磁性单层膜磁学性质的Monte Carlo模拟

采用类Ising模型,利用Monte Carlo方法研究了磁性单层膜中退磁性偶极作用和铁磁性交换作用对系统磁学性质的影响.结果显示,随着偶极相互作用的增加,系统在低温下的磁化出现平台现象,此时磁化曲线可分为2个阶段,在低外场下,温度升高,系统易磁化,在高外场下则反之.这种新奇的磁化行为导致系统的磁熵变在低温低外场下出现大于零的反常行为.在模拟过程中,对长程力作用采用了比较精确的处理方法.     

Two-stage magnetization reversal of ferromagnetic films

The two-stage magnetization relaxation of thin ferromagnetic films during the magnetization reversal process is considered in the context of the generalized Kolmogorov three-phasecrystallization model. The experimental data agree qualitatively with the considered model.     

Bifurcation magnetic resonance in films magnetized along hard magnetization axis

We study low-frequency ferromagnetic resonance in a thin film magnetized along the hard magnetization axis performing an analysis of magnetization precession dynamics equations and numerical simulation. Two types of films are considered: polycrystalline uniaxial films and single-crystal films with cubic magnetic anisotropy. An additional (bifurcation) resonance initiated by the bistability, i.e. appearance of two closely spaced equilibrium magnetization states is registered. The modification of dynamic modes provoked by variation of the frequency, amplitude, and magnetic bias value of the ac field is studied. Both steady and chaotic magnetization precession modes are registered in the bifurcation resonance range.     

Steplike magnetization in a spin-chain system:Ca3Co2O6

Because of a ferromagnetic in-chain coupling between Co3+ ions at trigonal sites, Co2O6 chains are considered as large rigid spin moments. The antiferromagnetic Ising model on the triangular lattice is applied to describe an interchain ordering. An evolution of metastable states in a sweeping magnetic field is investigated by the single-flip technique. At the first approximation two steps in the magnetization curve and a plateau at 1/3 of the saturation magnetization are found. Four steps in magnetization are determined in high-order approximations in agreement with experimental results.     

Der Einfluß der Dipolwechselwirkung auf die Magnetisierung dünner ferromagnetischer Schichten

The magnetization of thin films is calculated for low temperatures, taking into account the exchange interaction, an external magnetic field, and the dipole interaction. The calculations are performed within a quantized phenomenological spin wave theory. For thin enough films, within the temperature range considered, only the lowest spin wave band contributes to the decrease of the magnetization. The influence of the dipole interaction is as follows: The magnetization decreases less rapidly with growing temperature than predicted by calculations within the Heisenberg model; the decrease depends considerably on the angle between the magnetization and the film plane; even atT=0K there is a small increase of the magnetization with growing external field.     

Kapitza pendulum effect in an amorphous magnet with a weak disorder

The mechanism of magnetization orientation in an amorphous ferromagnet under the effect of a weak anisotropic disorder of the random anisotropy type is considered. The disorder orients the magnetization so as to minimize the transverse magnetization fluctuations. The effect of the disorder is found to be analogous to the effect of vibrations of the suspension point on the orientation of a mechanical pendulum (the Kapitza pendulum). The disorder types ensuring the presence of the orienting effect are specified; simultaneously with the effect, the continuous degeneracy is lifted and the Larkin-Imry-Ma mechanism of long-range order collapse is suppressed.     

Persistent currents and magnetization in two-dimensional magnetic quantum systems

Persistent currents and magnetization are considered for a two-dimensional electron (or gas of electrons) coupled to various magnetic fields. Thermodynamic formulae for the magnetization and the persistent current are established and the “classical” relationship between current and magnetization is shown to hold for systems invariant both by translation and rotation. Applications are given, including the point vortex superposed onto an homogeneous magnetic field, the quantum Hall geometry (an electric field and an homogeneous magnetic field) and the random magnetic impurity problem (a random distribution of point vortices).     

The description of the domain structure in thin ferromagnetic films with thickness near the critical value

The magnetization distribution is considered in a thin ferromagnetic film of a thickness near the critical value, for which the continuous phase transition from the homogeneous magnetization state to the domain structure appears. The easy axis of the magnetization is perpendicular to the film plane and the anisotropy constant β < 4π. On the basis of the self-consistent theory the vortex-like magnetization distribution in the interdomain wall was obtained. The value of the critical thickness, the period of the domain structure and the amplitudes of the magnetization vector components as dependent on the film parameters were derived.     

