Angle-dependent x-ray magnetic circular dichroism from (Ga,Mn)As: anisotropy and identification of hybridized states

Remarkably anisotropic Mn L2,3 x-ray magnetic circular dichroism spectra from the ferromagnetic semiconductor (Ga,Mn)As are reported. States with cubic and uniaxial symmetry are distinguished by careful analysis of the angle dependence of the spectra. The multiplet structures with cubic symmetry are qualitatively reproduced by calculations for an atomiclike d5 configuration in tetrahedral environment, and show zero anisotropy in the orbital and spin moments within the experimental uncertainty. However, hybridization with the host valence bands is reflected by the presence of a preedge feature with a uniaxial anisotropy and a marked dependence on the hole density.     

Ferromagnetic resonance in systems with competing uniaxial and cubic anisotropies

We develop a model for ferromagnetic resonance in systems with competing uniaxial and cubic anisotropies. This model applies to (i) magnetic materials with both uniaxial and cubic anisotropies, and (ii) magnetic nanoparticles with effective core and surface anisotropies; We numerically compute the resonance frequency as a function of the field and the resonance field as a function of the direction of the applied field for an arbitrary ratio of cubic-to-uniaxial anisotropy. We also provide some approximate analytical expressions in the case of weak cubic anisotropy. We propose a method that uses these expressions for estimating the uniaxial and cubic anisotropy constants, and for determining the relative orientation of the cubic anisotropy axes with respect to the crystal principle axes. This method is applicable to the analysis of experimental data of resonance type measurements for which we give a worked example of an iron thin film with mixed anisotropy.     

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.     

Theoretical investigation on electronic structure and mechanical properties of cubic crystallographic structures with point defects in Al-based alloys

Electronic structure and mechanical properties of cubic crystallographic structures with point defects in Al-based alloys are investigated using the first-principles calculations. Equilibrium structural parameters and mechanical parameters such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio and anisotropy are calculated and agreed well with experimental values. Effects of point defects on the electronic structures and mechanical properties of such cubic phases are further analyzed and discussed in view of the charge density and the density of states.     

Fe-Ga磁致伸缩合金自旋轨道耦合效应研究

磁致伸缩和磁晶各向异性来源于自旋轨道耦合及晶体场效应,两种效应作用效果将对Fe-Ga大磁致伸缩合金研究方向将起指导作用.通过基于均匀梯度近似的线形缀加平面波法研究了Fe-Ga磁致伸缩合金中D03、B2-like、L12结构在施加自旋轨道耦合效应前后Fe原子3d轨道劈裂情况,轨道电子数及磁晶各向异性.结果表明,自旋轨道耦合效应可以进一步劈裂D03及L12立方结构晶体场,D03和L12结构中Fe原子3d轨道电子数,尤其是dxy和d(x2-y2)轨道下自旋电子数由于自旋轨道耦合作用发生改变;但自旋轨道耦合对B2-like结构作用微弱.Fe-Ga磁致伸缩合金中磁晶各向异性主要由晶体场效应决定.     

Magnetic domain structures and magnetotransport properties in Co-Ag granular thin films

The magnetic microstructures and magnetotransport properties in granular Co_xAg_1-x films with 17%≤x≤62% were studied. Magnetic force microscopy (MFM) observations showed the presence of magnetic stripe domains in as-deposited samples with x≥45% and the evolution of the magnetic domain patterns to in-plane domains with annealing. A perpendicular magnetic anisotropy as high as about 8×10⁵ ergs/cc for as-deposited Co₆₂Ag₃₈ and about 6×10⁵ ergs/cc for as-deposited Co₄₅Ag₅₅ was observed by magnetization and torque measurements. With increasing annealing temperature, the perpendicular magnetic anisotropy became negative. The origin of the perpendicular magnetic anisotropy may be attributed to a rhombohedral distortion of the cubic cell due to residual substrate-film stresses. The magnetic stripe domains are the consequence of the interplay of the indirect or direct exchange, perpendicular magnetic anisotropy and dipolar interactions. Finally, magnetoresistance (MR) curves displayed training behaviours and different shapes when measured with different configurations (parallel, transverse and perpendicular). It is proposed that the existence and the evolution of the magnetic domain structures strongly affect the magnetotransport properties due to the extra contribution of the electron scattering at the domain walls. Furthermore, an anisotropic MR also contributes to the overall MR curves. Received: 2 March 2000 / Accepted: 28 March 2000 / Published online: 23 May 2001     

Why is polonium simple cubic and so highly anisotropic?

Using the state-of-the-art ab initio electronic structure calculations, we explain why alpha-Po prefers the simple cubic structure (it is due to the relativistic mass-velocity and Darwin terms), elucidate its extreme elastic anisotropy (this is an intrinsic property of the simple cubic crystal structure), and predict a transformation to a mixture of two trigonal structures at pressures of 1-3 GPa.     

