Driven spin wave modes in XY ferromagnet: non-equilibrium phase transition

The dynamical responses of XY ferromagnet driven by linearly polarised propagating and standing magnetic field wave have been studied by Monte Carlo simulation in three dimensions. In the case of propagating magnetic field wave (with specified amplitude, frequency and the wavelength), the low temperature dynamical mode is a propagating spin wave and the system becomes structureless (or random) in the high temperature. A dynamical symmetry breaking phase transition is observed at a finite (non-zero) temperature. This symmetry breaking is confirmed by studying the statistical distribution of the angle of the spin vector. The dynamic non-equilibrium transition temperature was found to decrease as the amplitude of the propagating magnetic field wave increased. A comprehensive phase boundary is drawn in the plane formed by temperature and amplitude of propagating field wave. The phase boundary was observed to shrink (in the low temperature side) for longer wavelength of the propagating magnetic wave. In the case of standing magnetic field wave, the low temperature excitation is a standing spin wave which becomes structureless (or random) in the high temperature. Here also, like the case of propagating magnetic wave, a dynamical symmetry breaking non-equilibrium phase transition was observed. A comprehensive phase boundary was drawn. Unlike the case of propagating magnetic wave, the phase boundary does not show any systematic variation with the wavelength of the standing magnetic field wave. In the limit of vanishingly small amplitude of the field, the phase boundaries approach the recent Monte Carlo estimate of equilibrium transition temperature.     

动态外场作用下Ising自旋体系的非平衡动态相变

系统地考察了Ising自旋体系的动力学方程对三种不同性质的驱动外场（正弦波 、方波和锯齿波）的动态响应及其相应的非平衡动态相变特征.在正弦波和方波的驱动场 作用下，体系存在分别对应于低温对称破缺的铁磁有序态和高温对称顺磁无序态的动态非平衡转变，相应的动态转变相界上存在区分连续转变和非连续转变的三临界点；而锯齿波驱动 场情形下体系始终维持对称性破缺的有序态.体系动态转变表现出的上述差异与作用外场的驱动特征有关.确定了表征相应动态相变相界的临界驱动外场振幅h0C和频率 ωc、体系的温度tc, 并给予了分析讨论 关键词： Ising自旋体系 非平衡动态相变 对称性 平均场     

Monte Carlo study of dynamic phase transition in Ising metamagnet driven by oscillating magnetic field

The dynamical responses of Ising metamagnet (layered antiferromagnet) in the presence of a sinusoidally oscillating magnetic field are studied by Monte Carlo simulation. The time average staggered magnetisation plays the role of dynamic order parameter. A dynamical phase transition was observed and a phase diagram was plotted in the plane formed by field amplitude and temperature. The dynamical phase boundary is observed to shrink inward as the relative antiferromagnetic strength decreases. The results are compared with that obtained from pure ferromagnetic system. The shape of dynamic phase boundary observed to be qualitatively similar to that obtained from previous meanfield calculations.     

Nonequilibrium dynamical phase transition of 3D kinetic Ising／Heisenberg spin system

We have studied the nonequilibrium dynamic phase transitions of both three-dimensional (3D) kinetic Ising and Heisenberg spin systems in the presence of a perturbative magnetic field by Monte Carlo simulation. The feature of the phase transition is characterized by studying the distribution of the dynamical order parameter. In the case of anisotropic Ising spin system (ISS), the dynamic transition is discontinuous and continuous under low and high temperatures respectively, which indicates the existence of a tri-critical point (TCP) on the phase boundary separating low-temperature order phase and high-temperature disorder phase. The TCP shifts towards the higher temperature region with the decrease of frequency, i.e. T_{TCP}=1.33×exp(-ω/30.7). In the case of the isotropic Heisenberg spin system (HSS), however, the situation on dynamic phase transition of HSS is quite different from that of ISS in that no stable dynamical phase transition was observed in kinetic HSS after a threshold time. The evolution of magnetization in the HSS driven by a symmetrical external field after a certain duration always tends asymptotically to a disorder state no matter what an initial state the system starts with. The threshold time τ depends upon the amplitude H_{0}, reduced temperature T/T_C and the frequency ω as τ=C·ω^α·H_0^{-β}·(T/T_C)^{-γ}.     

