Academic literature on the topic 'Low-dimension ferromagnetism'

With the seemly limit of scaling on CMOS microelectronics fast approaching, spintronics has received enormous attention as it promises next-generation nanometric magnetoelectronic devices; particularly, the electric field control of ferromagnetic transition in dilute magnetic semiconductor (DMS) systems offers the magnetoelectronic devices a potential for low power consumption and low variability. Special attention has been given to technologically important group IV semiconductor based DMSs, with a prominent position for Mn doped Ge. In this paper, we will first review the current theoretical understanding on the ferromagnetism in MnxGe1−x DMS, pointing out the possible physics models underlying the complicated ferromagnetic behavior of MnxGe1−x. Then we carry out detailed analysis of MnxGe1−x thin films and nanostructures grown by molecular beam epitaxy. We show that with zero and one dimension quantum structures, superior magnetic properties of MnxGe1−x compared with bulk films can be obtained. More importantly, with MnxGe1−x nanostructures, such as quantum dots, we demonstrate a field controlled ferromagnetism up to 100 K. Finally we provide a prospective of the future development of ferromagnetic field effect transistors and magnetic tunneling junctions/memories using dilute and metallic MnxGe1−x dots, respectively. We also point out the bottleneck problems in these fields and rendering possible solutions to realize practical spintronic devices.

The systematic effective Lagrangian method was first formulated in the context of the strong interaction; chiral perturbation theory (CHPT) is the effective theory of quantum chromodynamics (QCD). It was then pointed out that the method can be transferred to the nonrelativistic domain—in particular, to describe the low-energy properties of ferromagnets. Interestingly, whereas for Lorentz-invariant systems the effective Lagrangian method fails in one spatial dimension (ds=1), it perfectly works for nonrelativistic systems in ds=1. In the present brief review, we give an outline of the method and then focus on the partition function for ferromagnetic spin chains, ferromagnetic films, and ferromagnetic crystals up to three loops in the perturbative expansion—an accuracy never achieved by conventional condensed matter methods. We then compare ferromagnets in ds=1, 2, 3 with the behavior of QCD at low temperatures by considering the pressure and the order parameter. The two apparently very different systems (ferromagnets and QCD) are related from a universal point of view based on the spontaneously broken symmetry. In either case, the low-energy dynamics is described by an effective theory containing Goldstone bosons as basic degrees of freedom.

The phase diagram of the single-orbit double exchange model for manganites with ferromagnetic Hund coupling between mobile eg electrons and spins of localized t2g electrons as well as antiferromagnetic superexchange coupling between t2g electrons is investigated with a large scale Monte Carlo simulation in one dimension. The phase boundary is determined based on the internal energy, the electron density and the structure factor. In particular, low-temperature properties at quarter filling are studied in detail.

On the basis of Bethe ansatz solution of two-component bosons with SU(2) symmetry and δ-function interaction in one dimension, we study the thermodynamics of the system at finite temperature by using the strategy of thermodynamic Bethe ansatz (TBA). It is shown that the ground state is an isospin "ferromagnetic" state by the method of TBA, and at high temperature the magnetic property is dominated by Curie's law. We obtain the exact result of specific heat and entropy in strong coupling limit which scales like T at low temperature. While in weak coupling limit, it is found there is still no Bose-Einstein Condensation (BEC) in such 1D system.

Smart materials combine sensors, intelligence, and actuators to allow a material to respond to its environment. Magnetostrictive materials can be used as both the sensors and actuators in such materials. High-power magnetostrictive actuators can deliver forces greater than 50 MPa with strains of up to 0.6%, while other magnetostrictive sensor materials can provide hundreds of times the sensitivity of semiconductor strain gages. Magnetoelastic materials also have adaptable elastic moduli which may be varied by external magnetic fields.Magnetostriction is the change in any dimension of a magnetic material caused by a change in its magnetic state. In this article we concentrate on ferromagnetic materials exhibiting Joule magnetostriction, which is a change in linear dimension parallel to an applied magnetic field (see Figure 1), and the reciprocal effect in which the material changes its magnetic state under the influence of applied stress.The phenomenon of magnetostriction has been known for well over a century, since Joule discovered in 1847 the change in length of an iron rod when magnetized. The modern era of magnetostrictive materials began in 1963 with the measurement of nearly 1% magnetostrictive strains at low temperatures in the basal planes of Dy and Tb. A search for magnetostrictive materials with high magnetostriction at room temperature led to the alloying of rare earths with transition metals, culminating in the discovery in 1971 of giant room-temperature magnetostriction in the Laves phase compound TbFe2.

