arXiv subjects

Hamza Polat

Publications and source records attributed to Hamza Polat.

At least 19 recordsLinked to original sources

arXiv2020

Thermal Properties of Rung Disordered Two-leg Quantum Spin Ladders: Quantum Monte Carlo Study

A two-leg quenched random bond disordered antiferromagnetic spin$-1/2$ Heisenberg ladder system is investigated by means of stochastic series expansion (SSE) quantum Monte Carlo (QMC) method. Thermal properties of the uniform and staggered susceptibilities, the structure factor, the specific heat and the spin gap are calculated over a large number of random realizations in a wide range of disorder strength. According to our QMC simulation results, the considered system has a special temperature point at which the specific heat take the same value regardless of the strength of the disorder. Moreover, the uniform susceptibility is shown to display the same character except for a small difference in the location of the special point. Finally, the spin gap values are found to decrease with increasing disorder parameter and the smallest gap value found in this study is well above the weak coupling limit of the clean case.

cond-mat.str-el↗

arXiv2015

Dynamic phase transitions in a ferromagnetic thin film system: A Monte Carlo simulation study

Dynamic phase transition properties of ferromagnetic thin film system under the influence both bias and time dependent magnetic fields have been elucidated by means of kinetic Monte Carlo simulation with local spin update Metropolis algorithm. The obtained results after a detailed analysis suggest that bias field is the conjugate field to dynamic order parameter, and it also appears to define a phase line between two antiparallel dynamic ordered states depending on the considered system parameters. Moreover, the data presented in this study well qualitatively reproduce the recently published experimental findings where time dependent magnetic behavior of a uniaxial cobalt films is studied in the neighborhood of dynamic phase transition point.

cond-mat.stat-mech↗

arXiv2015

Magnetic response of a disordered binary ferromagnetic alloy to an oscillating magnetic field

By means of Monte Carlo simulation with local spin update Metropolis algorithm, we have elucidated non-equilibrium phase transition properties and stationary-state treatment of a disordered binary ferromagnetic alloy of the type $A_{p}B_{1-p}$ on a square lattice. After a detailed analysis, we have found that the system shows many interesting and unusual thermal and magnetic behaviors, for instance, the locations of dynamic phase transition points change significantly depending upon amplitude and period of the external magnetic field as well as upon the active concentration of $A-$ type components. Much effort has also been dedicated to clarify the hysteresis tools, such as coercivity, dynamic loop area as well as dynamic correlations between time dependent magnetizations and external time dependent applied field as a functions of period and amplitude of field as well as active concentration of of $A-$ type components, and outstanding physical findings have been reported in order to better understand the dynamic process underlying present system.

cond-mat.stat-mech↗

arXiv2015

Monte Carlo simulations of dynamic phase transitions in ferromagnetic thin-films

By means of detailed Monte Carlo (MC) simulations, we have presented dynamic phase transition (DPT) properties of ferromagnetic thin-films. Thermal variations of surface, bulk and total dynamical order parameters (DOP) for a film and total order parameter for the films with different thicknesses have been examined. Opposite regimes of the critical value of reduced exchange interaction (surface to bulk ratio) $R_{c}$ at which the critical temperature becomes independent of film thickness $L$ has been also taken into consideration. The average magnetizations of each layer is reversed in these regimes. Based on the results, we have confirmed that the system represents a crossover behavior in between ordinary to extraordinary transition in the presence of surface exchange enhancement.

cond-mat.stat-mech↗

arXiv2014

Nonequilibrium dynamics of a mixed spin-1/2 and spin-3/2 Ising ferrimagnetic system with a time dependent oscillating magnetic field source

Nonequilibrium phase transition properties of a mixed Ising ferrimagnetic model consisting of spin-1/2 and spin-3/2 on a square lattice under the existence of a time dependent oscillating magnetic field have been investigated by making use of Monte Carlo simulations with single-spin flip Metropolis algorithm. A complete picture of dynamic phase boundary and magnetization profiles have been illustrated and the conditions of a dynamic compensation behavior have been discussed in detail. According to our simulation results, the considered system does not point out a dynamic compensation behavior, when it only includes the nearest-neighbor interaction, single-ion anisotropy and an oscillating magnetic field source. As the next-nearest-neighbor interaction between the spins-1/2 takes into account and exceeds a characteristic value which sensitively depends upon values of single-ion anisotropy and only of amplitude of external magnetic field, a dynamic compensation behavior occurs in the system. Finally, it is reported that it has not been found any evidence of dynamically first-order phase transition between dynamically ordered and disordered phases, which conflicts with the recently published molecular field investigation, for a wide range of selected system parameters.