Quantum spin model of a cubic ferromagnet in a magnetic field

The zero temperature phase diagram of a one-dimensional ferromagnet with cubic single ion anisotropy in an external magnetic field is studied. The mean-field approximation and the density-matrix renormalization group method are applied. Two phases at finite magnetic fields are identified: a canted phase with spontaneously broken symmetry and a phase with magnetization along the magnetic field. Both methods predict that the canted phase exists even for the single-ion anisotropy strong enough to destroy the magnetic order at zero magnetic field. In contrast to the mean-field theory, the density-matrix renormalization group predicts a reentrant behavior for the model. The character of the phase transition at finite magnetic field has also been considered and the critical index has been found. Received 9 May 2000 and Received in final form 5 July 2000     

Evidence for non-cubic magnetostriction in epitaxial bubble garnets

Epitaxial bubble garnet films grown on non-magnetic garnet substrates exhibit a dominant growth or stress induced uniaxial anisotropy, which is responsible for the stripe and bubble domain structures, and the intrinsic cubic magnetocrystalline anisotropy which can affect bubble device performance. The anisotropy constants have been deduced from measurements of stripe domain nucleation in the garnet films. We extend this measurement technique and its interpretation so that it also yields values of the magnetoelastic interactions.The measurement is based on observing the details of the topography of the nucleating domain structure, specifically the orientation of the nucleating stripe domains as a function of the orientation and magnitude of the applied magnetic field.The interpretation is based on a micromagnetic analysis of the conditions for homogeneous second order stripe domain nucleation. The contributions to the phenomena of the cubic anisotropy and of the magnetostriction are included in the analysis as perturbations.The theory produces predictions which are compatible with qualitative earlier experiments reported in the literature. It provides a satisfactory quantitative account of systematic new observations we have made on a GdTmY bubble garnet film with the specific objective of measuring magnetostriction.Analysis of the experimental data yields strong evidence for a non-cubic component of the magnetostriction possibly associated with the same growth-kinetic mechanism that gives rise to the non-cubic anisotropy. The sign and magnitude of the macroscopic non-cubic magnetoelastic constant is estimated from the experimental results.     

Sudden critical current drops induced in S/F structures

In the search for new physical properties of S/F structures, we have found that the superconductor critical current can be controlled by the domain state of the neighboring ferromagnet. The superconductor is a thin wire of thickness d_s ≈2ξ_S. Nb/Co and Nb/Py (Permalloy Ni₈₀Fe₂₀) bilayer structures were grown with a significant magnetic anisotropy. Critical current measurements of Nb/Co structures with ferromagnet thickness d_F > 30 nm show sudden drops in two very defined steps when the measurements are made along the hard axes direction (i.e. current track parallel to hard anisotropy axes direction). These drops disappear when they are made along the easy axis direction or when the ferromagnet thickness is below 30 nm. The drops are accompanied by vortex flux flow. In addition magnetorestistance measurements close to T_C show a sharp increase near saturation fields of the ferromagnet. Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet thickness d_F ~ 50 nm along the easy anisotropy axes. These results are explained as being due to spontaneous vortex formation and flow induced by Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain walls produce a 2D vortex-antivortex lattice structure.     

Negative Poisson’s ratio for cubic crystals and nano/microtubes

The paper systemizes numerous cubic crystals which can have both positive and negative Poisson’s ratios (the so-called partial auxetics) depending on the specimen orientation in tension. Several complete cubic auxetics whose Poisson’s ratio is always negative are indicated. The partial cubic auxetics are classified with the use of two dimensionless elastic parameters. For one of the parameters, a critical value is found at which the orientation behavior of the crystals changes qualitatively. The behavior of mesotubes obtained by rolling up plates of cubic crystals (crystals with rectilinear anisotropy) is considered in detail. Such mesotubes with curvilinear cubic anisotropy can have micron and nanometer lateral dimensions. It is shown that uniform tension of nano/microtubes of cubic crystals is possible only in the particular case of zero chirality angle (the angle between the crystallographic axis and the axis of a stretched tube). It is demonstrated by the semi-inverse Saint-Venant method that solution of the axial tension problem for cylindrically anisotropic nano/microtubes of cubic crystals with a non-zero chirality angle is possible with radially inhomogeneous fields of three normal stresses and one shear stress. In the examples considered, the cylindrically anisotropic nano/microtubes of cubic crystals are auxetics even if they are initially non-auxetics with rectilinear anisotropy.     

High-frequency dynamics of Co/Fe multilayers with stripe domains

Within the framework of two-dimensional (2D) numerical micromagnetic simulations, the equilibrium magnetization configuration and the high-frequency (0.1–30 GHz) linear response of Co/Fe multilayers have been investigated in detail. Due to the perpendicular anisotropy of Co layers, a stripe domain pattern can develop through the whole multilayer, the characteristics of which depend on the magnitude of the perpendicular anisotropy, the respective thicknesses of Co and Fe layers and the number of Co/Fe bilayers in the stack. One of the most striking features associated with the layering effect is the ripening aspect of the static magnetization configuration across the multilayers which induces complicated dynamic susceptibility spectra including surface modes and volume modes strongly confined within the inner Fe layers. The effect of the cubic magnetocrystalline anisotropy of Fe layers and the influence of a nonuniform perpendicular magnetic anisotropy within the Co layers on the static and dynamic magnetic properties of Co/Fe multilayers are then analyzed quantitatively.     