Magnetic Properties of Transverse Ising Model under a Time Oscillating Longitudinal Field

A transverse Ising spin system, in the presence of time-dependentlongitudinal field, is studied by the effective-field theory (EFT). Theeffective-field equations of motion of the average magnetization are givenfor the simple cubic lattice (Z = 6) and the honeycomb lattice (Z = 3).The Liapunov exponent λ is calculated for discussing the stability of the magnetization and it is used to determine the phase boundary. Thedynamic phase transition diagrams in h₀/ ZJ -Γ/ZJ plane and in h₀/ZJ-T/ZJ plane have been drawn, and there is no dynamical tricritical point on the dynamic phase transition boundary. The effect of the thermal fluctuations upon the dynamic phase boundary has been discussed.     

Electromagnetic soliton propagation in an anisotropic Heisenberg helimagnet

We study the nonlinear spin dynamics of Heisenberg helimagnet under the effect of electromagnetic wave (EM) propagation. The basic dynamical equation of the spin evolution governed by Landau–Lifshitz equation resembles the director dynamics of the twist in a cholestric liquid crystal. With the use of reductive perturbation technique the perturbation is invoked for the spin magnetization and magnetic field components of the propagating electromagnetic wave. A steady-state solution is derived for the weakly nonlinear regime and for the next order, the components turn around s plane perpendicular to the propagation direction. It is found that as the electromagnetic wave propagates in the medium, both the magnetization and magnetic field modulate in the form of kink soliton modes by introducing amplitude fluctuation in the tail part of the same.     

The spin density waves in chromium detected by hyperfine field measurements

The magnetic hyperfine field of tantalum nuclei in a high purity chromium matrix has been measured using the Time Differential Perturbed Angular Correlation technique. The spectra show that the hyperfine field is proportional to the amplitude of the spin density wave of chromium and that the tantalum probe nuclei do not clamp the phase of the spin density wave. The incommensurate antiferromagnetic first order phase transition as well as the spin flip transition have been observed. The temperature dependence of the hyperfine field is shown to deviate from the temperature dependence of the maximum magnetization of the spin density wave.     

基于伊辛模型的Fe0.5Mn0.1Al0.4 合金磁化强度和磁熵变蒙特卡洛模拟

基于伊辛模型的单自旋反转蒙特卡洛算法,考虑了粒子间的最近邻以及次近邻相互作用,研究了无序 合金的磁化强度和磁熵变。首先,强调了粒子间的次近邻相关作用对体系的磁性和热力学性质的影响,明确了次近邻相互作用系数,证实了低温合金阻挫的存在;其次,研究了在相变温度处(不同磁场下)磁化强度随外加磁场（温度）的变化情况以及磁性粒子对磁化强度的贡献,发现反铁磁性粒子Mn在低温区对 合金的相变起了主要作用,而高温区体系的相变是由铁磁性粒子Fe贡献的;最后,分析了体系在相变温度处磁熵变数值随外加磁场的变化情况以及磁熵变在不同的外磁场下随温度的变化情况,当外加磁场h=0.14时,Mn粒子在冻结温度处的平均磁化强度为零,体系处于最无序的状态,对应的磁熵变 达到了正向最大值,极值的位置对应于体系的相变温度。     

