Yusuf Yüksel

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 has 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 investigating the 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 to the influence of the particle size on the thermal and magnetic properties, as well as magnetic features such as coercivity, remanence and the 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.

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

Department of Physics, Dokuz Eylu¨l University, TR-35160 Izmir, Turkey(Dated: June 26, 2012)A cylindrical magnetic nanowire system composed of ferromagnetic core and shell layers has beeninvestigated by using effective field theory with correlations. Both ferromagnetic and antiferromag-netic exchange couplings at the core-shell interface have been considered. Main attention has beenfocused on the effects of the quenched disordered shell bonds, as well as interface bonds on themagnetic properties of the system. A complete picture of the phase diagrams and magnetizationprofiles has been represented. It has been shown that for the antiferromagnetic nanowire system,the magnetization curves can be classified according to N´eel theory of ferrimagnetism and it hasbeen 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 literaturebefore for the equilibrium properties of nanoscaled magnets. As another interesting feature of thesystem, it has been found that a compensation point can be induced by a bond dilution process inthe surface. Furthermore, we have not found any evidence of neither the first order phase transitioncharacteristics, nor the reentrance phenomena.I. INTRODUCTION

Effective field investigation of dynamic phase transitions for site diluted Ising ferromagnets driven by a periodically oscillating magnetic field

Dynamic behavior of a site diluted Ising ferromagnet in the presence of a periodically oscillating magnetic field has been analyzed by means of the effective field theory (EFT). The dynamic equation of motion has been solved for a honeycomb lattice (z=3) with the help of a Glauber type stochastic process. The global phase diagrams and the variation of the corresponding dynamic order parameter as a function of the Hamiltonian parameters and temperature has been investigated in detail and it has been shown that the system exhibits reentrant phenomena, as well as a dynamic tricritical point which disappears for sufficiently weak dilution.

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 based on a decoupling approximation. A 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 the temperature versus field amplitude plane resemble the corresponding phase diagrams of the 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, the coexistence region disappears depending on the values of the system parameters.

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 kBTc/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 △/J on the phase diagrams of the system. The system exhibits clear distinctions in a qualitative manner with coordination number q for random crystal fields with △/J,D/J≠0. 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.

An introduced effective-field approximation and Monte Carlo study of a spin-1 Blume-Capel model on a square lattice

The magnetic properties of a spin-1 Blume‐Capel (BC) model on a square lattice (q = 4) with a ferromagnetic interaction have been examined here by the use of Monte Carlo (MC) simulation technique and an introduced effective-field approximation (IEFT), which includes the correlations between different spins that emerge when expanding the identities. The effects of the external magnetic field and crystal field on the magnetic properties of the spin system are discussed in detail. In order to obtain credible results, a detailed comparison of the results obtained by the two methods has been made with those of the other methods in the literature. A number of interesting phenomena originating from the temperature, crystal field and external field have been found.

Stationary State Solutions of a Bond Diluted Kinetic Ising Model: An Effective-Field Theory Analysis

We examined the stationary state solutions of a bond diluted kinetic Ising model under a time dependent oscillating magnetic field within the effective-field theory (EFT) for a honeycomb lattice (q=3). The effects of the Hamiltonian parameters on the dynamic phase diagrams have been discussed in detail. Bond dilution process on the kinetic Ising model causes a number of interesting and unusual phenomena such as reentrant phenomena and has a tendency to destruct the first-order transitions and the dynamic tricritical point. Moreover, we have investigated the variation of the bond percolation threshold as functions of the amplitude and frequency of the oscillating field.

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 the recently published studies is represented and it is shown that there exists a qualitatively good agreement.

Effective-field-theory analysis of the three-dimensional random-field Ising model on isometric lattices.

An Ising model with quenched random magnetic fields is examined for single-Gaussian, bimodal, and double-Gaussian random-field distributions by introducing an effective-field approximation that takes into account the correlations between different spins that emerge when expanding the identities. Random-field distribution shape dependencies of the phase diagrams and magnetization curves are investigated for simple cubic, body-centered-cubic, and face-centered-cubic lattices. The conditions for the occurrence of reentrant behavior and tricritical points on the system are also discussed in detail.

Universality aspects of layering transitions in ferromagnetic Blume–Capel thin films

Abstract Critical phenomena and universality behavior of ferromagnetic thin films described by a spin-1 Blume–Capel Hamiltonian has been examined for various thickness values ranging from 3 to 40 layers. Using effective field theory, we have found that crystal field interactions significantly affects the critical value of surface to bulk ratio of exchange interactions Rc at which the critical temperature becomes independent of film thickness L. Moreover, we have extracted the shift exponent λ from computed data. Based on the results, we have shown that in the presence of surface exchange enhancement, the system may exhibit a dimensional crossover. We have also found that presence of crystal field interactions does not affect the value of λ. Hence, a ferromagnetic spin-1/2 thin film is in the same universality class with its spin-1 counterpart.