Magnetoelectric effect and magnetic dynamics in the Gd2CuO4 antiferromagnet

The magnetoelectric effect and the magnetic dynamics in Gd₂CuO₄ have been studied using a phenomenological approach and group-theory methods. Vector order parameters are introduced based on four magnetic sublattices. Invariant products of the order parameters are determined, from which the thermodynamic potential density is constructed. Using the spin-wave representation, the calculations can be significantly simplified and the ground orientation magnetic state can be presumably determined. The magnetic dynamics is described by the Landau-Lifshitz equations, from which the antiferromagnetic resonance frequency and the dynamic susceptibilities, namely, magnetic, antiferromagnetic, magnetoelectric, and antiferroelectric susceptibilities are found. The frequency and the susceptibility are shown to be controlled by applied electric field.     

Magnetic Ordering of Strongly Magnetized Massive Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B Magnets

The magnetic ordering of a strongly magnetized massive rectangular magnet is studied via magnetic force microscopy. The magnetic ordering structures on the magnet’s surface are determined. An attempt is made to interpret the observed magnetic ordering by analyzing the simplest magnetic ordering models theoretically.     

Single-domain magnetic nanoparticles as force generators for the nanomechanical control of biochemical reactions by low-frequency magnetic fields

The dynamics of single-domain magnetic nanoparticles cross-linked into multiparticle aggregates by organic ligands is considered. Mechanical factors of the effect of low frequency magnetic field on macromolecules attached to magnetic nanoparticles/aggregates within a suspension or gel are analyzed. The optimum conditions ensuring the best control over biochemical reactions in suspension by an external magnetic field (i.e., the ranges of frequency and magnetic field intensities, and the size of magnetic nanoparticles and shells covering them) are determined.     

Features of motion of interacting magnetic liquid drops

The features of simultaneous motion of two identical drops of magnetic liquid, which are determined by their hydrodynamic and magnetic interaction, are considered. The changes in the trajectories of drops initially moving in the same direction, caused by a variation of direction and magnitude of the applied dc and ac magnetic field, are studied experimentally and substantiated theoretically. The possibility of effective control of the motion of magnetic liquid drops by applying magnetic fields is demonstrated.     

Light scattering by aggregates of magnetite nanoparticles in a magnetic field

The effect of variation of the scattered light intensity in a magnetic fluid with aggregates of magnetite particles several microns in size under the action of a magnetic field is studied. The effect relaxation times are determined when the magnetic field is turned on and off. This effect is found to be caused by the orientation of anisotropically magnetized aggregates. Experimental data are used to calculate the average anisotropy of the magnetic susceptibility of the aggregate and estimate its magnetic permeability.     

Magnetization reversal process at atomic scale in systems with itinerant electrons

The magnetic response of itinerant electrons systems to an external magnetic field is investigated on the basis of a microscopic Hamiltonian from which the spin-polarized electronic structure is determined. The magnetic moment and grand thermodynamic potential of the d-electronic subsystem on a particular atomic site in the presence of the external field are calculated as a function of the moment's orientation for fixed electron configuration of its local environment. Self-consistent magnetic solutions strongly depend on the d-electron number, determined by the position of the d level relative to the Fermi energy. For parameters corresponding to α-Fe, two branches of self-consistent solutions with high and low magnetic moments are found. For parameters corresponding to bulk Cr, a Fe impurity in the Cr matrix and a Cr impurity in the Fe matrix, there are only low-spin solutions. The theory is also applied for describing magnetization reversal processes in exchange spring magnets. A slab of Fe was considered as a soft magnetic layer. The influence of the hard magnet is modeled by the inclusion of an external magnetic field applied to the interface Fe layers. The dependence of the hysteresis loop on the thickness of the Fe slab and on the value of the interface field is investigated.     

Plastic deformation modeling of backward extrusion process for Nd–Fe–B ring magnets

3D finite element-based software (3D DEFORM) was used to simulate the thermal extrusion process of nanocrystalline magnetic ring. The effective stresses and effective strains for a ring magnet at different stages of the extrusion process were determined by simulation. The effective strains at different stages are displayed. The effective stresses on the cross section are determined by simulation. The test results of magnetic properties were of good validation of the three-dimensional finite element analysis for nanocrystalline backward extruded ring. 3D finite element-based plastic deformation simulation is proved to be an effective way to analyze the hot extrusion process of nanocrystalline magnetic ring, and to provide guiding for the mold design of thermal extrusion.     

