交换偏置系统中的反铁磁磁化与自旋波

采用自由能变分法研究了铁磁/反铁磁双层膜系统在反铁磁层存在净磁化下的自旋波谱.本模型中铁磁薄层具有单轴磁晶各向异性和立方磁晶各向异性,反铁磁层仅具有单轴磁晶各向异性,但厚度有限,推导出了系统铁磁共振频率的表达式.结果表明：系统的自旋波谱分光学模和声学模两种,其中光学模仅在反铁磁层存在净磁化时得到激发.自旋波谱可按外磁场强度的变化情况分为强弱两支;区分强磁场和弱磁场的临界场依赖于铁磁/反铁磁间的交换作用,反铁磁层的磁化强度以及反铁磁层的厚度等.交换偏置场对光学模的影响明显于声学模,而反铁磁的净磁化和其厚度对系统的影响紧密联系,难以区分.但当反铁磁层净磁化很小可忽略时,系统只存在声学模激发. 关键词： 铁磁/反铁磁双层膜 反铁磁层净磁化 光学模 声学模     

混合Heisenberg自旋体系动态相变的滞后标度

采用Monte Carlo方法对离散混合经典Heisenberg自旋体系在周期性外场驱动下动态相变行为进行了模拟计算.在典型Heisenberg自旋体系的哈密顿量基础上,引入表征非晶相的随机各向异性能项(比例为X)和表征晶体相的单轴各向异性能项(比例为1-X),考察了该混合自旋体系磁滞后回线面积A_area随X和单轴各向异性常数A及随机各向异性常数D的变化规律,并确定了该类自旋体系动态相变新的滞后标度关系A_area-A^δD^{η关键词： 海森堡模型 Monte Carlo模拟 磁滞标度 非晶纳米晶}     

具有单轴各向异性场的无规横向伊辛模型(S=1)的临界行为

应用我们提出的对近似和离散路径积分技术相结合的方法,研究了具有单轴各向异性场的无规横场伊辛模型(自旋s=1)的临界行为,并且讨论了单轴各向异性场和横场无规分布对系统相图结构的影响。 关键词：     

铁磁/反铁磁双层薄膜中自旋波特性研究

本文在一维海森堡模型的基础上,采用界面参数化方法,研究了非周期性边界条件下,铁磁/反铁磁双层薄膜中自旋波低温激发问题.重点讨论了表面各项异性对薄膜中自旋波谱、色散关系的影响.结果表明:表面各向异性对铁磁层中自旋波谱影响较大,对色散关系影响甚微。     

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     

小尺寸正方形铁磁薄膜边界效应对磁化翻转过程的影响

用自旋动力学方法系统地研究磁偶极相互作用表现的边界效应对小尺寸正方形铁磁薄膜的磁化翻转过程的影响.在确定的磁偶极相互作用强度下,针对不同的单轴各向异性强度和不同的磁化角(外磁场与易轴间的夹角),具体给出矫顽场与磁化角及单轴各向异性强度之间的依赖关系和-些有代表性的磁滞回线,并给出磁化翻转过程中-些有代表性的微观磁结构.模拟结果表明:磁偶极相互作用表现的边界钉扎作用与单轴各向异性场之间的竞争决定磁滞回线的形状和矫顽场的大小,从而在不同磁化角情况下会导致不同的矫顽场机理.本文提出可有效地描述正方形铁磁性薄膜复杂微观磁畴结构的形成与演变的五磁畴模型.这种五磁畴模型既能直接揭示单轴各向异性正方形铁磁薄膜的几何特性和物理特性,也方便于磁化翻转过程的分析.     

有限铁磁—反铁磁线链中的自旋波谱及其激发

本文研究了由交换作用耦合起来的有限长铁磁和反铁磁链组成的自旋系统。计算了系统的自旋波的波谱及其在微波场中的激发强度。在所得结果的基础上,讨论了具有反铁磁氧化层的铁磁薄膜的自旋波共振。阐明了等效地使用唯象普遍边界条件的可能性。根据理论,对等效的表面各向异性能作了估计,得到的数值与实验值很好地符合。     

量子顺电EuTiO_3材料基态磁性的第一性原理研究

EuTiO_3是钙钛矿结构的量子顺电体,实验发现其基态具有平面各向异性G类反铁磁结构,本文运用基于密度泛函理论的第一性原理计算研究了EuTiO_3处于量子顺电相和应力作用下处于铁电四方相时可能的自旋取向和自旋交换耦合作用,分析了自旋耦合作用的路径,探讨了应力对磁性交换路径的作用,结果发现:当体系自由时,EuTiO_3具有自旋沿[110]方向平面内单轴各向异性的G类反铁磁结构,该结构下Eu离子4f电子自旋通过处于面心位置的O 2p实现自旋反铁磁性的超交换耦合,而在外加应力诱导的铁电四方结构下,由于自旋交换路径中Eu—O—Eu键角改变,Eu 4f电子自旋实现了[110]方向的铁磁交换耦合。     

