交换偏置双层膜中的反铁磁自旋结构及其交换各向异性

研究了交换偏置双层膜中界面存在二次以及双二次交换耦合下反铁磁磁矩转动及其交换各向异性.结果表明，其反铁磁膜中的磁矩转动存在可逆“恢复行为”、不可逆“半转动行为”、不可逆“倒转行为”以及不可逆“半倒转行为”四种情形，四种情形的出现强烈地依赖于界面二次、双二次耦合以及反铁磁膜厚度.其中可逆恢复行为情况下，系统出现交换偏置，而不可逆的半转、半倒转以及倒转情形，系统不出现交换偏置.特别地,在界面处仅存在双二次耦合的情形下，其界面双二次耦合常数J₂≤0.1 σ关键词： 反铁磁自旋结构 交换各向异性 界面双二次耦合 交换偏置     

Critical temperature of an anisotropic ferromagnet in the presence of biquadratic exchange

A formulation, using the double time Green's function method, is presented for the anisotropic spin-one ferromagnetic system in the presence of biquadratic exchange and an expression is given for the critical temperature T_c in terms of parameters associated with the uniaxial anisotropy (treated exactly) and the biquadratic interaction (only the longitudinal part, treated in RPA). The prediction that T_c increases with the biquadratic exchange is found to be consistent with the recent calculation.     

Intrinsic biquadratic exchange and sublattice magnetizations in canted ferrimagnets

We investigate in a molecular field approximation the influence of an intrinsic biquadratic exchange term on the sublattice magnetizations of a canted ferrimagnet. A comparison is given with an approximate treatment only valid when the origin of the biquadratic term is in magnetostrictive phenomena. The approximate treatment is found to give the correct sign, but to overestimate the effects of the biquadratic term.     

Surface modes with biquadratic coupling and uniaxial anisotropy in semi-infinite Heisenberg antiferromagnet

We address the role and the influence of both biquadratic exchange and uniaxial anisotropy in the study of bulk and surface magnetic excitations for a two-sublattices semi-infinite Heisenberg antiferromagnet. The calculations apply to combined effect and are based on a matching technique within the framework of both non-interacting spin-wave theory and random phase approximation. Emphasis has been laid upon the advantage of handling bulk and surface spin precessional fields, characterized by their symmetry in particular. Analytic expressions for the bulk and surface equations of motion are also derived which enables us to analyze qualitatively the nature of the modes by observing the response of the energy branches to the variation of both biquadratic exchange and single-ion anisotropy field as well as exchange parameters occurring on the surface. Two numerical applications of the theoretical formalism developed in this work are described. First concerns the situation when the free surface model is assumed. The second application considers the existence of surface perturbations and treats the effects of the presence of biquadratic exchange term on the bulk and surface spin-wave spectra showing two kinds of surface propagating modes which depend on the presence or not of the uniaxial anisotropy contribution and also on the variations of the bulk-surface exchange parameters. A significant conclusion which this work reached relates to the checking of the results obtained in the literature by using our own theoretical formalism. We show that this formalism is very well adapted to calculations of localized bulk and surface spin-wave modes associated with the simultaneous presence of biquadratic exchange and uniaxial anisotropy. Also, the results obtained for the bulk spin fluctuations field as well as for the magnetic surface waves, are found to be in qualitative agreement with those obtained in the literature. The analytic formulation presented in this work provides an accurate and practical scheme for calculating the surface spin dynamics under the influence of surface exchange parameters, biquadratic coupling and uniaxial anisotropy terms.     

Field induced biquadratic exchange in hard/soft ferromagnetic bilayers

The appearance of biquadratic exchange coupling between soft Fe and hard SmCo thin layers is found. The remanent magnetization in the Fe layer reorients from parallel to perpendicular with respect to the SmCo easy axis after application of large enough negative field. To explain such an unexpected behavior in contacting ferromagnetic layers a model is proposed based on Slonczewski's fluctuating exchange mechanism. In our samples a partial remagnetization of the hard layer creates fluctuations of the bilinear interactions. The intralayer exchange averaging of the resulting magnetization fluctuations in the soft layer causes the observed biquadratic coupling.     

Solitary excitations in one-dimensional ferromagnetic spin chains with biquadratic exchange interaction

By using the traveling wave method, the solutions of the elliptic function wave and the solitary wave are obtained in a ferromagnetic spin chain with a biquadratic exchange interaction, a single ion anisotropic interaction and an anisotropic nearest neighbour interaction. The effects of the biquadratic exchange interaction and the single ion anisotropic interaction on the properties (width, peak and stability) of the soliton are investigated. It is also found that the effects vary with the strengths of these interactions.     

