Dissertations / Theses on the topic 'SCIENCE / Physics / Magnetism'

Palladium nanoparticles have been synthesized using a systematic variation of the two-phase Burst-Schiffrin reaction to specifically tailor their physical properties. Furthermore, hexanethiolate and phenylethanethiolate ligands have been added to kinetically stabilize the nanoparticles and as a consequence the magnetic properties have been altered due the change in ligand-nanoparticle exchange interaction. The magnetic properties of the nanoparticles were then studied via the vibrating sample magnetometer and subsequently compared with similar experiments in the nanomagnetism literature. A distinctive increase in magnetic saturation, remanence and coercivity has been evidenced by comparing the phenylethanethiolate ligand group samples to the hexanethiolate ligand group samples, indicating the importance of capping agents within this popular subject.

The present thesis is concerned to the application of first-principles self-consistent total-energy calculations within the density functional theory on different topics in materials science.

Crystallographic phase-transitions under high-pressure has been study for TiO2, FeI2, Fe3O4, Ti, the heavy alkali metals Cs and Rb, and C3N4. A new high-pressure polymorph of TiO2 has been discovered, this new polymorph has an orthorhombic OI (Pbca) crystal structure, which is predicted theoretically for the pressure range 50 to 100 GPa. Also, the crystal structures of Cs and Rb metals have been studied under high compressions. Our results confirm the recent high-pressure experimental observations of new complex crystal structures for the Cs-III and Rb-III phases. Thus, it is now certain that the famous isostructural phase transition in Cs is rather a new crystallographic phase transition.

The elastic properties of the new superconductor MgB2 and Al-doped MgB2 have been investigated. Values of all independent elastic constants (c11, c12, c13, c33, and c55) as well as bulk moduli in the a and c directions (Ba and Bc respectively) are predicted. Our analysis suggests that the high anisotropy of the calculated elastic moduli is a strong indication that MgB2 should be rather brittle. Al doping decreases the elastic anisotropy of MgB2 in the a and c directions, but, it will not change the brittle behaviour of the material considerably.

The three most relevant battery properties, namely average voltage, energy density and specific energy, as well as the electronic structure of the Li/LixMPO4 systems, where M is either Fe, Mn, or Co have been calculated. The mixing between Fe and Mn in these materials is also examined. Our calculated values for these properties are in good agreement with recent experimental values. Further insight is gained from the electronic density of states of these materials, through which conclusions about the physical properties of the various phases are made.

The electronic and magnetic properties of the dilute magnetic semiconductor Mn-doped ZnO has been calculated. We have found that for an Mn concentration of 5.6%, the ferromagnetic configuration is energetically stable in comparison to the antiferromgnetic one. A half-metallic electronic structure is calculated by the GGA approximation, where Mn ions are in a divalent state leading to a total magnetic moment of 5 μB per Mn atom.

The magnetic properties of Metallo-organic heterostructure interfaces are studied. These heterostructures are built with manganese phthalocyanine (MnPc) and iron phthalocyanine (FePc). Previously, the powder of each material is reported to be an Ising-like chain magnet with Arrhenius relaxation. The relaxation is slow enough to exhibit magnetic hysteresis at low temperatures. Each layer of the heterostructure is investigated separately by depositing a thin film of either iron phthalocyanine (FePc) or manganese phthalocyanine (MnPc) on a Silicon substrate heated to 150 °C. FePc thin films show magnetic hysteresis below 5K with a typical coercivity of 1850 ± 50 Oe and moment of about 1.9 µB in agreement with values from the literature. Similarly, the MnPc thin film deposited at 150 °C shows magnetic hysteresis at 2.5 K, and no hysteresis at 5K and 10 K. A coercive field of 390 Oe is recorded at 2.5 K. The saturation magnetization is near 9 emu cm^–3, which corresponds to an effective magnetic moment per Mn ion of about 0.5 µB. For the MnPc/FePc thin film bilayer, the FePc is deposited at 150 °C onto the Silicon substrate, the sample is cooled to room temperature followed by the MnPc deposition in situ. The magnetic moment of this heterostructure is consistent with contributions from the FePc layer only, since the room temperature deposited MnPc has antiferromagnetic characteristics. This heterostructure has magnetic hysteresis with a coercivity of 910 Oe. No measurable shift of the hysteresis loops—as expected for an antiferromagnetic-ferromagnetic coupled interface—is observed in this set of bilayers.