Domain structure of silicon iron single crystals

The model of a new domain structure arising after the magnetization of silicon iron single crystals in planes of the (110) type at an angle of 0°<Θ<-55° to the axis of easy magnetization is considered. Using this model the angular dependence of the domain-structure characteristics is established; it agrees closely with direct observations. On magnetizing a single crystal in the angular range 55° <Θ≤ 90° to the easy axis, layers with a uniform resultant magnetization parallel to the [001] direction are formed.     

On the magnetization of a thin-walled cylindrical sheath with a magnetic field from the Rayleigh range inside

In terms of the general theory of magnetization of thin-walled sheaths, magnetization of a thin-walled indefinitely long circular cylindrical sheath that is placed in an external magnetic field and experiences the action of an internal magnetic field from the Rayleigh range is considered. It is shown that the solution of the problem can be reduced to solving infinite sets of nonlinear equations in the coefficients of Fourier series representing the components of the magnetization and internal magnetic field strength that are averaged over the thickness of the sheath and tangent to its surface. Approximate solutions to these sets of equations are obtained, and expressions for the coefficients multiplying the first terms of the series are given.     

Ordered states and structural transitions in a system of Abrikosov vortices with periodic pinning

A system of Abrikosov vortices in a quasi-two-dimensional HTSC plate is considered for various periodic lattices of pinning centers. The magnetization and equilibrium configurations of the vortex density for various values of external magnetic field and temperature are calculated using the Monte Carlo method. It is found that the interaction of the vortex system with the periodic lattice of pinning centers leads to the formation of various ordered vortex states through which the vortex system passes upon an increase or a decrease in the magnetic field. It is shown that ordered vortex states, as well as magnetic field screening processes, are responsible for the emergence of clearly manifested peaks on the magnetization curves. Extended pinning centers and the effect of multiple trapping of vortices on the behavior of magnetization are considered. Melting and crystallization of the vortex system under the periodic pinning conditions are investigated. It is found that the vortex system can crystallize upon heating in the case of periodic pinning.     

Non-Langevin high-temperature magnetization of nanoparticles in a weak magnetic field

Experimental evidence and theoretical substantiation are presented for the asymptotic behavior of high-temperature magnetization of an ensemble of nanoparticles in a weak magnetic field, which was predicted earlier and which differs qualitatively from the “Langevin” limit for ideal superparamagnetic particles. It is shown that the physical reason for the new asymptotic behavior is the temperature-independent “positive” tilt of the uniform magnetization vector at local energy minima in the direction of the field; this asymptotic behavior is associated with the nonstandard thermodynamics of single-domain particles, which depends on the ratio of characteristic frequencies of regular precession and random diffusion of this vector. An alternative approach is proposed for describing the magnetic dynamics of an ensemble of nanoparticles in a magnetic field, and the precession orbits of the magnetization vector are considered as stochastic states of each particle, whereas each state is characterized by the trajectory-averaged value of magnetization.     

Magnetization reversal processes of single nanomagnets and their energy barrier

Micromagnetic simulations were performed to investigate the influence of geometry and magnetic anisotropy constant on energy barrier and magnetization reversal mechanism of individual bits important for the bit patterned media concept in magnetic data storage. It is shown that dependency of the energy barrier on magnetic and geometric properties of bits can be described by an analytical approach in the case of quasi-coherent magnetization rotation process. However, when the bit size exceeds a critical size, for which an incoherent magnetization reversal is preferred, the analytical approach becomes invalid and no self-consistent theory is available. By systematically investigating the influence of bit size on the magnetization reversal mode, it was found that the transition from quasi-coherent to incoherent magnetization reversal mode can still be described analytically if an activation volume is considered instead of the bit volume. In this case, the nucleation volume is an important parameter determining thermal stability of the bit. If the volume of the bit is larger than twice the activation volume, the energy barrier stays nearly constant; with further increase in bit size, no gain in thermal stability can be achieved.     