Anisotropic ferromagnet with biquadratic exchange

A formulation, using the double time Green’s function method, is presented for the anisotropic spin-one ferromagnetic system in the presence of Ising part of the biquadratic exchange and an expression is given for the critical temperatureT _c in terms of two parameters. One is associated with the uniaxial anisotropy (treated exactly) and the other one corresponds to the biquadratic interaction (treated in RPA). The bilinear Heisenberg exchange term is treated in Callen’s approximation emphasizing the role of intersite correlation for small parameters. The prediction thatT _c increases with the biquadratic exchange is found to be consistent with the recent calculation based on high temperature series expansion.     

Renormalization group treatment of a spin-one Ising model with biquadratic exchange interaction

The critical behavior of a spin-one Ising model with biquadratic exchange interactions is studied using the mean field renormalization group method. The phase diagram for various lattices is obtained. The nature of the phase transition with biquadratic interaction is analysed and comparison with other methods is made.     

铁磁/反铁磁/铁磁三层膜系统中反铁磁自旋结构及其交换各向异性

研究铁磁/反铁磁/铁磁三层膜中界面存在二次以及双二次交换耦合下反铁磁磁矩转动及其交换各向异性.结果表明,其反铁磁膜中的磁矩转动存在可逆"恢复行为"、不可逆"连续倒转行为"以及不可逆"中断倒转行为"三种情形,三种情形的出现强烈地依赖于两界面处的线性耦合和双二次耦合.钉扎界面的交换耦合与旋转界面的交换耦合相互竞争,当钉扎界面耦合占主导时,反铁磁磁矩发生可逆"恢复行为",系统出现交换偏置.在旋转界面耦合占主导情形下,其线性耦合与双二次耦合也相互竞争,分别导致反铁磁磁矩发生不可逆"连续倒转行为"和不可逆"中断倒转行为",系统都不出现交换偏置,但矫顽场都得以增强.相关结论为实验上观测的磁滞能耗以及界面垂直耦合提供了可能的解释.     

Reentrant behaviour in Ising models with biquadratic exchange interaction

The critical behaviour for the spin-1 Ising model with biquadratic interaction is investigated in two and three dimensions using the Monte Carlo method. The phase diagram for the three-dimensional system exhibits a reentrant behaviour for large positive values of the biquadratic exchange coupling. The temperature dependence of magnetization and susceptibility in the reentrant region is also presented.     

Green function theory of anisotropic Heisenberg Ferromagnet with biquadratic exchange

The effects of biquadratic exchange interaction and the uniaxial anisotropy on the curie temperature of the Heisenberg Ferromagnet with spin-one has been studied. Green function technique with random phase approximation has been used. The variation of curie temperature with biquadratic and anisotropic parameters for different lattices has been discussed.     

On the mean-field theory of magnetic multilayers with bilinear and biquadratic Heisenberg exchange

A system consisting of several layers of magnetic ions interacting by both bilinear and biquadratic Heisenberg exchange is studied within the framework of the mean-field approximation. It is shown that for S = 1 there exist two types of ordering: ferromagnetic and ferroquadrupolar. The stability of phases as the function of temperature, biquadratic exchange and surface exchange is discussed analytically and numerically and it was shown that similar to bulk samples there appear first- and second-order transitions and a tricritical point may appear depending on system parameters.     

Surface-induced ordering in thin uniaxial liquid crystal films

The interface localization transition in thin uniaxial liquid crystal films with competing surface fields has been studied using Metropolis Monte Carlo simulations. The model is constructed from a lattice of continuously orientable interacting spins, and the Hamiltonian contains both bilinear and biquadratic contributions. The biquadratic contribution to the Hamiltonian is familiar from the Lebwohl-Lasher model, and accounts for the particle anisotropy in a liquid crystal. The head-tail asymmetry of the molecules in a uniaxial liquid crystal is taken into account through a bilinear contribution familiar from the classical ferromagnetic Heisenberg model with exchange anisotropy Lambda. The critical temperature T(c), characterizing the interface localization transition within the uniaxial liquid crystal film, depends strongly on the relative magnitudes of the bilinear and biquadratic interactions between the spins. For systems dominated by the biquadratic interaction, T(c) is found to be close to the bulk critical temperature of the system. But as the biquadratic interaction strength is reduced, T(c) departs markedly from the bulk critical temperature of the system.     