Magnetic relaxation describes the process by which a magnetic system prepared in a non-equilibrium state returns to an equilibrium distribution. Thin film samples of iron phthalocyanine (FePc) are deposited onto heated substrates. Substrates are made of either silicon, or smooth gold on silicon. FePc molecules self-assemble into small crystalline structures. Due to the planar shape of the molecules, iron chains are formed. The length of these chains depends on the deposition temperature of the sample. Here, FePc thin films are saturated in an applied magnetic field of 3 T at low temperatures. The magnetic field is then reduced to 0 T at a rate of either 54 Oe/s or 100 Oe/s. The change of the magnetization at zero-field and constant temperature is recorded over a time interval of 5000 s. A series of 200 nm thick FePc samples are prepared at varying deposition temperatures onto silicon substrates. Based on the separation distance between iron chains, the inter-chain interactions—probably based on dipole interactions—is expected to be small. The intra-chain interactions are modified by the grain size. Using the stretched exponential model, a non-vanishing asymptotic remnant magnetization is found. The value of this asymptote is shown to decay exponentially with measurement temperature, and vanish near 4.5 K. The dynamic response has a peak which becomes higher in temperature with larger grain size up to 180°C, where we expect a phase transition in the thin film morphology. Above 3.2 K, the relaxation time appears activated, but the data is inconclusive at this moment. From these results, we find that both static and dynamic magnetic responses play an important role in FePc thin films in the measured temperature range of 2.5 K to 4.0 K. Asymptotic remnant magnetization, the static variable, is only non-zero below 4 K and importantly depends on the grain size as larger grains tend to make the inter-chain interactions more important.

The Langmuir-Blodgett deposition process is investigated for creating polystyrene nanosphere monolayers on a hydrophilic silicon substrate. The monolayers are fabricated over areas ~1 cm² and sputter coated with 100Å of permalloy. The quality of the monolayers is analyzed with optical microscope image processing, and 2D Fourier transforms of electron microscope images. The magnetic switching behavior of the sputtered samples is measured using an alternating gradient magnetometer, and compared to completely flat permalloy. The magnetic hysteresis measurements are done at different angle between the easy and hard axis of the flat permalloy films. The measurements show different hysteresis shapes for nanosphere patterned permalloy and flat permalloy, with the difference becoming greater nearer the hard axis of the flat permalloy samples. The ambiguity of an easy or hard axis on a curved surface is likely to contribute to the difference in magnetic switching behavior between the two sample types.

The concept of the geometric Berry phase of the quantum mechanical wave function has led to a better theoretical understanding of natural phenomena in all fields of fundamental physics research. In condensed matter physics, the impact of this theoretical discovery has been particularly profound: The quantum Hall effect, the anomalous Hall effect, the quantum spin Hall effect, magnetic skyrmions, topological insulators, and topological semimetals are but a few subfields that have witnessed rapid developments over the three decades since Michael Berry's landmark paper. In this thesis, I will present and discuss the results of three experiments where Berry's phase leads to qualitatively new transport behavior of electrons or magnetic spin excitations in solids.

We introduce the theoretical framework that leads to the prediction of a thermal Hall effect of magnons in Cu(1,3-bdc), a simple two-dimensional layered ferromagnet on a Kagomé net of spin S = 1/2 copper atoms. Combining our experimental results measured down to very low temperatures T = 0.3 K with published data from inelastic neutron scattering, we report a quantitative comparison with the theory. This confirms the expected net Berry curvature of the magnon band dispersion in this material.

Secondly, we have studied the thermal Hall effect in the frustrated pyrochlore magnet Tb₂Ti₂O₇, where the thermal Hall effect is large in the absence of long-range magnetic order. We establish the magnetic nature of the thermal Hall effect in Tb₂Ti₂O₇, introducing this material as the first example of a paramagnet with non-trivial low-lying spin excitations. Comparing our results to other materials with zero thermal Hall effect such as the classical spin ice Dy₂Ti ₂O₇ and the non-magnetic analogue Y₂Ti₂O ₇, we carefully discuss the experimental limitations of our setup and rule out spurious background signals.

The third and final chapter of this thesis is dedicated to electrical transport and thermopower experiments on the half-Heusler material GdPtBi. A careful doping study of the negative longitudinal magnetoresistance (LMR) establishes GdPtBi as a new material platform to study the physical properties of a simple Weyl metal with only two Weyl points (for magnetic field along the crystallographic ⟨111⟩ direction). The negative LMR is associated with the theory of the chiral anomaly in solids, and a direct consequence of the nonzero Berry curvature of the energy band structure of a Weyl semimetal. We compare our results to detailed calculations of the electronic band structure. Moving beyond the negative LMR, we report for the first time the effect of the chiral anomaly on the longitudinal thermopower in a Weyl semimetal.