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.     

Singularity in high-frequency susceptibility of thin magnetic films with uniaxial anisotropy

A sharp peak of magnetic susceptibility has been observed in the ferromagnetic resonance spectra of uniaxial magnetic films placed in a planar field directed orthogonal to the easy magnetization axis, along which a pumping high-frequency magnetic field has been oriented. The peak width is considerably narrower than the line width of the uniform ferromagnetic resonance, and its position in a field equal to the film anisotropy field does not depend on the pumping frequency. The nature of the peak is associated with a drastic increase in the static transverse susceptibility of the film in the vicinity of the anisotropy field. It is shown phenomenologically that the peak can be observed only for quality samples with small angular and amplitude dispersion of the uniaxial anisotropy.     

On the magnetic properties of dislocations in silicon

The anisotropy of a group of equidistant lines in the EPR spectrum of plastically deformed Si can be described as line splitting in a nearly axial crystal field, the axis being parallel to the Burgers vector of the dislocations. We suppose that the spins of the unpaired electrons in the core of the 60° dislocations are coupled along limited segments giving rise to superparamagnetic resonance.     

Effect of thermal annealing on the magnetic anisotropy of GaMnAs ferromagnetic semiconductor

We have systematically investigated the influence of annealing on the magnetic anisotropy properties of GaMnAs film using an epilayer with a Mn concentration of 6.2%. The GaMnAs epilayer was grown by molecular beam epitaxy and the planar Hall effect measurement was used to monitor the magnetic anisotropy of the film. We found significant annealing-induced changes in the magnetic anisotropy properties of the GaMnAs film that depended on the annealing conditions. For example, the cubic anisotropy that gave a four-fold symmetry of magnetic easy axes decreased while the uniaxial anisotropy that gave a two-fold symmetry of magnetic easy axes increases in the samples annealed temperature below 300 °C. In particular, the uniaxial anisotropy along the [010] direction in as-grown GaMnAs film changed to the [100] direction by rotating by 90° after the sample was annealed at 300 °C for 3 h. This investigation thus indicates that the magnitude and the direction of the magnetic anisotropy in the GaMnAs film can be effectively controlled by choosing an appropriate annealing time and temperature.     

Specific features in precession dynamics of magnetization of a uniaxial magnetic film

The precession dynamics of the magnetization of a film with in-plane uniaxial anisotropy in the case of its biasing along the hard magnetization axis has been analyzed by numerically solving the Landau-Lifshitz equations. It has been revealed that the ferromagnetic resonance spectrum near the magnetic anisotropy field exhibits an additional peak, which is associated with the angular bistability due to the presence of two symmetric angular equilibrium positions. The trajectories of precession motion during biasing along the hard magnetization axis differ substantially from the trajectories corresponding to the biasing along the easy axis.     

Magnetic anisotropy of Fe films on GaAs(113)A substrates

We employ superconducting quantum interference device magnetometry to study the thickness dependence of in-plane and out-of-plane magnetic anisotropic properties of Fe films grown on high-index GaAs(113)A substrates by molecular beam epitaxy. The evolution of the in-plane magnetic anisotropy with film thickness is distinguished into two regions. First, for Fe film thicknesses ≤50 MLs, we observe an in-plane uniaxial magnetic anisotropy with the easy axis along the in-plane 〈332̄〉 axes. Second, for Fe film thicknesses ≥70 MLs, we observe a four-fold magnetic anisotropy with the easy axis along the in-plane 〈031̄〉 axes. The existence of an out-of-plane perpendicular magnetic anisotropy is also detected in ultrathin Fe films. Similar to Fe on GaAs(001), our results provide evidence for the interfacial origin of the in-plane uniaxial and out-of-plane perpendicular magnetic anisotropy. Both the uniaxial and the perpendicular interface anisotropy are found to be independent of the epitaxial orientation and are hence an intrinsic property of the Fe/GaAs interface. PACS 75.70.-i; 75.50.Bb; 81.15.Hi     

晶格失配应力对单晶(BiTm)_3(GaFe)_5O_(12)膜磁畴结构的影响

利用液相外延工艺在钆镓石榴石衬底上制得了单晶(BiTm)_3(GaFe)_5O_(12)膜,研究了晶格失配应力对其磁畴结构的影响.研究发现,生长速率越快,膜的晶格常数越大;晶格失配应力可以在一定范围内调整膜的垂直各向异性;随着晶格失配应力由较大张应力逐渐转变为较大压应力,磁畴形状先由磁泡畴转变成迷宫畴,然后转变为过渡态部分弯曲的条状畴,最终转变为整齐排列的条状畴;失配应力同时对畴宽也有影响,膜受到的失配应力越大,畴宽越大.这一实验研究对基于控制晶格失配应力来调控单晶膜的各向异性和磁畴结构有指导意义.     