Magnetic properties of nonmagnetic metals with randomly distributed paramagnetic impurities

A generalized mean field theory for disordered systems with the RKKY interaction is constructed on the basis of calculation and analysis of distribution functions for random magnetic fields produced by magnetic moments with an irregularly spatial distribution. These distribution functions are determined by two methods: (i) analytically and (ii) numerically by statistical processing of the results of calculation of random fields in a model system. For metals diluted by magnetic impurities, it is shown that the ground state of the system becomes magnetically ordered when the impurity concentration exceeds a certain critical value depending on the type of crystal lattice of the metal and the sample shape. The magnetic phase diagram of the system is determined and the temperature dependence of its magnetic susceptibility, the concentration dependence of the Curie temperature, and the temperature and concentration dependences of the magnetization and magnetic part of the heat capacity of the system are established.     

A new class of surface magnetic TM polaritons in crystals with linear magnetoelectric effect

The conditions necessary for the formation of a new type of a surface magnetic TM polariton at the magnetic dielectric-nonmagnetic metal or the magnetic dielectric-nomagnetic dielectric interfaces are determined. The formation of a surface electromagnetic wave of this type is caused by the effect of spatial dispersion of a magnetic dielectric medium.     

Magnetic properties of amorphous alloys

The magnetic properties of existing amorphous iron based materials, i.e., Fe--metalloid, Fe--early transition metals and Fe--rare earth alloys, are briefly discussed for some representative alloys. The spin orientation of amorphous Fe--metalloid alloys has been determined by the angular dependence of hyperfine interactions. It is shown that in iron--early transition metals ferromagnetic order is not long-ranged, but determined by magnetic clusters. The magnetic hyperfine field distributions of Fe-rich iron--early transition metals consist of a high and a low field tail. The magnetic structure has been investigated for two representative Fe--RE (RE = Er, Ce) amorphous alloys. For the first time, the magnetic coupling phenomenon in amorphous/crystalline multilayers has been discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

W(110)基底上的铁纳米岛初始自发磁化态的微磁学模拟

用微磁学模拟研究W(110)基底上铁纳米岛的初始自发磁化态的磁畴结构,确定了不规则形状、椭圆形和矩形岛中不同磁畴态之间的各向异性常数的临界点,得到了纳米岛的磁化态作为各向异性常数和厚度函数的完整相图,相图中存在一较宽的过渡区,把双畴态与涡旋态和菱形态分开,过渡区两侧的边界是不确定的.计算结果表明,初始自发磁化态的磁畴结构主要由各向异性及岛的厚度决定,而且岛的边沿形状对涡旋态和菱形态的磁畴结构有重要影响.准确的铁纳米岛的各向异性常数仍有待于进一步确定. 关键词： 初始自发磁化磁畴结构 铁纳米岛 微磁学模拟 各向异性     

Equilibrium values and dynamics of the net magnetic moment of a system of magnetic dipoles

Equilibrium states of different systems formed by coupled spherical bodies with dipole magnetic moments have been investigated using a numerical analysis. The bistable states and the corresponding values of the net magnetic moment are determined for a number of planar and three-dimensional systems of dipoles, and the conditions providing the existence of orientational configurations of coupled dipoles involved in the bistability are analyzed. The disturbances of the magnetic moment due to the quasi-static passage of an additional dipole and the dynamic modes excited by a homogeneous alternating magnetic field and represented by periodic, quasi-periodic, and chaotic oscillations of the magnetic moment of the system are considered for several types of systems. The bifurcation diagrams of the dynamic modes are constructed, and the specific features typical of the systems under consideration are revealed.     

High-frequency magnetic permeability of nanocomposite film

The high-frequency magnetic permeability of nanocomposite film consisting of the single-domain spherical ferromagnetic particles in the dielectric matrix is studied. The permeability is assumed to be determined by rotation of the ferromagnetic inclusion magnetic moments around equilibrium direction in AC magnetic field. The composite is modeled by a cubic array of ferromagnetic particles. The magnetic permeability tensor is calculated by solving the Landau–Lifshits–Gilbert equation accounting for the dipole interaction of magnetic particles. The permeability tensor components are found as functions of the frequency, temperature, ferromagnetic inclusions density and magnetic anisotropy. The obtained results show that nanocomposite films could have a rather high value of magnetic permeability in the microwave range.     