Exact solution of the impure one-dimensional n-vector model with bilinear and biquadratic exchange interactions

We demonstrate explicitly that the impure one-dimensional n-vector model with bilinear and biquadratic exchange interactions can be solved exactly. Results are provided for the free energy, equal-time spin-spin correlation functions and the initial susceptibility of the corresponding bond and site model in the annealed and quenched limit. A numerical analysis of the thermodynamic response functions in the n = 3 case is performed, and therein some interesting consequences of the simultaneous influence of the biquadratic interactions and impurities are established. The presence of the biquadratic interaction is interpreted as an effect of the lattice compressibility.     

Influence of magnetic dipole and magnetoelastic interactions on the phase states of 2D non-Heisenberg ferromagnetic with complex exchange interactions

The phase states of the 2D non-Heisenberg ferromagnetic with anisotropic bilinear and biquadratic exchange interactions are investigated. The limiting cases of the system under consideration are the two-dimensional XY-model with biquadratic exchange interaction and the isotropic Heisenberg ferromagnetic. The account of the magnetic dipole interaction leads to the realization of spatially inhomogeneous quadrupolar phase. The stability regions of various phase transitions for different values of the material parameters are studied. The phase diagram is built. Besides, the temperature phase transitions are investigated. The influence of the magnetoelastic interaction on the formation of the long-range quadrupolar order is determined.     

Green's function theory of biquadratic exchange in a heisenberg ferromagnet

In this paper the effect of biquadratic exchange on Curie temperature has been discussed on the basis of double-time temperature-dependent Green-function formalism with the utilization of Callen decoupling scheme. Calculation has been done for a spin-1 bcc Heisenberg ferromagnet by the method of Flax and Raich and it has been observed that the variation of Curie temperature with the biquadratic exchange parameter α obeys a law prescribed by the previous authors.     

4f–3d exchange coupling in Gd/X/Co (X=Pt, Cr) multilayers

We have studied the interlayer exchange coupling between Gd and Co through Pt and Cr spacers in a series of multilayers. Magnetization curves have been analyzed with a model, taking into account bilinear and biquadratic interlayer exchange couplings between Gd and Co. The interlayer exchange coupling constants have been determined as a function of the Pt or Cr spacer layer thickness.     

Magnetic anisotropy of Co/Cu/Co films with indirect exchange coupling

The effect of isothermal annealing on the magnetic anisotropy, bilinear and biquadratic exchange coupling energies, and domain structure of Co/Cu/Co trilayer fiilms with d_Co=6 nm and d_Cu=1.0 and 2.1 nm prepared by magnetron sputtering has been studied. It is shown that, under isothermal annealing, the biquadratic coupling energy decreases by more than an order of magnitude in films with d_Cu=1.0 nm and increases in films with d_Cu=2.1 nm. The fourth-order magnetic anisotropy is shown to be related to the existence of biquadratic exchange energy.     

Phase transitions as a function of material constants and temperature in intermetallic compounds of the terfenol-D type

A model of magnetic and magnetoelastic properties of intermetallic compounds has been considered with the inclusion of the influence of the “giant” magnetoelastic coupling and the biquadratic exchange interaction. The phase transitions as a function of material constants and temperature have been investigated in the framework of the proposed model. It has been demonstrated that the ferromagnetic and quadrupole phases can be formed in the system under consideration. In this case, the phase transition between these phases is a first-order transition and occurs through the intermediate, i.e., quadrupole-ferromagnetic, state. The dependences of the phase transition temperature on the Heisenberg and biquadratic exchange interaction constants have been obtained for compounds of the terfenol-D type.     

Renormalization Group Study of the Mixed Spin-1 and Spin-3/2 Blume-Emery-Griffiths Model with Attractive Biquadratic Coupling

The mixed spin-1 and spin-3/2 Blume-Emery-Griffiths model with attractive biquadratic coupling is investigated in the framework of the Migdal-Kadanoff Renormalization Group method. By changing the ratio R > 0 of biquadratic and bilinear exchange interactions and according to the different values of crystal field interactions, we have determined six main types of phase diagrams. The full flow in the parameters space of the Hamiltonian was established and the fixed points obtained are drawn up in a table. In addition, we have determined the eigenvalues of the transformation of the group in the vicinity of the critical points. Finally, the introduction of a positive biquadratic interaction was discussed.