准晶体的磁性

在对准晶体结构和物性作简要介绍的基础上，回顾了目前准晶磁性的理论和实验研究现状，介绍了磁性比热、磁结构相变、自旋波谱．自旋玻璃态冻结及穆斯堡尔谱等实验结果及其解释．同时讨论了准晶体磁性材料的各向异性及可能出现的磁化特点、磁畴结构等，从而预估了铁磁性准晶体作为新材料的可能应用前景．     

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     

Anisotropic low temperature thermal resistance in polycrystalline magnetic thin films

The scattering of spin waves and the thermal resistance due to this scattering are calculated for polycrystalline magnetic thin films with a small overall uniaxial anisotropy. The scattering is caused by the random orientation of crystallites, and also by the magnetization ripple, which is likewise produced by this random orientation. The wavelength spectra of the ripple and of the spin waves at low energy are essentially influenced by the magnetostatic interaction, which is taken into account within Harte's linearized thin film approximation, and for comparison also within Hoffmann's approximation. Due to the anisotropy of the spectra, the contribution of the spin waves to the thermal resistance of permalloy films should be highly anisotropic at temperatures of 2 K and below.     

Spin waves in two-dimensional ferromagnet with large easy-plane anisotropy

Spin waves in easy-plane two-dimensional ferromagnet when anisotropy is much stronger than exchange are investigated. The spectra of magnons, the spin–spin and quadrupolar correlation functions have been derived. It is shown that in such a system there exist spin waves at low temperatures. Some properties of the quadrupolar ordering in ferromagnets are discussed.     

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.     

非线性偶极-交换表面自旋波的包络孤立子解

介绍了非线性表面自旋波理论研究的新进展 ,并对前人的理论做了推广 ,针对具有非均匀交换各向异性的偶极 交换铁磁介质的非线性表面自旋波的性质和行为进行了较为系统的理论研究. The new progress in the theoretical investigation on nonlinear surface spin waves is reviewed, and the theory mentioned above is generalized. A relatively systematical investigation on the behaviors and properties of the nonlinear surface spin waves in dipole-exchange ferromagnetic media with inhomogeneous exchange anisotropy is developed.     

Spin-wave spectrum of an ideal multilayer magnet upon modulation of all parameters of the Landau-Lifshitz equation

The spectrum of exchange spin waves in a multilayer magnet is theoretically investigated without regard for dissipation upon periodic modulation of all magnetic material parameters (uniaxial anisotropy constant, exchange interaction constant, saturation magnetization, gyromagnetic ratio) entering into the Landau-Lifshitz equation. A graphical method is proposed for analyzing the dependence of the propagation of spin waves on the depth of modulation of the material parameters. Practical application of the results obtained and the effect of dissipation on the propagation of spin waves in the system are discussed.     

All-optical probe of coherent spin waves

A novel, all-optical method to excite and detect spin waves in magnetic materials is presented. By exploiting the temperature dependence of the magnetic anisotropy, an ultrashort laser pulse is efficiently converted in a picosecond "anisotropy field" pulse that triggers a coherent precession of the magnetization. Recording the temporal evolution of the precessing spins by a time-delayed probe-pulse provides a quantitative method to study locally the magnetic anisotropy, as well as switching and damping phenomena in micromagnetic structures. Applications to nickel and permalloy ( Ni80Fe20) films are discussed, particularly showing the possibility to explore standing spin waves in thin films.     

Magnetization dynamics in thin films and multilayers

The behavior of spin waves is influenced by essentially all the parameters which characterize a magnetic material – exchange interactions, anisotropy, surface effects, dipolar interactions, phase transitions and imperfections. Thus measurements of spin wave frequencies can give important information on characterizing different magnetic materials and structures. In this paper we outline the major results and calculational methods for long-wavelength spin waves in thin films and multilayers. While the primary attention is on ferromagnet-based structures, long-wavelength spin waves in antiferromagnets are also discussed. We indicate how particular measurements of spin wave frequencies can be used to extract the fundamental parameters of the different structures.     

Spin Waves in a Semi-Infinite Heisenberg Ferromagnet with Magnetocrystalline Anisotropy

A Green function formalism is employed to investigate the effect of magnetocrystalline anisotropy on surface spin waves in semi-infinite Heisenberg ferromagnets at low temperatures T << T_c. The dispersion relations are investigated in detail. The density of states (DOS) of surface spin waves and layered magnetizations are also derived.     

Intrinsic Localized Spin Wave Modes and Modulational Instability in a Two-Dimensional Heisenberg Ferromagnet

An analytical study on the properties of intrinsic localized modes and modulational instability in a quantum two-dimensional ferromagnet with single-ion uniaxial anisotropy is completed in the semiclassical limit. By making use of the semidiscrete multiple-scale method, we obtain a line localized solution and a radially symmetric localized solution, and analyze their existence conditions. Taking into account that the existence of bright localized solutions is closely linked to the modulational instability of plane waves, we analytically study the discrete modulational instability of plane spin waves. The result of the modulational instability analysis show that the uniaxial anisotropy plays a key role in the appearance of our intrinsic localized spin wave modes.