Direct measurement of effective magnetic diffusivity in turbulent flow of liquid sodium

The first direct measurements of effective magnetic diffusivity in turbulent flow of electroconductive fluids (the so-called β effect) under the magnetic Reynolds number Rm?1 are reported. The measurements are performed in a nonstationary turbulent flow of liquid sodium, generated in a closed toroidal channel. The peak level of the Reynolds number reached Re≈3×10(6), which corresponds to the magnetic Reynolds number Rm≈30. The magnetic diffusivity of the liquid metal was determined by measuring the phase shift between the induced and the applied magnetic fields. The maximal deviation of magnetic diffusivity from its laminar value reaches about 50%.     

Magnetic order in an MnF2 epitaxial layer with the orthorhombic structure

The magnetic structure of MnF₂ is determined by the neutron diffraction method in the metastable orthorhombic phase grown in the form of thin (～1 μm) film on a CaF₂ buffer layer by the molecular beam epitaxy method. The magnetic moments of Mn⁺⁺ form a simple two-sublattice antiferromagnetic structure and are directed along the c crystallographic axis parallel to the film plane. Using the temperature dependence of magnetic reflections, a Néel temperature of 67.19(7) K and a critical index of 0.50(2), which corresponds to the mean field approximation, are determined.     

Relaxation effects in Fe17 and Fe19 oxo-bridged iron clusters

The magnetic properties of a new type of iron clusters are studied by means of Mössbauer spectroscopy in an applied magnetic field. The results obtained agree with the ones obtained from magnetic susceptibility measurements. They are well explained in terms of relaxation determined by fluctuations in the hyperfine field due to spin-spin interaction. The triangular arrangement of the iron ions plays an essential role in this mechanism giving rise to spin frustration effects.     

Anisotropy of antiferromagnetic 180 degrees domains in LiCoPO4 and LiNiPO4

Unexpected three-dimensional distributions of antiferromagnetic 180 degrees domains are observed in LiCoPO4 and LiNiPO4 by optical second harmonic generation. Domains in LiCoPO4 are isotropic in spite of the quasi-two-dimensional magnetic structure whereas domains in LiNiPO4 are distinctly anisotropic, but in contrast to the anisotropy of the magnetic structure. The diversity reveals a potential for fine-tuning magnetic properties determined by the distribution of domains or domain walls and the urgent need for an improved understanding of spatial correlations in antiferromagnets.     

Friedel oscillations of a magnetic field penetrating into a normal metal and a size-quantized system

A magnetic field applied to a size-quantized system causes persistent equilibrium currents nonuniformly distributed across this system. For a quantum film and a two-dimensional strip, the distributions of the dia-and paramagnetic currents and magnetic field are determined. The possibility of observing field distribution by NMR is discussed.     

Stepwise magnetization of dispersed ferromagnets due to magnetic interparticle interactions

The main features of stepwise magnetization of dispersed ferromagnets caused by magnetic interparticle interactions are studied using a two-particle model. The ranges of values of the magnetic anisotropy constants of particles and of the dipole-dipole interaction between them are determined over which a reproducible jumpwise change in the magnetization of the system occurs in an external positive magnetic field. The proposed model is shown to explain the main specific features of the fine structure of the ferromagnetic resonance spectra.     

Induced and parametric excitation of spin waves by a light field with a discrete spectrum and optically induced spin echo

Induced and parametric excitation of spin waves by a light field and light-induced spin echo are considered. The static and steady-state dynamic magnetizations in the light field are determined, and the phase diagrams of the system in magnetic and light fields are constructed. The threshold of parametric excitation, the amplitude of stationary oscillations, and the trajectories described by the end point of the magnetic moment vector are calculated for the case of light-induced nonlinear ferromagnetic resonance. The trajectories can be topologically different. The transition between states with different trajectories occurs as a phase transition. The parameters of the light-induced spin echo are determined.