cond-mat.stat-mech↗

arXiv2014

Dynamic hysteretic features of the Ising-type thin films

In order to elucidate the important characteristics of hysteretic response, such as types of the frequency dispersion curves, the decay of hysteresis loop area, the mechanism of domain nucleation and/or growth in the dynamic process, etc., a followup examination after the original work has been presented. The additive complementary treatment is essentially based on the results of recent preprint arXiv:1302.2727 concerning the effect of oscillatory perturbation on the surface enhancement phenomenon in magnetic thin films. Throughout the analysis, the best appropriate parameter values have been chosen since they would allow us to achieve striking results. The topology of the response for different layer indices in two regimes of modified surface exchange has been particularly emphasized.

cond-mat.stat-mech↗

arXiv2013

Hysteretic response characteristics and dynamic phase transition via site dilution in the kinetic Ising model

The decay of the hysteresis loop area of the system, which is obeying a site diluted kinetic Ising model, is considered by the disorder parameter using the effective field theory analysis. The exhibition focuses on the understanding of external field frequency, amplitude and the site concentration dependency of the hysteresis loop area for several powerful treatments. Important characteristics of the hysteretic response, such as frequency dispersion, effect of domain nucleation phenomenon on the dynamic process etc. has been introduced together with well known other characteristics. An attempt has been made to explain the relations between the competing time scales (intrinsic microscopic relaxation time of the system and the time period of the external oscillatory field) and the shape of the response. As a result of the detailed investigations, existence of essentially three, particularly four types of dispersion curves have been propounded.

cond-mat.stat-mech↗

arXiv2013

Effective-field theory study of the dynamical Ising-type thin films

The stationary state solutions of the Ising-type thin films with different layers in the presence of an external oscillatory field have been examined within the effective field theory. The exhibition focuses on understanding of the external field frequency and amplitude effect on the overall behavior of system for several powerful treatments. The particular attention has been paid on the dynamical parametric evolution of the special point at which the critical temperature of the three-dimensional infinite bulk system, where the surface and the modified exchange parameter are of no importance. Important characteristics of the films, such as surface enhancement phenomenon, effect of thickness on the dynamic process etc. have been introduced together with well known other characteristics. An attempt has been made to explain the relations between the competing time scales (intrinsic microscopic relaxation time of the system and the time period of the external oscillatory field) and frequency dispersion of the critical temperature coordinate of the special point.

cond-mat.stat-mech↗

arXiv2013

Time dependent magnetic field effects on the $\pm J$ Ising model

Nonequilibrium phase transition properties of the $\pm J$ Ising model under a time dependent oscillating perturbation are investigated within the framework of effective field theory for a two-dimensional square lattice. After a detailed analysis, it is found that the studied system exhibits unusual and interesting behaviors such as reentrant phenomena, and the competition between ferromagnetic and antiferromagnetic exchange interactions gives rise to destruct the dynamic first order phase transitions as well as dynamic tricritical points. Furthermore, according to Néel nomenclature, the magnetization profiles have been found to obey Q-type, L-type and P-type classification schemes under certain conditions. Finally, it is observed that the treatment of critical percolation with applied field amplitude strongly depends upon the frequency of time varying external field.

cond-mat.stat-mech↗

arXiv2013

Monte Carlo investigation of a spherical ferrimagnetic core-shell nanoparticle under a time dependent magnetic field

Monte Carlo simulation based on Metropolis algorithm has been used with a great success to analyze the dynamic phase transition properties of a single spherical core-shell nanoparticle system with a spin-3/2 core surrounded by a spin-1 shell layer with antiferromagnetic interface coupling under the influence of a time dependent oscillating magnetic filed. It has been found that the dynamic phase boundaries strongly depend on the Hamiltonian parameters such as for the high amplitude and period values of the external field, the phase transition temperature sharply changes whereas it tends to slowly alter as the reduced magnitude of interlayer parameter $\mathrm{J_{int}/J_{sh}}$ increases. Moreover, it is observed that the magnetization curves of the particle have been found to obey P-type, N-type and Q-type classification schemes under certain conditions. Much effort has also been paid to the influence of the particle size on the thermal and magnetic properties of the particle. Finally, a comparison of our observations with those of recently published study including dynamic treatments of a nanocubic core-shell system is represented and the findings indicate that there exists a qualitatively good agreement with some relatively distinct differences.