Lateral and vertical ordered one-dimensional InGaAs/GaAs quantum structures

Lateral and vertical ordered one-dimensional quantum structures, i.e. InGaAs/GaAs(001) quantum dot chains and quantum wires, have been obtained using molecular beam epitaxy. It was found that the InGaAs wires or dot chains sit on two-dimensional wetting layers and run along the [-110] direction, as the result of anisotropic strain and in-plane adatom diffusion. This anisotropic nature produces a model system for studying the electronic properties of one-, two-, and three-dimensional quantum confinements and related optical responses. The strain anisotropy is of importance in determining the electronic states of the quantum structures and the surrounding strained barrier. The strain-induced effects, such as change of band-gap and splitting of heavy–light hole states, were studied experimentally and theoretically. Optical anisotropy of these quantum structures is also discussed.     

Nonlinear dynamics of vortex-like asymmetric domain walls in magnetic triaxial films

On the basis of numerical solution of the Landau-Lifshitz equation, we investigated the nonlinear dynamic behaviour of 90-degree and 180-degree asymmetric vortex-like domain walls in single-crystal films with cubic symmetry. Films with surfaces of (100) and (110) types are considered. The nonlinear dynamic rearrangement of the internal structure of a wall was investigated. The surface uniaxial magnetic anisotropy is shown to strongly influence the very process of dynamic rearrangement in the internal structure of a wall even up to the complete suppression of the whole process.     

Modulation of photonic band structures with dielectric anisotropy

By the multiple scattering method and the extended Mie theory, we have calculated the photonic band structure of the photonic crystals consisting of the dielectric spheres with uniaxial/biaxial anisotropy. The results demonstrate that for fcc lattice structure there exist two partial photonic band gaps which does not appear in the isotropic case. Among them, the lower one, lying between the second and the third bands, exists in one third of the first Brillouin zone, while the upper one, opening between the fourth and fifth bands, can appear simultaneously in the rest two thirds of the first Brillouin zone. The effects of anisotropy on the band structures are studied as well, which suggests the biaxial anisotropy are much more flexible than the uniaxial anisotropy in modulating the band structures.     

Instantons and solitons in a monolayer film of ferromagnetic grains with biaxial anisotropy

We investigate domain wall and domain structure in a monolyer film consisting of ferromagnetic grains with biaxial anisotropy, which are seen to be the static versions of instanton and soliton, respectively. The equation of motion of the magnetization vector is reduced to the (1 + 2)-dimensional sine-Gordon field equation in strong anisotropy limit and the instanton and soliton configurations are obtained analytically. Various new domain structures in the ferromagnetic film are found.     

Mixed magnetic phases in (Ga,Mn)As epilayers

Two different ferromagnetic-paramagnetic transitions are detected in (Ga,Mn)As/GaAs(001) epilayers from ac susceptibility measurements: transition at a higher temperature results from (Ga,Mn)As cluster phases with [110] uniaxial anisotropy and that at a lower temperature is associated with a ferromagnetic (Ga,Mn)As matrix with 100 cubic anisotropy. A change in the magnetic easy axis from [100] to [110] with increasing temperature can be explained by the reduced contribution of 100 cubic anisotropy to the magnetic properties above the transition temperature of the (Ga,Mn)As matrix.     

NbSi2奇异高压相及其热力学性质的第一性原理研究

采用基于粒子群优化算法的结构预测程序CALYPSO, 并结合第一性原理的VASP程序, 在175 GPa发现NbSi₂的奇异立方高压相. 在此结构中, Nb原子形成金刚石结构, 而Si原子则形成正四面体镶嵌在金刚石结构中. 声子谱计算结果表明该结构是动力学稳定的. 电子结构分析表明, 六角相和立方相NbSi₂均为金属, 对金属性贡献较大的是Nb原子, 而且Nb和Si原子之间存在明显的p-d杂化现象, 电荷更多地聚集在Si四面体中. 利用“应力应变”方法, 计算了NbSi₂的弹性常数, 分析了其体积模量、剪切模量、杨氏模量和德拜温度等热动力学性质随压力的变化并进行了详细的讨论. 根据剪切模量和体积模量的比值分析了NbSi₂两种相结构的脆性和延展性, 发现压力会导致六角相NbSi₂的延展性增加, 但对立方相结构的延展性影响较小; 采用经验算法计算了NbSi₂两种相结构硬度变化情况, 结合这一比值进行了详细的分析. 弹性各向异性计算结果表明, 随着压力增加, 六角结构的各向异性增强, 而立方结构的各向异性减小.     

Surface contribution to the anisotropy of magnetic nanoparticles

We calculate the contribution of the Néel surface anisotropy to the effective anisotropy of magnetic nanoparticles of spherical shape cut out of a simple cubic lattice. The effective anisotropy arises because deviations of atomic magnetizations from collinearity and thus the energy depends on the orientation of the global magnetization. The result is second order in the Néel surface anisotropy, scales with the particle's volume, and has cubic symmetry with preferred directions [+/- 1, +/-1 , +/-1].