量子自旋波本征值及易轴型各向异性对其的影响

本文考虑体单轴各向异性利用界面重参数化方法从理论上严格推导了两层异质铁磁质中量子自旋波的本征值,并模拟计算了简立方晶格结构 (100) 第一布里渊区的能带结构,特别研究了易轴型体单轴各向异性对本征值的影响.结果发现在这样的体系中存在体模、禁闭模和界面模;各种本征模与体交换作用、界面交换作用、晶格自旋、原子层数有关,而且各向异性直接影响各种本征模的存在条件. 关键词： 铁磁薄膜 自旋波 各向异性     

异质双层磁性薄膜中自旋波的波形演化

在一维海森堡模型的基础上,采用界面参数化方法,将双层异质磁薄膜中自旋波本征值问题归结为联立求解能量约束方程和界面参数化方程．重点讨论体系中体模和完全禁闭模的波形演化过程,发现体系中体模波形随自旋波矢呈余弦变化,会出现局域共振现象．激发能对两子层中体模有较大的影响,不仅影响体模的振幅,而且还影响体模的波长．另外,激发能对体系中的完全禁闭模也有较大影响,随着激发能增大,铁磁层中完全禁闭模波长变短,波速变小,但振幅不变．     

The energy bands of quantized spin waves and the uniaxial anisotropy in a biferromagnetic system

The eigenproblems of quantized spin waves in a (100) ferromagnetic bilayer system have been investigated theoretically using the interface rescaling approach. The energy-band structure of the system has been obtained and the effect of the uniaxial bulk anisotropy field of easy-axis type on the energy bands are explored thoroughly. It is found that all kinds of eigenmodes of the spin waves in the biferromagnetic system are determined by the bulk exchange coupling constant, the interface exchange coupling constant, the spin of lattice, the number of atomic layers and the uniaxial bulk anisotropy. Especially, we explore the effect of the uniaxial bulk anisotropy field of easy-axis type on the energy bands of first Brillouin zone and the existence condition of all kinds of the eigenmodes.     

外场作用下铁电/铁磁双层膜的极化磁化性质

建立了铁电/铁磁双层膜模型,铁电层的电矩用连续标量描述,而铁磁层的自旋应用经典矢量描述.利用蒙特卡罗方法模拟了体系的热力学性质和极化、磁化行为.给出了零场下体系的内能、比热、极化和磁化随温度变化的关系,并分别研究了体系在外磁场和外电场下的极化和磁化行为.模拟结果表明,双层膜体系的内能、比热、极化和磁化性质因层间耦合系数的不同而明显不同,当界面耦合较弱时,双层膜表现出各自的热力学性质,当层间耦合增强到一定程度时,双层膜耦合为一个整体,表现出统一的热力学性质.该双层膜在外场中形成电滞回线和磁滞回线,并表现出偏置特性,界面耦合强度和温度影响滞后回线和偏置现象.     

外应力场下铁磁/反铁磁双层膜系统中的自旋波

采用自由能极小的方法研究了铁磁/反铁磁双层膜系统在外应力场下的一致进动自旋波性质，即铁磁共振现象. 本模型中铁磁层很薄可看成单畴结构，但具有单轴磁晶各向异性和立方磁晶各向异性；而反铁磁层仅具有单轴磁晶各向异性，但其厚度趋于半无穷. 推导出了该系统的铁磁共振频率和频谱宽度的解析式. 结果表明，外应力场和界面交换耦合或反铁磁磁强度仅在弱磁场下对系统的铁磁共振有影响，且系统的铁磁共振行为按磁场强度可分为两支，其区分弱磁场和强磁场的临界场依赖于外应力场的方向. 另一方面，应力场方向的改变可借助于反铁磁层磁畴变化对铁磁层磁晶各向异性轴有影响. 关键词： 铁磁/反铁磁双层膜 界面耦合强度 铁磁共振 应力场     

Surface dependence of the magnetic configurations of the ordered B2 FeCr alloy

Tight-binding linear muffin tin orbitals calculations with generalized gradient approximation were carried out for the magnetic configurations at the surface of the ferromagnetic ordered B2 FeCr alloys. For both (001) and (111) crystallographic phases, non ferromagnetic configurations are shown to be more stable than the ferromagnetic configuration of the bulk alloy. For (001) surface we display a c ground state for either Cr or Fe at the surface. For Cr top layer the magnetic moments are larger than in the bulk B2 FeCr while they are slightly enhanced for Fe top layer. For (111) surface an antiferromagnetic coupling between surface and subsurface is always obtained i.e. for either Fe or Cr at the surface. This change of coupling between Fe and Cr (from ferromagnetic to antiferromagnetic) is expected to be fundamental to any explanation of the experimental results obtained for the interface alloying at the Fe/Cr interfaces. Received 23 March 1998     