cond-mat.stat-mech↗

arXiv2012

Dynamic phase transition properties and hysteretic behavior of a ferrimagnetic core-shell nanoparticle in the presence of a time dependent magnetic field

We have presented dynamic phase transition features and stationary-state behavior of a ferrimagnetic small nanoparticle system with a core-shell structure. By means of detailed Monte Carlo simulations, a complete picture of the phase diagrams and magnetization profiles have been presented and the conditions for the occurrence of a compensation point $T_{comp}$ in the system have been investigated. According to Néel nomenclature, the magnetization curves of the particle have been found to obey P-type, N-type and Q-type classification schemes under certain conditions. Much effort has been devoted to investigation of hysteretic response of the particle and we observed the existence of triple hysteresis loop behavior which originates from the existence of a weak ferromagnetic core coupling $J_{c}/J_{sh}$, as well as a strong antiferromagnetic interface exchange interaction $J_{int}/J_{sh}$. Most of the calculations have been performed for a particle in the presence of oscillating fields of very high frequencies and high amplitudes in comparison with exchange interactions which resembles a magnetic system under the influence of ultrafast switching fields. Particular attention has also been paid on the influence of the particle size on the thermal and magnetic properties, as well as magnetic features such as coercivity, remanence and compensation temperature of the particle. We have found that in the presence of ultrafast switching fields, the particle may exhibit a dynamic phase transition from paramagnetic to a dynamically ordered phase with increasing ferromagnetic shell thickness.

cond-mat.mtrl-sci↗

arXiv2012

Effects of the quenched random crystal field on the non-equilibrium relaxation process of the spin-3/2 Blume-Capel model

In this work, the relaxation process of the spin-3/2 Blume-Capel model with quenched random crystal field on a two dimensional square lattice has been investigated by a method which combines the statistical equilibrium theory and the thermodynamic of linear irreversible processes. Generalized force and flux have been defined in irreversible thermodynamics limit. The kinetic equation for the magnetization has been obtained by using linear response theory. In order to understand effect of the first-order phase transition on the relaxation process, low temperature regions have been analyzed. And also, dependence on the temperature and random crystal field of the relaxation process has been investigated near the second-order and isolated critical phase transition points. Moreover, evidence of the reentrant behavior has been observed on the relaxation process. Finally, critical exponents of the relaxation time and magnetization have been found by benefiting from log-log plot.

cond-mat.stat-mech↗

arXiv2012

Nonequilibrium dynamics of a spin-3/2 Blume Capel model with quenched random crystal field

The relaxation and complex magnetic susceptibility treatments of a spin-3/2 Blume-Capel model with quenched random crystal field on a two dimensional square lattice are investigated by a method combining the statistical equilibrium theory and the thermodynamics of linear irreversible processes. Generalized force and flux are defined in irreversible thermodynamics limit. The kinetic equation for the magnetization is obtained by using linear response theory. Temperature and also crystal field dependencies of the relaxation time are obtained in the vicinity of phase transition points. We found that the relaxation time exhibits divergent treatment near the order-disorder phase transition point as well as near the isolated critical point whereas it displays cusp behavior near the first order phase transition point. In addition, much effort has been devoted to investigation of complex magnetic susceptibility response of the system to changing applied field frequencies and it is observed that the considered disordered magnetic system exhibits unusual and interesting behaviors. Furthermore, dynamical mean field critical exponents for the relaxation time and complex magnetic susceptibility are calculated in order to formulate the critical behavior of the system. Finally, a comparison of our observations with those of recently published studies is represented and it is shown that there exists a qualitatively good agreement.

cond-mat.stat-mech↗

arXiv2012

Investigation of oscillation frequency and disorder induced dynamic phase transitions in a quenched-bond diluted Ising ferromagnet

Frequency evolutions of hysteresis loop area and hysteresis tools such as remanence and coercivity of a kinetic Ising model in the presence of quenched bond dilution are investigated in detail. The kinetic equation describing the time dependence of the magnetization is derived by means of effective-field theory with single-site correlations. It is found that the frequency dispersions of hysteresis loop area, remanence and coercivity strongly depend on the quenched bond randomness, as well as applied field amplitude and oscillation frequency. In addition, the shape of the hysteresis curves for a wide variety of Hamiltonian parameters is studied and some interesting behaviors are found. Finally, a comparison of our observations with those of recently published studies is represented and it is shown that there exists a qualitatively good agreement.