基于伊辛模型的Fe_(0.5)Mn_(0.1)Al_(0.4)合金磁化强度和磁熵变蒙特卡洛模拟

基于伊辛模型的单自旋反转蒙特卡洛算法,考虑了粒子间的最近邻以及次近邻相互作用,研究了无序Fe0.5Mn0.1Al0.4合金的磁化强度和磁熵变.首先,强调了粒子间的次近邻相关作用对体系的磁性和热力学性质的影响,明确了次近邻相互作用系数,证实了低温合金阻挫的存在;其次,研究了在相变温度处(不同磁场下)磁化强度随外加磁场(温度)的变化情况以及磁性粒子对磁化强度的贡献,发现反铁磁性粒子Mn在低温区对Fe0.5Mn0.1Al0.4合金的相变起了主要作用,而高温区体系的相变是由铁磁性粒子Fe贡献的;最后,分析了体系在相变温度处磁熵变数值随外加磁场的变化情况以及磁熵变在不同的外磁场下随温度的变化情况,当外加磁场H=0.14(a.u.)时,Mn粒子在冻结温度处的平均磁化强度为零,体系处于最无序的状态,对应的磁熵变ΔS(0.1,0.14)达到了正向最大值,极值的位置对应于体系的相变温度.     

Dynamic behaviors of the hexagonal Ising nanowire

By utilizing the effective-field theory based on the Glauber-type stochastic dynamics, the dynamic behaviors of the hexagonal Ising nanowire (HIN) system in the presence of a time dependent magnetic field are obtained. The time variations of average order parameters and the thermal behavior of the dynamic order parameters are studied to analyze the nature of transitions and to obtain the dynamic phase transition points. The dynamic phase diagrams are introduced in the plane of the reduced temperature versus magnetic field amplitude. The dynamic phase diagrams exhibit coexistence phase region, several ordered phases and critical point as well as a reentrant behavior.     

随机外磁场作用下Ising自旋体系的随机共振

通过采用数字仿真手段，研究了经平均场处理的Ising自旋体系在弱确定性周期外场和随机外场（噪声场）混合驱动下的动态响应行为.着重考察了不同强度混合驱动外场作用下，Is ing自旋体系的非平衡动态转变所表现出区别于单纯确定性周期场作用下的新特征——随机 共振.选定体系非平衡动态转变的动态序参量Q为表征参量，系统模拟计算了混合驱动外场在多种参数组合下体系的动态响应特征，高场低温下的非连续动态转变和低场高温下的连续动态转变.模拟计算表明在适当混合驱动外场的作用下，Ising自旋体系具有随机共振现象，并诱发形成非 关键词： Ising自旋体系 随机共振 动态相变 对称性     

Ising自旋体系的非平衡动态相变

在平均场理论架构下, 以含时金兹堡-朗道和Glauber动力学这两类动态模型为基础，研究 了受外场和温度共同驱动的Ising自旋体系的非平衡动态相变.确定了界定动态无序(动态顺 磁相P)和动态有序(动态铁磁相F)转变的动态相界.并根据动态序参量Q和Binder参数U随系统 温度t(r₀)、驱动外场频率ω和振幅h₀的变化规律，就上述两类模 型的动态相界上是否存在区分连续动态转变和非连续动态转变的三临界点这一引发争议的问 题做出了进一步分析说明. 关键词： Ising自旋体系 非平衡动态相变 含时金兹堡-朗道模型 Glauber 动力学模型     

Expansion Around the Mean Field in Quantum Magnetic Systems

We introduce a new definition of ordered phase in a magnetic system based on properties of the local spin state probability. A statistical functional associated to this quantity depends both on amplitude and direction of the local magnetization. We show that it is possible to introduce an expansion at fixed magnetization amplitude in the inverse of lattice coordination number if the direction is selected by an extremum condition. In the limit of infinite coordination number we recover the mean field results. First order corrections are derived for the Ising model in the presence of a transverse field and for the XY model. Our results concerning critical temperature and order parameter compare favorably with other approaches.     

Ground-state magnetic properties of the spin-2 transverse Ising model

The ground-state magnetic properties of the spin-2 transverse Ising model with a longitudinal crystal field are studied within the framework of mean-field theory (MFT) and effective-field theory (EFT), respectively. The phase diagrams and magnetization curves are examined in detail. It is found that the system exhibits a tricritical behavior in the ground-state phase diagrams. Some interesting phenomena have been found, especially the first-order phase transition from one ordered phase to the other ordered phase, which is due to the high spin. The spin correlation has important effect on the magnetic properties of the system. We also find that the ground-state phase diagrams of the spin-2 transverse Ising model are very different from those of the spin-3/2 transverse Ising model.     