Dependence of the Interface Magnetism in a Ferromagnetic Superlattice on the Geometric Structure

A new method which combines the mean-field renormalization group with the discretized path-integral representation is proposed to study interface magnetism in a ferromagnetic superlattice consisting of two alternating materials. A simple spin-1/2 transverse Ising model with the nearest-neighbour coupling is assumed. The dependence of the interface magnetism in the superlattice on the geometric structure and the coupling strengths between the interface and bulk layers are studied.     

Brillouin-zone mapping of the spin waves in a ferromagnetic bilayer system

The eigenproblems of spin waves in a heterogeneous ferromagnetic bilayer system with periodic boundary conditions are solved using the interface-rescaling approach. Brillouin zone mapping and the eigenmodes of the system are investigated. We find three types of spin waves may exist in the system: the bulk mode, the interface mode, and the perfect confined mode. The fine structure of the energy band in the heterogeneous bilayer system is first given for the whole two-dimensional Brillouin zone. Conditions for the existence of the interface mode are discussed. Finally, we analyze the resonant-confined spin waves in bulk modes and their oscillating behavior.     

Collective mode dispersions of organic chain compounds

We investigate the collective mode dispersions for the tight-binding dielectric matrix with two one-dimensional electron bands per donor and acceptor chains, and the three-dimensional long-range Coulomb electron-electron interaction within the random phase approximation. The hybridized collective modes are the result of the strong coupling between the intraband plasmon and the interband dipolar modes due to strong dipole Coulomb interactions. Our calculations show the existence of the low-energy renormalized plasmon mode above the electron-hole quasi-continuum in the long wavelength limit. The obtained modes are brought into correspondence with the optical data of quasi-one-dimensional organic conductor tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ). Namely, the renormalized plasmon and the dipolar mode are assigned to the observed excitations at respective energy scales of roughly 10 meV and 0.75 eV, explaining why lower excitation is eliminated while higher excitation persists below the temperature of the Peierls phase transition.     

Glassy phases in a layered ising model with random interlayer exchange

Thermodynamics of a layered Ising model with infinite-range ferromagnetic intralayer interaction and random nearest-neighbor interlayer coupling is considered. A detailed analysis of the model with vanishing average interlayer coupling is presented. The Gibbs free energy is found in the critical region, and the existence of many metastable states is demonstrated. Thermodynamic parameters of the system are found for periodic states. As the mean square interlayer coupling increases, the equilibrium state of the system undergoes an infinite sequence of first-order phase transitions, the number of magnetic planes and the distance between them change discontinuously, and so do both bulk magnetization and magnetic susceptibility.     

Spin triplet Andreev reflection induced by interface spin-orbit coupling in half-metal/superconductor junctions

We show that with interface spin-orbit coupling, triplet pairing can occur in the half-metal/superconductor junction. The tunneling conductance is different from the usual Andreev reflection and strongly depends on the polarisation orientation. The probability of triplet pairing for different incident angles and zero-biased conductance are also calculated. The mechanism for the formation of the triplet pairing is that the interface spin-orbit coupling provides an effective spin-flip barrier, which couples all the transport modes in spin Nambu space. Because of its unique particle hole symmetry, this spin-orbit coupling interface effect is different from the spin-flip ferromagnetic barrier which induces zero-bias conductance vanishing and finite V-shape conductance within the energy gap.     

Interlayer exchange coupling in ferromagnet-semiconductor digital magnetic alloys

The interlayer exchange coupling in ferromagnet-semiconductor digital magnetic alloys in which monolyers (submonolayers) of transition metals are embedded into a semiconductor matrix is studied theoretically. A mechanism of an indirect exchange between ferromagnetic δ layers is proposed; it is based on the confinement of carriers in two-dimensional spin-polarized states inside the energy gap of the semiconductor. These appear due to strong potential and exchange carrier scattering by the δ layers. The interlayer exchange coupling is shown to occur through a nondegenerate semiconductor interlayer because of virtual electron excitations through an energy barrier separating these partly filled two-dimensional spin-polarized states and the edge of the bulk semiconductor band. The interlayer coupling intensity decreases exponentially with increasing distance between neighboring δ layers, and the type of this coupling can change from ferromagnetic into antiferromagnetic or vice versa as the interlayer thickness or the degree of filling the two-dimensional states increases.     

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.     

