of related compounds was developed that serve as models for understanding the role of magnetic exchange interactions and anisotropy in nanomagnetic materials. The work presented in Chapter II describes the successful incorporation of the [Fe(CN)6]3- building block into planar geometries with nuclearities ranging from three to ten metal centers. In Chapter III, this methodology was optimized to yield two pentanuclear FeIII/NiII clusters, namely, the trigonal bipyramidal unit, {[Ni(tmphen)2]3[Fe(CN)6]2}, and the extended square, {[Ni(bpy)2(H2O)][Ni(bpy)2]2-[Fe(CN)6]2}. Magnetic measurements on pure phases of these samples revealed that each system exhibits ferromagnetic coupling between the L.S. FeIII and NiII centers, but neither exhibits slow paramagnetic relaxation effects down to T=2K. In Chapter IV, this chemistry was extended to the [Mn(CN)6]3-building block in order to increase magnetic exchange coupling and anisotropy in this cluster type, efforts that resulted in the isolation of the molecule, {[Mn(tmphen)2]3[Mn(CN)6]2}. This cluster exhibits intramolecular antiferromagnetic exchange interactions between the Mn centers which lead to an S=11/2 ground state and a negative ZFS value (D=-0.348 cm-1), parameters that support the experimental observation of Single-Molecule Magnet (SMM) behavior at low temperatures. A detailed investigation of the physical and structural properties of {[Co(tmphen)2]3[Fe(CN)6]2} in Chapters V and VI led to the realization that the cluster exhibits sensitivity to temperature and humidity. The molecule exists in three different electronic isomeric forms in the solid state and undergoes a charge-transfer induced spin-transition (CTIST) under the influence of temperature. The results presented in Chapter VI describe the behavior of this same cluster in solution, the highlight of which is the discovery that water reacts with the cluster to form a fourth electronic isomer. Finally, it is described in Chapter VII that this Co/Fe trigonal bipyramidal unit can be used as a building block for systematically incorporating three metal types into a family of 1-D chain and cluster compounds.
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Phelps, Gretchen. "MAGNETO-OPTICAL EFFECTS AND PRECISION MEASUREMENT PHYSICS: ACCESSING THE MAGNETIC FARADAY EFFECT OF POLARIZED 3HE WITH A TRIPLE MODULATION TECHNIQUE." UKnowledge, 2014. http://uknowledge.uky.edu/physastron_etds/15.
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This work is comprised of the study of two magneto-optical phenomena: the Kerr effect and the Faraday effect. Neutron physics experiments often utilize polarized neutrons, and one method to generate or guide polarized neutrons involves the use of a system of magnetic super-mirrors. Experience shows that the magnetization of the super-mirror may decay with time; therefore, we implemented the surface magneto-optical Kerr effect (SMOKE) to study the temporal behavior of the magnetization of a magnetized remnant super-mirror sample, where a sensitivity of 0.1 mrad was obtained. Unique to our set-up was the method in which the various magnetization directions were probed. The sample was magnetized prior to insertion into the set-up, and a high precision rotational stage was used to manually rotate the sample to effectively generate a reversal of the magnetic field. Multiple samples from a larger super-mirror specimen were tested, in which no change in the magnetization was detected for one month after sample magnetization. Further studies could increase the sensitivity of the experiment, potentially rendering the method as an application for real-time magnetization monitoring.
Polarized 3He nuclei are often used as an effective polarized neutron target at various laboratories, including Jefferson Lab, through the use of spin-exchange optical pumping in a glass cell constructed of GE-180. Utilizing the nuclear spin optical rotation to measure the Faraday effect of polarized 3He would develop a new procedure for polarization monitoring, establish a powerful tool to diagnose the wall properties and thicknesses of the cells used, and the determination of the frequency independent magnetic component of the polarizability would ultimately lead to the extraction of the spin polarizability of 3He. Furthermore, this study has the future implications of being the pioneer experiment for terrestrial dark matter studies. A new triple modulation technique was devised, where a sensitivity of 60 nrad was obtained, and the first ever extraction of the Verdet constant of GE-180 was recorded, an important factor in wall thicknesses and diagnostic investigations for Jefferson Lab. However, a measurement of the nuclear spin optical rotation of a polarized 3He target was not realized, as the measured polarization suggests a Faraday rotation just below the 60 nrad threshold. Nevertheless, the devised triple modulation method proves to be a very sensitive probe in Faraday effect studies, and additional examination of the polarized target for the production of a larger polarization, should yield a measurement of the nuclear spin optical rotation of polarized 3He.