Magnetization dynamics of molecular magnets in a circular polarized electromagnetic wave of millimeter range

The appearance of magnetization in a crystal of molecular magnets under the action of a circular polarized electromagnetic wave of millimeter range is considered. The time dependence of magnetization is obtained for the case of resonance and for approximation of the single relaxation time.     

铁磁球/反铁磁纳米体系磁滞回线的Monte Carlo模拟

利用经典Heisenberg模型和Monte Carlo方法研究外磁场和反铁磁磁晶各向异性、交换相互作用对铁磁球均匀嵌入到反铁磁基体中的铁磁/反铁磁纳米体系磁滞回线的影响.模拟结果显示,外加反向最大磁场不同时,磁滞回线形状不同.当磁场正向增加时,体系的磁化强度会产生一个跃变,但跃变高度与反向场最大值无关.反铁磁磁晶各向异性越大,体系的交换偏置现象越明显,且磁化强度回到饱和值所需的外磁场越大.随着反铁磁基体交换相互作用的增大,在正向和负向磁场区域还可能出现新的磁滞现象.     

Spin reorientation transition in (111) textured L10 CoPt layers

In this work, a spin reorientation transition from [001] axis to an in-plane direction occurs near Curie temperature under a small external field for (111) textured L1₀ CoPt layers in an AlN/CoPt multilayer film, indicating the dominant role of the shape anisotropy at elevated temperatures over the magnetocrystalline anisotropy. A large in-plane residual magnetization is also observed after cooling the sample from a temperature above the Curie point. The formation of magnetization during cooling is considered due to the alignment of magnetic moments along the easy axis by the small field in the spin reorientation transition temperature region. Our work reveals the importance of shape anisotropy for the formation of magnetization in the heat assisted magnetic recording process.     

高温超导块脉冲磁化实验研究与理论模拟

通过磁化熔融织构超导块材俘获远高于常规永磁体的磁场,可以用于磁分离、制作电机等,如何磁化是其中关键技术问题之一.脉冲磁化目前被认为是最有前景的超导块材的磁化方式之一.我们通过霍尔元件测量脉冲磁化过程中超导块材表面磁场的变化,并运用磁通蠕动流动模型和幂指数模型分别描述脉冲磁化过程,理论与实验相比较,结果表明磁通蠕动流动模型能够更好地描述脉冲磁化,热量的产生、局域的温升直接关系到俘获磁场的大小,通过合理的铁轭设置和利用多脉冲磁化技术有助于提高最终的磁化效果.     

Magnetic equation of state for thin films

The magnetization fluctuations are considered in thin films treated as a set of monoatomic layers parallel to the film surfaces. The correlation functions have been determined and their forms are discussed. The influence of inhomogeneity of magnetization on the equation of state for a magnetic system described by Valenta's model is studied. The behaviour of the spontaneous magnetization as well as the magnetic susceptibility near the Curie point is described.This work is partially supported by the Institute of Physics of the P.A.S.     

On the influence of a horizontal magnetic field on the Rosensweig instability of a nonlinear magnetizable ferrofluid

The Rosensweig instability induced by magnetic forces of the flat free surface of the layer of a stationary nonlinearly magnetizable ferrofluid is considered. The fluid covers a horizontal plate of a nonmagnetic material, located in a tilted magnetic field. The critical value of the vertical component of the magnetization vector is calculated in the linear formulation for a deep magnetic fluid for any physically admissible magnetization law. The influence of the horizontal component of the applied magnetic field on the onset of instability upon the modified Langevin magnetization is analyzed.     

Magnetization and Lyapunov exponents on a kagome chain with multi-site exchange interaction

The Ising approximation of the Heisenberg model in a strong magnetic field, with two, three and six spin exchange interactions is studied on a kagome chain. The kagome chain can be considered as an approximation of the third layer of ³He absorbed on the surface of graphite (kagome lattice). By using dynamical approach we have found one- and multi-dimensional mappings (recursion relations) for the partition function. The magnetization diagrams are plotted and they show that the kagome chain is separating into four sublattices with different magnetizations. Magnetization curves of two sublattices exhibit plateaus at zero and 2/3 of the saturation field. The maximal Lyapunov exponent for multi-dimensional mapping is considered and it is shown that near the magnetization plateaus the maximal Lyapunov exponent also exhibits plateaus.