铁磁和反铁磁双层膜中铁磁共振的研究

采用微磁学理论研究了铁磁/反铁磁双层膜中的铁磁共振现象.本模型将铁磁薄层抽象为一个单晶,具有立方磁晶各向异性和单轴磁晶各向异性,而反铁磁层视为厚度趋近于半无穷,且只有单轴磁晶各向异性.推导出了该系统的铁磁共振频率和频率谱宽度的解析式.数值计算表明,铁磁共振模式分两支,取决于立方磁晶各向异性.而界面的交换耦合,是磁易轴具有单向性的起因.     

First-principles prediction of half-metallic ferromagnetism in Cd(TM)O2 (TM=Cr, Mn, Fe, Co, Ni) compounds

We report the electronic structure of Cd(TM)O₂ (TM=Cr, Mn, Fe, Co, Ni) in the chalcopyrite structures. From this study we find that Cd(TM)O₂ is a half-metallic ferromagnetic compound. From the energy consideration we find that Cd(TM)O₂ is more stable in chalcopyrite structure rather than in rock salt structure. A careful analysis of the spin density reveals the ferromagnetic coupling between the p-d states and the cation dangling-bond p states, which is believed to be responsible for the stabilization of the ferromagnetic phase. The calculated heat of formation, bulk modulus and cohesive energy are reported.     

Modulation instability of magnetostatic waves in a magnetically coupled two-layer structure

Modulation instability of direct bulk magnetostatic waves in structures consisting of two magnetically coupled planar films separated by a nonmagnetic layer is studied. It is found that, depending on the type of excitation of the waveguide modes, mode coupling can either change the characteristics of the modulation wave instability or ensure their stability with respect to the corresponding perturbations by affecting the dispersion properties of the structure.     

Tunable interlayer exchange coupling energy by modification of Schottky barrier potentials

We theoretically demonstrate that the interlayer exchange coupling (IEC) energy can be manipulated by means of an external bias voltage in a F₁/NM/F₂/S (F₁: ferromagnetic, NM: nonmagnetic metallic, F₂: ferromagnetic, S: semiconductor layers) four-layer system. It is well known that the IEC energy between two ferromagnetic layers separated by nanometer thick nonmagnetic layer depends on the spin-dependent electron reflectivities at the interface in F₁/NM/F₂ trilayer system. We apply such dependence to the F₁/NM/F₂/S four-layer system, where the reflectivity of NM/F₂ interface also depends on F₂/S interface due to the multiple reflection of an electron like optics. Finally, the IEC energy depends on the spin-dependent electron reflectivity not only at the interfaces of F₁/NM/F₂, but also at the interface of F₂/S. Naturally the Schottky barrier is formed at the interface between metallic ferromagnetic layer and semiconductor, the Schottky barrier height and thickness can be tailored by an external bias voltage, which causes the change of the spin-dependent reflectivity at F₂/S interface. We show that the IEC energy between two ferromagnetic layers can be controlled by an external bias voltage due to the electron-optics nature using a simple free-electron-like one-dimensional model.     

多原子半无限晶体中表面极化子的内部激发态

研究多原子半无限晶体中电子与表面光学(SO)声子耦合强,而与体纵光学(LO)声子耦合弱的极化子的激发态性质.采用线性组合算符和幺正变换方法导出与SO声子耦合强而与LO声子耦合弱情形下极化子的基态能量、第一内部激发态能量和激发能量.结果表明,多原子半无限晶体中与SO声子耦合强,而与LO声子耦合弱的极化子的基态能量、第一内部激发态能量不仅包含不同支LO声子和不同支SO声子与电子耦合的能量,而且也包含不同支SO声子之间相互作用贡献的附加能量.激发能量与体纵光学声子无关.     

Linear–quadratic order parameter coupling and multiferroic phase transitions

The coupling between order parameters in systems with several instabilities has been analysed within Landau theory. The dominant term considered in this paper is linear in one order parameter and quadratic in the second order parameter ~QP2; other coupling terms have been treated previously. Typical examples for Q are proper or pseudo-proper ferroelastic instabilities, while P might be octahedral tilting in a perovskite, (anti-)ferromagnetic ordering or (anti-)ferroelectric soft modes. Coupling of this type is common in fluorites, Verwey transitions, Jahn-Teller systems, pnictide superconductors, etc. Analytical solutions and characteristic phase diagrams of the stable configurations are compiled. The coupling can lead to stepwise phase transitions even when the uncoupled systems would show continuous transitions. Mixed phases are common, so that many 'intermediate phases' described in the literature may be the result of this linear-quadratic coupling.