The study of thin ferromagnetic films has become a field of great interest due to their importance for applications in magnetic-recording media, in order to create ultrahigh density magnetic data storage. Such an interest is also due to numerous specific phenomena related to the low dimension of these systems. In this paper we investigate the magnetic properties of bilayer thin film. The specific behavior of the mentioned system type is simulated using the Monte Carlo technique, in the extended anisotropic Heisenberg model. The magnetization, out-of-plane and in-plane magnetic susceptibilities behaviors according to temperature are investigated in order to find out the contingent magnetic ordering phase and the critical temperature, for two easy-axis anisotropy set parameter assignments. We have also presented the specific heat as a function of temperature for different direct exchange parameter values and we have found out a single-peak shape of the graphs, the critical point traveling to lower temperatures as the direct exchange parameter decreases, the similarity of the two presented cases being evident.

"This paper aims to analyze the impact of using a thin magnetic shield placed in the space between the primary and secondary winding of a simplified, low power, single-phase transformer used in energy harvesting applications that demand power transformers not only in the energy conditioning stage but also in the energy harvesting stage. By using magnetic shields, the saturation of the ferromagnetic core and, in some particular cases, the destruction of electronic devices is avoided. For this purpose two scenarios are studied: one which doesn't take into account the magnetic shield, as it considers only the air space between the primary and secondary windings, respectively, and the second case study which considers a magnetic screen placed in the centre of the air space domain. The size of the air space domain, d, is varied as the secondary winding distance itself from the primary one until it reaches the core. The number of turns in the primary and secondary winding is equal, N1 = N2 = 300 turns. By moving the secondary winding away from the primary winding, the variation of the distance d between the coils is achieved, thus keeping the same cross-section of the secondary winding. The thickness of the magnetic shield is chosen arbitrarily, as thin as possible, with a dimension of 400 µm. The idealy, 1:1, simplified, low-power, single-phase transformer powered by a harmonic voltage supply at V1 = 20 V and at a frequency, f = 50 Hz, with load resistance of Rs = 100 Ω, is analyzed in a time-dependent study and its computational domain is taken from literature [4]. Different materials can be used for realizing this magnetic shieling, even copper and aluminum, but in this paper a magnetic sheet metal material is considered because of its small, almost nonexistent electrical conductivity. Our goal is to analyze the effect of magnetic shielding on the saturation of the ferromagnetic core, and the reactance and resistance values of the primary and secondary winding, respectively, for different dimensions of the air space, d. For comparison purposes, the second model, the one in which we have the magnetic sheet metal, an analysis is performed in the permanent harmonic regime, in addition to the one performed in the dynamic one."

AbstractIn the last few years there has been significant interest in the field of thin films, due to numerous specific phenomena related to the low dimension of these systems, and to the large opportunities in development of high technologies based on their specific magnetic and electronic properties. When dealing with systems of reduced dimensionality it is important to take into account the influence of magnetic anisotropies. In this paper we investigate the magnetic properties of bilayer thin film. This behavior is modeled using Monte Carlo simulations, in the Extended Anisotropic Heisenberg Model. The magnetization, out-of-plane and in-plane magnetic susceptibilities, and also the specific heat bearings according to temperature are investigated in order to find the potential magnetic ordering phases and the critical temperatures, for two sets parameter assignments. For quasi-uniform anisotropy parameters of the film we detect the ferromagnetism-paramagnetism transition and then, by changing the model parameters values, we relieve a short range ferromagnetic ordering phase arising from the antiferromagnetic base layer coupling influence and from easy-plane anisotropy discontinuity on the layers interface.

AbstractWe consider the ferromagnetic quantum Heisenberg model in one dimension, for any spin $$S\ge 1/2$$ S ≥ 1 / 2 . We give upper and lower bounds on the free energy, proving that at low temperature it is asymptotically equal to the one of an ideal Bose gas of magnons, as predicted by the spin-wave approximation. The trial state used in the upper bound yields an analogous estimate also in the case of two spatial dimensions, which is believed to be sharp at low temperature.

