Critical behavior of low dimensional magnetic systems

Mixed spin Ising models have less translations symmetry than their 'single spin' counterparts and are well adapted for the study of a certain type of ferromagnetism. In this articles the high-temperature series expansion work of Schofield and Bowers (1981)-who studied mixed spin-1/2-spin-1 Ising models-is generalised by replacing the spin-1 objects by arbitrary spin-S ones. The new series are used to investigate the spin dependence or independence of critical parameters in a novel setting. There is no evidence to suggest that the exponents studied are spin dependent.

The classification of disordered phases of mixed spin (2,1/2) Ising model and the chaoticity of the corresponding dynamical system

Fracture analysis for a penny-shaped crack problem of a superconducting cylinder in a parallel magnetic field

Recent advances of experimental nanofabrication techniques draw increasing attention to the non-equilibrium behavior of low dimensional systems. Of particular interest are the strongly interacting one dimensional systems whose description in out of equilibrium situations remains a theoretical challenge. This thesis contributes to the understanding of important elementary processes in the non-equilibrium physics of one dimensional electron systems. Tunneling of an electron into a Luttinger liquid leads to partitioning of its charge and energy into counter-propagating modes. This thesis studies the partitioning of the energy which had previously remained unexplored. It turns out that energy partitioning is essentially independent of the charge partitioning and one can even reach conditions such that energy and charge propagate in opposite directions. Another important difference is their experimental accessibility. In contrast to the charge, energy partitioning provides a measurable characteristic of the tunneling process even in dc setups and we propose experimental geometries that allow for tuning and detecting energy partitioning. At higher excitation energies it becomes necessary to include curvature effects of the electron dispersion. Another part of this thesis discusses the consequences of curvature induced three-body collisions on the relaxation in quantum wires. This is particularly interesting due to the integrability of the Luttinger model which does not allow for thermalization within this paradigm of one dimensional systems. In this thesis we derive energy relaxation rates due to three-body processes beyond the Luttinger model within a well-defined perturbative approach. It turns out that the electron spin and the long range Coulomb interaction are important ingredients for a quantitative description of recent experiments which we provide in this thesis. Furthermore, we study the influence of three-body collisions on the energy relaxation in integer quantum Hall edge states. We specifically address different interaction induced edge reconstruction scenarios and find that edge reconstruction strongly enhances the energy relaxation. This is particularly pronounced when the reconstruction creates additional counter-propagating modes. Finally, we discuss another system which is crucially controlled by non-equilibrium effects. The so called nanoelectromechanical systems show a coupling between the electronic and mechanical degrees of freedom. The electron current can thus influence the mechanical motion which leads to a number of interesting applications. Previous theoretical studies on the basis of non-equilibrium Green's functions showed that these current induced forces can be expressed in terms of intuitive scattering matrix expressions. This thesis sheds considerable light on this observation by providing a much more satisfactory and concise derivation of the scattering theory of current induced forces.

Magnetic properties, reentrant phenomenon, and phase diagrams in mixed half-integer spin (1/2, 5/2) Ising model under a random crystal field

The magnetic properties of the double perovskite oxide Sr2FeMoO6 are analyzed using a mixed-spin Ising model with spins ( 1 2 , 5 2 ) in the presence of a random crystal field Δ and exchange interactions J on a three-dimensional cubic lattice. The study employs both the mean-field approximation based on the Bogoliubov inequality for Gibbs free energy and Monte Carlo simulations using the Metropolis algorithm to provide a comprehensive analysis of the system’s phase transitions and magnetization behavior, with focusing on the role of Fe and Mo sublattices. We establish the ground-state phase diagram, identifying multiple stable magnetic configurations and first-order transitions at low temperatures. Indicate compensation temperature T comp . This work provides deeper insight into the thermodynamic, statistical physics, and magnetic properties of Sr2FeMoO6,with implications for future applications in spintronics and magnetic storage technologies. revealing multiple stable magnetic states, first-order transitions, and with high Curie temperatures ( T c = 420 K) above room temperature, and a distinct compensation temperature T comp , underscoring its potential for spintronic applications and advanced magnetic storage technologies.

