Critical properties of random anisotropy magnets

The problem of critical behaviour of three dimensional random anisotropy magnets, which constitute a wide class of disordered magnets is considered. Previous results obtained in experiments, by Monte Carlo simulations and within different theoretical approaches give evidence for a second order phase transition for anisotropic distributions of the local anisotropy axes, while for the case of isotropic distribution such transition is absent. This outcome is described by renormalization group in its field theoretical variant on the basis of the random anisotropy model. Considerable attention is paid to the investigation of the effective critical behaviour which explains the observation of different behaviour in the same universality class.     

Hysteresis and rotational hysteresis of textured polycrystalline magnets

A system of magnetic particles with uniaxial anisotropy is considered. The orientation of the particles is described by a distribution function, representing the texture by a single integer n. In each particle, two elementary processes of the magnetization reversal of the particles are taken into account, the coherent rotation of the magnetization and the pinning of domain walls. In the framework of this model the hysteresis loops including minor loops and virgin curves and the rotational hysteresis are computed, where arbitrarv angles between the texture axis and the external field are taken into consideration.     

Coercive field of a polycrystalline ferrimagnet with uni-axial anisotropy

Unlike the coercive field _Hc$H_{\mathrm{c}}$ of a bulk ferrimagnet, which diverges at the compensation temperature _Tcomp$T_{\mathrm{comp}}$, the coercive field of a polycrystalline ferrimagnet with uni-axial anisotropy is shown to have a minimum at _Tcomp$T_{\mathrm{comp}}$. Despite this behavior, the field required for domain-wall motion still diverges at the compensation temperature. These ideas are used to treat a ferrimagnetic class of molecule-based magnets, the bimetallic oxalates, that exhibit a minimum coercivity at _Tcomp$T_{\mathrm{comp}}$.     

Hydrostatic pressure and hexagonal magnetic anisotropy of hematite

The effect of hydrostatic pressure on the hexagonal basal magnetic anisotropy of a rhombohedral easy-plane weak ferromagnet is investigated in terms of the thermodynamic theory. It is shown that the contribution to the hexagonal anisotropy is determined by the effective constants of the uniaxial and cubic anisotropy, which include the additions proportional to the hydrostatic pressure. The hydrostatic pressure, which leads to the basal anisotropy observed in experiments for stressed samples, is determined for hematite.     

On the susceptibility exponent of random anisotropy magnets

The temperature dependence of the non-linear susceptibility ≈₂(T) of random anisotropy magnets in the Ising limit (speromagnets) is calculated for temperatures above the freezing temperature T_f within the framework of the correlated molecular field theory. For the effective susceptibility exponent λ_s(T) = (T?T_f)≈₂d^-1_≈2/dT a non-monotonic temperature dependence is found as for the case of spin glasses. This must be taken into account in order to obtain reliable values for the critical susceptibility exponent from experimental data.     

Pair model of anisotropy and magnetostriction in soft amorphous magnets

A pair model for amorphous alloys has been developed, considering both the spherical symmetry (isotropic alloys) and the cylindrical one (alloys exhibiting pair ordering or microstructural anisotropy). The elastic energy has been first derived. Then starting from the Heisenberg exchange, the pseudodipolar (PSD), the cluster electric field (CEF) and the simple dipolar couplings, the magnetoelastic coupling and magnetic anisotropy energies have been written in the limiting case of a very small random anisotropy.     

Collective rearrangement at the onset of flow of a polycrystalline hexagonal columnar phase

Creep experiments on polycrystalline surfactant hexagonal columnar phases show a power law regime, followed by a drastic fluidization before reaching a final stationary flow. The scaling of the fluidization time with the shear modulus of the sample and stress applied suggests that the onset of flow involves a bulk reorganization of the material. This is confirmed by x-ray scattering under stress coupled to in situ rheology experiments, which show a collective reorientation of all crystallites at the onset of flow. The analogy with the fracture of heterogeneous materials is discussed.     

具有六方晶系结构的多晶体材料弹性常数——Y弹性常数

利用最近提出的新的物理参量——Ｙ弹性常数,将其应用于具有六方晶系结构的多晶体材料.推导了六方晶系结构的多晶体材料之Ｙ弹性常数,通过算例与具有六方晶系结构的多晶体材料之X射线弹性常数进行了比较.运用这个Ｙ弹性常数进一步推导出的多晶体材料整体之机械弹性常数的表达式与Kneer的研究结果中的表达式虽然形式不同,但针对具体材料所计算的结果却完全符合. 关键词： Y弹性常数 六方晶系 多晶体材料     

Localization of disordered bosons and magnets in random fields

We study localization properties of disordered bosons and spins in random fields at zero temperature. We focus on two representatives of different symmetry classes, hard-core bosons (XY magnets) and Ising magnets in random transverse fields, and contrast their physical properties. We describe localization properties using a locator expansion on general lattices. For 1d Ising chains, we find non-analytic behavior of the localization length as a function of energy at ω=0$\omega =0$, ξ⁻¹(ω)=ξ⁻¹(0)+A|ω|^α${\xi }^{-1}\left(\omega \right)={\xi }^{-1}\left(0\right)+A{\left|\omega \right|}^{\alpha }$, with α$\alpha$ vanishing at criticality. This contrasts with the much smoother behavior predicted for XY magnets. We use these results to approach the ordering transition on Bethe lattices of large connectivity K$K$, which mimic the limit of high dimensionality. In both models, in the paramagnetic phase with uniform disorder, the localization length is found to have a local maximum at ω=0$\omega =0$. For the Ising model, we find activated scaling at the phase transition, in agreement with infinite randomness studies. In the Ising model long range order is found to arise due to a delocalization and condensation initiated at ω=0$\omega =0$, without a closing mobility gap. We find that Ising systems establish order on much sparser (fractal) subgraphs than XY models. Possible implications of these results for finite-dimensional systems are discussed.     