SILVA, Wanêssa Façanha. Ondas de spin em redes decoradas. 2014. 62 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014.
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Low-dimensional systems have attracted much attention lately due to systems such as graphene and carbon nanotubes. Such systems have great potential for technological applications. In particular the creation of electronic devices due to their specific electronic properties. In this sense , the study of other systems in low dimension becomes urgent. More specifically , the study of magnetic properties of materials at low dimensionality also brings great new features in the behavior of ferromagnetic systems . The behavior of spin waves in such systems may be important to the study of spintronic and the development of new devices and magnetic memories . Thus in this work we aim to study the behavior of ferromagnetic spin waves in two-dimensional systems . For two-dimensional systems we consider here two-dimensional networks decorated . The decorations are introduced to generate networks with more than one basic atom in the unit cell of the system to study the richness of the spectrum of spin waves due to these changes . At first deal with a superimposition of square networks where the displacement of these networks depends on the control parameters alpha and beta . We also use the superposition of a square on a hexagonal network.
Sistema de baixa dimensionalidade têm atraído uma grande atenção ultimamente devido a sistemas como grafeno e nanotubos de carbono. Tais sistemas têm grandes possibilidades de aplicações tecnológicas, em particular na criação de dispositivos eletrônicos, devido às suas propriedades eletrônicas específicas. Nesse sentido, o estudos de outros sistemas em baixa dimensão se torna urgente. Mais especificamente, o estudo de propriedades magnéticas de materiais de materiais em baixa dimensionalidade também trás grandes novidades no comportamento de sistemas ferromagnéticos. O comportamento de ondas de spin em tais sistemas pode ser para o estudo da spintrônica e o desenvolvimento de novos aparelhos e memórias magnéticas. Dessa forma temos como objetivo nesse trabalho estudar o comportamento de ondas de spin em sistemas bidimensionais ferromagnéticos. Por sistemas bidimensionais consideramos aqui redes bidimensionais decoradas. As decorações são introduzidas para gerar redes com mais de um átomo na base da célula unitária da rede para estudarmos a riqueza do espectro das ondas de spin devido a essas modificações. A princípio tratamos com uma superposição de redes quadradas onde o deslocamento dessas redes depende dos parâmetros de controle α e β. Também usamos a superposição de um rede quadrada sobre um hexagonal.

Sistema de baixa dimensionalidade tÃm atraÃdo uma grande atenÃÃo ultimamente devido a sistemas como grafeno e nanotubos de carbono. Tais sistemas tÃm grandes possibilidades de aplicaÃÃes tecnolÃgicas, em particular na criaÃÃo de dispositivos eletrÃnicos, devido Ãs suas propriedades eletrÃnicas especÃficas. Nesse sentido, o estudos de outros sistemas em baixa dimensÃo se torna urgente. Mais especificamente, o estudo de propriedades magnÃticas de materiais de materiais em baixa dimensionalidade tambÃm trÃs grandes novidades no comportamento de sistemas ferromagnÃticos. O comportamento de ondas de spin em tais sistemas pode ser para o estudo da spintrÃnica e o desenvolvimento de novos aparelhos e memÃrias magnÃticas. Dessa forma temos como objetivo nesse trabalho estudar o comportamento de ondas de spin em sistemas bidimensionais ferromagnÃticos. Por sistemas bidimensionais consideramos aqui redes bidimensionais decoradas. As decoraÃÃes sÃo introduzidas para gerar redes com mais de um Ãtomo na base da cÃlula unitÃria da rede para estudarmos a riqueza do espectro das ondas de spin devido a essas modificaÃÃes. A princÃpio tratamos com uma superposiÃÃo de redes quadradas onde o deslocamento dessas redes depende dos parÃmetros de controle α e β. TambÃm usamos a superposiÃÃo de um rede quadrada sobre um hexagonal
Low-dimensional systems have attracted much attention lately due to systems such as graphene and carbon nanotubes. Such systems have great potential for technological applications. In particular the creation of electronic devices due to their specific electronic properties. In this sense , the study of other systems in low dimension becomes urgent. More specifically , the study of magnetic properties of materials at low dimensionality also brings great new features in the behavior of ferromagnetic systems . The behavior of spin waves in such systems may be important to the study of spintronic and the development of new devices and magnetic memories . Thus in this work we aim to study the behavior of ferromagnetic spin waves in two-dimensional systems . For two-dimensional systems we consider here two-dimensional networks decorated . The decorations are introduced to generate networks with more than one basic atom in the unit cell of the system to study the richness of the spectrum of spin waves due to these changes . At first deal with a superimposition of square networks where the displacement of these networks depends on the control parameters alpha and beta . We also use the superposition of a square on a hexagonal network.

In Chapter 14, the singular behavior of ferromagnetic systems with O(N) symmetry and short-range interactions, near a second order phase transition has been determined in the mean-field approximation, which is also a quasi-Gaussian approximation. The mean-field approximation predicts a set of universal properties, properties independent of the detailed structure of the microscopic Hamiltonian, the dimension of space, and, to a large extent, of the symmetry of systems. However, the leading corrections to the mean-field approximation, in dimensions smaller than or equal to four, diverge at the critical temperature, and the universal predictions of the mean-field approximation cannot be correct. Such a problem originates from the non-decoupling of scales and leads to the question of possible universality. In Chapter 9, the question has been answered in four dimensions using renormalization theory, and related renormalization group (RG) equations. Moreover, below four dimensions, in an expansion around the mean-field, the most singular terms near criticality can be also formally recovered from a continuum, low-mass φ⁴ field theory. More generally, following Wilson, to understand universality beyond the mean-field approximation, it is necessary to build a general renormalization group in the form of flow equations for effective Hamiltonians and to find fixed points of the flow equations. Near four dimensions, the flow equations can be approximated by the renormalization group of quantum field theory (QFT), and the fixed points and critical behaviours derived within the framework of the Wilson-Fisher ϵ expansion.