Anomalous inversion of voltage versus current curves in ZFC and FC conditions in BSCCO ceramic superconductors

Particle size and temperature dependence of exchange bias in NiO nanoparticles

Effect of AFM and FM exchange interaction on magnetic anisotropy properties of single domain SmFeO3 at nanoscale

Effect of iron substitution on structural, magnetic and nonlinear optical properties of zinc chromite nanoparticles

Multiferroic BaNiF4 has been fabricated by hydrothermal method. The bifurcation between zero field cooling (ZFC) and field cooling (FC) temperature dependent magnetization (M-T) curves starts at 150 K, indicating the 2D antiferromagnetic (FM) transition. A further upturn of magnetization has been observed below 68 K in FC M-T curve, corresponding to the emergence of 3D AFM structure. Wasp-waisted hysteresis loop was observed under 130 K, which is explained by that the AFM aligned net spins from the canting of neighboring AFM spins due to Dzyaloshinskii-Moriya interaction can be easily aligned by the magnetic field. Exchange bias effect was detected below 70 K, which has been interpreted by the magnetization pinned by the spontaneous polarization through magnetoelectric coupling.

We report on the interplay between surface morphology and the magnetic properties of Pt/Mn ultra-thin bilayers by measuring the temperature dependence of the resistivity of Hall bars cooled under application of an external magnetic field. The resistivity (ρ) versus temperature (T) curves measured under a magnetic field applied along the x-, y- and z- directions exhibit a clear difference between the zero-field-cooled (ZFC) and field-cooled (FC) traces. The disparity between the ZFC and FC traces allowed us to determine the direction of the magnetic easy-axis. The ρ vs. T characteristic of the Pt/Mn bilayers revealed the existence of antiferromagnetic (AF) order below 79 K, in agreement with the Néel temperature observed in temperature dependent susceptibility curves. We also investigated the morphological evolution of the Mn films as a function of Mn thickness by atomic force microscopy (AFM). We observed the formation of a Mn layered structure on a 2nm-thick Al2O3 buffer layer for films thicker than 2.5 nm. Our results suggest that bilayer structures consisting of non-magnetic noble metals on ultra-thin antiferromagnetic metals can be used as a probe to explore the complex magnetic properties of the AF layer.

The temperature dependence of the magnetization along with magnetization hysteresis loops were measured on dc sputtered La2∕3Ca1∕3MnO3[ferromagnetic(F)]∕YBa2Cu3O7−δ[superconductor(S)]∕La2∕3Ca1∕3MnO3(F) and La2∕3Ca1∕3MnO3(F)∕La1∕3Ca2∕3MnO3[antiferromagnetic(AF)]∕YBa2Cu3O7δ(S) trilayered structures. The magnetic fields were applied both parallel (in-plane) and perpendicular (out-of-plane) to the film plane. Magnetization hysteresis loops, recorded at 5K, shown a clear central peak around zero field when the magnetic field was applied perpendicular to the film plane and a typical ferromagnetic behavior when applied parallel. Additionally, the dependence of the magnetization on the temperature, performed under the zero-field-cooling (ZFC) and field-cooling (FC) regimens, evidenced the active presence of the diamagnetic and ferromagnetic phases both in the F∕S∕F and F∕AF∕S trilayers. Thus, the diamagnetic phase was clearly observed under the ZFC regimen, whereas a possible paramagnetic Meissner effect was observed under the FC regimen.

The effective-field theory studies of critical phenomena in a mixed spin-1 and spin-2 Ising model on honeycomb and square lattices

Dynamic compensation temperature in the mixed spin-1 and spin-2 Ising model in an oscillating field on alternate layers of a hexagonal lattice