Anisotropy of Two-Dimensional Electron and Hole Mobilities in (0 0 1) GaAs/Al x Ga 1− x As Heterostructures Under Uniaxial Stress

Transport properties of two-dimensional electron and hole gas at (0 0 1) GaAs/Al x Ga 1 m x As heterointerface in [1 1 0] and [1 m 1 0] directions have been investigated for the first time under in plane uniaxial compression up to 5 kbar. Resistivity, Shubnikov-de Haas oscillations and Hall effect were measured and carrier densities and mobilities were determined. The in-plane uniaxial compression significantly modifies the energy spectrum of p-GaAs/Al x Ga 1 m x As that leads to strong anisotropy of hole mobility under compression. In the case of the n-type heterostructure uniaxial compression causes only change of the carrier concentration and the corresponding small change of the initial mobility anisotropy, determined by anisotropic surface roughness scattering.     

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.     

Influence of the film thickness and additional elements (Al,O, and N) on the properties of FeCo film structures

The magnetic properties and domain structure of FeCoAlON thin films with thicknesses varying from 55 to 550 nm have been studied, and conditions favoring preparation of FeCoAlON films with uniaxial anisotropy in the direction normal to the film plane, which is required for designing “perpendicular” super-high-density information recording, have been established. In FeCoAlON films with a thickness up to 300 nm, the domain structure consists of cross-linked domain walls, because strong demagnetizing field suppresses formation of stripe domains. After the film thickness has reached 320 nm, cross-linked domain walls transform into stripe domains, with uniaxial anisotropy in the film plane disappearing, to become replaced by uniaxial anisotropy in the direction normal to the film plane, which can be assigned to magnetoelastic stresses induced by nitrogen atoms filling up interstitial space in the (110) plane. A further increase in the film thickness (up to 550 nm) leads to a rotational anisotropy due to the increase of nitrogen concentration in interstitials and the increase of magnetoelastic stresses.     

Two-dimensional micromagnetic simulation of domain structures in films with combined anisotropy

The transformation of the domain structure of micrometer-thick films with variations in the induced uniaxial anisotropy constant with the easy magnetization axis perpendicular to the film surface has been investigated using numerical micromagnetic simulation in the framework of a two-dimensional model of the magnetization distribution. The case where the tetra-axial crystallographic anisotropy exists in the film with uniaxial magnetic anisotropy has been considered. The transformation of the open domain structure into the structure with a magnetic flux closed inside the sample has been investigated in detail, and new types of 109-degree and 90-degree vortex-like domain walls and periodic domain structures have been obtained.     

Stripe domain statics in garnet films

Magnetic garnet films grown epitaxially on nonmagnetic garnet substrates exhibit a growth or stress-induced uniaxial anisotropy in addition to the cubic magnetocrystalline anisotropy associated with their crystal symmetry. When the uniaxial anisotropy is dominant over the cubic, such films exhibit stripe or bubble domain structures; even a small cubic anisotropy component can have a decisive effect on the behavior of the domains in applied fields. We report an experimental study of the quadistatic behavior of domains in fields applied to a (111) film in the film plane along (11$\text{2}$) and ($\text{1}$10). The experimental results are interpreted by a new theory that gives good agreement with the observed behavior, and yields an accurate measurement of the cubic and uniaxial anisotropy constants.The main qualitative features of the results are: In a ($\text{1}$10) field, the walls are Neél walls perpendicular to the field. In a (11$\text{2}$) field the walls are Bloch walls parallel to the field, the domain magnetization in adjacent stripes is not symmetrical about the film plane, and adjacent stripes are not of equal width; the domain period first shrinks and then expands with increasing field; and even though the applied field has no component perpendicular to the film plane, the film develops a net perpendicular magnetic moment.     

Ising-like spin anisotropy and competing antiferromagnetic-ferromagnetic orders in GdBaCo2O5.5 single crystals

In RBaCo2O5+x compounds (R is rare earth), a ferromagnetic-antiferromagnetic competition is accompanied by a giant magnetoresistance. We study the magnetization of detwinned GdBaCo2O5.5 single crystals and find a remarkable uniaxial anisotropy of Co3+ spins which is tightly linked with the chain oxygen ordering in GdO0.5 planes. Reflecting the underlying oxygen order, CoO2 planes also develop a spin-state order consisting of Co3+ ions in alternating rows of S=1 and S=0 states. The magnetic structure appears to be composed of weakly coupled ferromagnetic ladders with Ising-like moments, which gives a simple picture for magnetotransport phenomena.     