Magnetoresistance anisotropy in a hexagonal lattice of Co antidots obtained by thermal evaporation

Patterned soft magnetic materials are eligible for use in magnetic random access memories. A hexagonal-lattice pattern of circular antidots was produced by optical lithography in a Co film. In order to test the effect of geometry on the local magnetisation configuration of such a structure, we performed room-temperature angle-resolved magnetisation measurements aimed to check the pinning of domain walls by the pattern's lattice. Magnetoresistance (MR) room-temperature measurements were performed at various angles between the magnetic field direction and the macroscopic electrical current vector, to clarify whether and how the local current density configuration affects the MR response. We found that the magnetoresistance is of anisotropic type (AMR) and has a local origin. Furthermore, the largely unsaturating behaviour of MR at high fields may be explained only by considering that tiny portions of the pattern constitute highly frustrated regions and align their magnetisation at rather high fields. A simplified model based on a local anisotropy term is shown to account for the experimental results for both M and MR.     

Homogenization of random anisotropy properties in polycrystalline magnetic materials

This paper is devoted to the determination of the equivalent anisotropy properties of polycrystalline magnetic materials, modelled by an assembly of monocrystalline grains with a stochastic spatial distribution of easy axes. The mathematical theory of $\Gamma \text{-}\mathrm{convergence}$ is applied to homogenize the anisotropic term in the Gibbs free energy. The procedure is validated focusing on the micromagnetic computation of reversal processes in polycrystalline magnetic thin films.     

Current percolation and anisotropy in polycrystalline MgB(2)

The influence of anisotropy on the transport current in MgB(2) polycrystalline bulk samples and wires is discussed. A model for the critical current density is proposed, which is based on anisotropic London theory, grain boundary pinning, and percolation theory. The calculated currents agree convincingly with experimental data, and the fit parameters, especially the anisotropy, obtained from percolation theory agree with experiment or theoretical predictions.     

Effective anisotropy between magnetically soft and hard grains in nanocomposite magnets

Effect of grain size on the effective anisotropy constant between magnetically soft (α-Fe) and hard (Nd₂Fe₁₄B) grains in nanocomposite magnets, 〈K_sh〉, has been investigated. The results show that the values of 〈K_sh〉 increase with decreasing size of soft grain, D_s, for the given size of hard grain, D_h. For the given D_s, 〈K_sh〉 increases with increasing D_h, and then approaches a steady value. In order to get a high value of 〈K_sh〉, the hard grain size should be larger than 30 nm and the soft grain size should be about 10 nm. PACS 75.30.E, 75.30.G, 75.50.T     

Effect of exchange-coupling interaction on anisotropy of grain in nanoscaled magnets

The effect of inter-grain exchange-coupling interaction on the anisotropy of grain in nanoscaled magnets has been investigated by putting forward an expression of anisotropy at grain boundary, , which is suitable for different coupling conditions, and expresses well the coherency between soft and hard grains. The average anisotropy of grain 〈K^ij〉 has been calculated based on and the theory of partial exchange-coupling interaction. It has been found that the average anisotropy of hard or soft grain, 〈K^hh〉 or 〈K^ss〉, increases with increasing grain size D monotonously when hard-hard or soft-soft grains couple. When soft-hard grains touch each other, with increasing D, the variation of average anisotropy of soft-hard grain 〈K〉 depends on the anisotropy at grain interface , which denotes the affection degree of hard grain on the anisotropy of soft grain. Compared with other results, it is more reasonable that ranges from to . The variations of anisotropy with D we calculated are consistent with those of coercivities given by other authors when is fixed in a certain range.     

Pressure induced anisotropy of electrical conductivity in polycrystalline molybdenum disulfide

Anisotropic specimens of MoS₂ are obtained by pressing the microcrystalline powder into special die. This inelastic compression results in a rearrangement of the disulfide micro platelets observed by atomic force microscopy and reflected in the macroscopic anisotropy in electrical conductivity in these samples. The conductivity measured parallel and perpendicular to the direction of applied pressure exhibits an anisotropy factor of ∼10 at 1 GPa. This behaviour of the conductivity as a function of applied pressure is explained as the result of the simultaneous influence of a rearrangement of the micro platelets in the solid and the change of the inter-grain distances.     

Upper critical fields of single-crystalline and polycrystalline Ca-Sr-Bi-Cu-O compounds

The ac resistivity of a 110 K phase multiphase polycrystalline Ca-Sr-Bi-Cu-O compound and an 85 K phase single-crystalline Ca_0.9Sr_2.1Cu_2.0O_{8 +} has been measured in various magnetic fields up to 8 T. Values forB _c ^2/ (0) of 71.5 T and forB _c2 (0) of 542 T are found for the 85 K phase sample. A value forB _c2(0) of 57.9 T is estimated for the 110K phase compound.     

Mapping phases of singular scalar light fields

We implement experimentally a simple method for accurate measurements of phase distributions of scalar light fields. The method is based on the polarimetric technique for recording the polarization maps of vector fields, where coaxial superposition of orthogonally polarized reference and signal beams allows the signal phase to be reconstructed from the polarization map of the total field. We demonstrate this method by resolving topologically neutral pairs of closely positioned vortices in a speckle field and recovering the positions of vortices within a Laguerre-Gaussian beam with the topological charge three.