Quantized spin waves in antiferromagnetic Heisenberg chains

The quantized stationary spin wave modes in one-dimensional antiferromagnetic spin chains with easy axis on-site anisotropy have been studied by means of Landau-Lifshitz-Gilbert spin dynamics. We demonstrate that the confined antiferromagnetic chains show a unique behavior having no equivalent, neither in ferromagnetism nor in acoustics. The discrete energy dispersion is split into two interpenetrating n and n' levels caused by the existence of two sublattices. The oscillations of individual sublattices as well as the standing wave pattern strongly depend on the boundary conditions. Particularly, acoustical and optical antiferromagnetic spin waves in chains with boundaries fixed (pinned) on different sublattices can be found, while an asymmetry of oscillations appears if the two pinned ends belong to the same sublattice.     

Enhancement of next-nearest-neighbouring entanglement in quantum Ising spin chain

Using the method of the Jordan--Wigner transformation for solving different spin--spin correlation functions, we have investigated the generation of next-nearest-neighbouring entanglement in a one-dimensional quantum Ising spin chain with the Gaussian distribution impurities of exchange couplings and external magnetic fields taken into account. The maximal value of entanglement between the next-nearest-neighbouring qubits in the transverse Ising model was analysed in detail by varying the effectively controlled parameters such as interchange coupling, magnetic field and the system impurity. For such systems, where both exchange couplings and external magnetic field disorder appear, we show that it is possible to achieve next-nearest-neighbouring entanglement better than the previously discussed pure Ising spin chain case. We also show that the Gaussian distribution impurity can induce next-nearest-neighbouring entanglement, which can be used as a means to characterize quantum phase transition.     

Slow magnetic order-order transition in the spin chain antiferromagnet Ca3Co2O6

Using powder neutron diffraction, we have discovered an unusual magnetic order-order transition in the Ising spin chain compound Ca3Co2O6. On lowering the temperature, an antiferromagnetic phase with a propagation vector k=(0.5,-0.5,1) emerges from a higher temperature spin density wave structure with k=(0,0,1.01). This transition occurs over an unprecedented time scale of several hours and is never complete.     

周期性调制稀释Ising体系的相变

利用关联有效场方法，研究了正弦调制稀释Ｉｓｉｎｇ铁磁体系在无横场和有横场下的相变，侧重于体系的临界温度随调制幅度的变化以及临界温度对横场的依赖关系．有趣的结论是：体系磁性原子浓度的这种非均匀性可以提高其临界温度．但是，调制幅度的进一步上升可以引起体系由整体磁有序向部分磁有序的跃变 关键词：     

Multicanonical sampling of the space of states of ℋ(2, <Emphasis Type="Italic">n</Emphasis>)-vector models

Problems of temperature behavior of specific heat are solved by the entropy simulation method for Ising models on a simple square lattice and a square spin ice (SSI) lattice with nearest neighbor interaction, models of hexagonal lattices with short-range (SR) dipole interaction, as well as with long-range (LR) dipole interaction and free boundary conditions, and models of spin quasilattices with finite interaction radius. It is established that systems of a finite number of Ising spins with LR dipole interaction can have unusual thermodynamic properties characterized by several specific-heat peaks in the absence of an external magnetic field. For a parallel multicanonical sampling method, optimal schemes are found empirically for partitioning the space of states into energy bands for Ising and SSI models, methods of concatenation and renormalization of histograms are discussed, and a flatness criterion of histograms is proposed. It is established that there is no phase transition in a model with nearest neighbor interaction on a hexagonal lattice, while the temperature behavior of specific heat exhibits singularity in the same model, in case of LR interaction. A spin quasilattice is found that exhibits a nonzero value of residual entropy.     

Monte Carlo simulation of a random-field Ising antiferromagnet

Phase transitions in the three-dimensional diluted Ising antiferromagnet in an applied magnetic field are analyzed numerically. It is found that random magnetic field in a system with spin concentration below a certain threshold induces a crossover from second-order phase transition to first-order transition to a new phase characterized by a spin-glass ground state and metastable energy states at finite temperatures.