Micromagnetic calculation of the equilibrium distribution of magnetic moments in thin films

A new approach has been proposed for determining an equilibrium configuration of magnetic moments in condensed matter in terms of its discrete model. The solution to this problem is reduced to a system of linear inhomogeneous equations with undetermined Lagrange multipliers. The possibility of numerically solving these systems has been shown using a modified power method. The efficiency of the method has been demonstrated for the model of a thin magnetic film with a nonuniform distribution of the uniaxial magnetic anisotropy over the area. The dependence of the coercive force on the uniaxial anisotropy of nanocrystallites, their exchange interaction, and other parameters of samples has been investigated.     

Magnetic structure,phase diagram,and a new type of spin-flop transition dominated by higher order interaction in a localized 5f system U3Pd20Si6

The magnetic structure of the localized-5f uranium intermetallic compound U3Pd20Si6 has been determined by means of a neutron diffraction experiment. Our data demonstrate that this compound has a collinear coupling of the sublattice ordering of the uranium spins on the 4a and 8c sites. We conclude that higher-order exchange and/or quadrupole interactions are necessary to stabilize this unique collinear structure. We discovered a new type of spin-flop transition against the uniaxial anisotropy induced by this collinear coupling.     

用铁磁共振方法同时测定单晶外延膜的Ku和K1

本文导出了兼具沿晶面法向感生单轴各向异性(K_u)和立方磁晶各向异性(K₁)的任一晶面沿任一方向的薄膜共振场公式,从而提出了同时测定K_u和K₁的一种方法。以(111)晶面为例,给出了具体方法和实验结果。     

垂直各向异性Ho3Fe5O12薄膜的外延生长与其异质结构的自旋输运

垂直磁各向异性稀土-铁-石榴石纳米薄膜在自旋电子学中具有重要应用前景.本文使用溅射方法在(111)取向掺杂钇钪的钆镓石榴石(Gd_0.63Y_2.37Sc₂Ga₃O₁₂,GYSGG)单晶衬底上外延生长了2—100 nm厚的钬铁石榴石(Ho₃Fe₅O₁₂,HoIG)薄膜,并进一步在HoIG上沉积了3 nm Pt薄膜.测量了室温下HoIG的磁各向异性和HoIG/Pt异质结构的自旋相关输运性质.结果显示,厚度薄至2 nm的HoIG薄膜(小于2个单胞层)在室温仍具有铁磁性,且由于外延应变,2—60 nm厚HoIG薄膜都具有很强的垂直磁各向异性,有效垂直各向异性场最大达350 mT;异质结构样品表现出非常可观的反常霍尔效应和“自旋霍尔/各向异性”磁电阻效应,前者在HoIG厚度小于4 nm时开始缓慢下降,而后者当HoIG厚度小于7 nm时急剧减小,说明相较于反常霍尔效应,磁电阻效应对HoIG的体磁性相对更加敏感;此外,自旋相关热电压随HoIG厚度减薄在整个厚度范围以指数方式下降,说明遵从热激化磁振子运动规律的自旋塞贝克效应是其主要贡献者.本文结果表明HoIG纳米薄膜具有可调控的垂直磁各向异性,厚度大于4 nm的HoIG/Pt异质结构具有高效的自旋界面交换作用,是自旋电子学应用发展的一个重要候选材料.     

Distortions of magnetic order parameters due to “charged” edges of atomic steps at the ferromagnet—antiferromagnet interface

Distortions of the magnetic order parameters due to the presence of uncompensated antiferromagnetic spins near the edges of atomic steps at the interface between the ferromagnetic film and the antiferromagnetic substrate with compensated atomic planes parallel to the interface have been studied. It has been demonstrated that such “charged” linear defects do not give rise to domains in the film or substrate, as was suggested in some models of the exchange bias. The cases of large and small strengths of random fields arising due to the presence of uncompensated spins near the step edges have been investigated.     

Analysis of the boundary problem of SSWR in asymmetric isotropic insulator films

The influence of various combinations of the uniaxial surface anisotropy constantsK _s1 andK _s2 on the angular dependence of the standing spin-wave resonance (SSWR) spectrum is studied for an isotropic insulator film with asymmetrically pinned spins. It is shown that the (K _s1,K _s2)-plane can be divided into zones in which the angular dependence of the SSWR spectrum has different characteristic features. These features are studied in detail.Dedicated to Academician Vladimír Hajko on the occasion of his 65th birthday.     

Optical transitions in antiferromagnets in the saturated paramagnetism region

The behaviour of the spins of the photoexcited ions and its nearest antiferromagnetic neighbours in a two-sublattice antiferromagnet with isotropic Heisenberg interaction in the saturated paramagnetism region has been theoretically studied. It has been shown that the magnetic moments of the ions may be oriented in this region noncollinear to the magnetic field in the case whether the exchange integral in the excited state is larger than in the ground state or there is strong uniaxial single-ion “easy plane” anisotropy in the excited state. The exciton-magnon transitions intensity may, as a result, also alter in the saturated paramagnetism region.