cond-mat.stat-mech↗

arXiv2012

Investigation of bond dilution effects on the magnetic properties of a cylindrical Ising nanowire

A cylindrical magnetic nanowire system composed of ferromagnetic core and shell layers has been investigated by using effective field theory with correlations. Both ferromagnetic and antiferromagnetic exchange couplings at the core-shell interface have been considered. Main attention has been focused on the effects of the quenched disordered shell bonds, as well as interface bonds on the magnetic properties of the system. A complete picture of the phase diagrams and magnetization profiles has been represented. It has been shown that for the antiferromagnetic nanowire system, the magnetization curves can be classified according to Néel theory of ferrimagnetism and it has been found that under certain conditions, the magnetization profiles may exhibit Q-type, P-type, N-type and L-type behaviors. The observed L-type behavior has not been reported in the literature before for the equilibrium properties of nanoscaled magnets. As another interesting feature of the system, it has been found that a compensation point can be induced by a bond dilution process in the surface. Furthermore, we have not found any evidence of neither the first order phase transition characteristics, nor the reentrance phenomena.

cond-mat.stat-mech↗

arXiv2012

Nonequilibrium phase transitions and stationary state solutions of a three-dimensional random-field Ising model under a time dependent periodic external field

Nonequilibrium behavior and dynamic phase transition properties of a kinetic Ising model under the influence of periodically oscillating random-fields have been analyzed within the framework of effective field theory (EFT) based on a decoupling approximation (DA). Dynamic equation of motion has been solved for a simple cubic lattice ($q=6$) by utilizing a Glauber type stochastic process. Amplitude of the sinusoidally oscillating magnetic field is randomly distributed on the lattice sites according to bimodal and trimodal distribution functions. For a bimodal type of amplitude distribution, it is found in the high frequency regime that the dynamic phase diagrams of the system in temperature versus field amplitude plane resemble the corresponding phase diagrams of pure kinetic Ising model. Our numerical results indicate that for a bimodal distribution, both in the low and high frequency regimes, the dynamic phase diagrams always exhibit a coexistence region in which the stationary state (ferro or para) of the system is completely dependent on the initial conditions whereas for a trimodal distribution, coexistence region disappears depending on the values of system parameters.

cond-mat.stat-mech↗

arXiv2011

A new effective field theory for spin-S (S<=1) dilute Ising ferromagnets

Site diluted spin-1/2 Ising and spin-1 Blume Capel (BC) models in the presence of transverse field interactions are examined by introducing an effective-field approximation that takes into account the multi-site correlations in the cluster of a considered lattice with an improved configurational averaging technique. The critical concentration below which the transition temperature reduces to zero is determined for both models, and the estimated values are compared with those obtained by the other methods in the literature. It is found that diluting the lattice sites by non magnetic atoms may cause some drastic changes on some of the characteristic features of the model. Particular attention has been paid on the global phase diagrams of a spin-1 BC model, and it has also been shown that the conditions for the occurrence of a second order reentrance in the system is rather complicated, since the existence or extinction of reentrance is rather sensitive to the competing effects between D/J, Ω/J and c.

cond-mat.stat-mech↗

arXiv2011

Critical behavior and phase diagrams of a spin-1 Blume-Capel model with random crystal field interactions: An effective field theory analysis

A spin-1 Blume-Capel model with dilute and random crystal fields is examined for honeycomb and square lattices by introducing an effective-field approximation that takes into account the correlations between different spins that emerge when expanding the identities. For dilute crystal fields, we have given a detailed exploration of the global phase diagrams of the system in $k_{B}T_{c}/J-D/J$ plane with the second and first order transitions, as well as tricritical points. We have also investigated the effect of the random crystal field distribution characterized by two crystal field parameters $D/J$ and $\triangle/J$ on the phase diagrams of the system. The system exhibits clear distinctions in qualitative manner with coordination number $q$ for random crystal fields with $\triangle/J,D/J\neq0$. We have also found that, under certain conditions, the system may exhibit a number of interesting and unusual phenomena, such as reentrant behavior of first and second order, as well as a double reentrance with three successive phase transitions.

cond-mat.stat-mech↗