Magnetic anisotropy-induced spin-reorientation in spinel FeCr2S4

In this paper, the low-field magnetic behavior of polycrystalline FeCr₂S₄ was investigated by thermal circling the sample from 5 to 200 K at a constant magnetic field. We show that, upon cooling the sample at 50 Oe to a temperature below the Curie temperature then warming back, the magnetization displays irreversibility between cooling and warming sequence. The irreversible behavior was suppressed gradually with increasing magnetic field. By considering spin-reorientation due to the increase of magnetic anisotropy upon cooling, the irreversible behavior has been explained qualitatively.     

Monte Carlo simulation of the néel relaxation of a single-domain particle

The magnetization reversal kinetics in a single-domain particle with uniaxial magnetic anisotropy was simulated using the Monte Carlo method. The algorithm developed here yields good agreement with the analytical estimates for superparamagnetic relaxation and qualitatively describes some features of magnetic behavior of fine-particle systems.     

High-Moment-Low-Moment Description of Magnetovolume Effects in Y(Mn x Al 1− x ) 2 and Y x Sc 1− x Mn 2

Based on the assumption of a high-moment-low-moment instability of the Mn atom, we construct a simple spin model with coupled magnetic and spatial degrees of freedom to describe the Laves phase systems Y(Mn x Al 1 m x ) 2 and Y x Sc 1 m x Mn 2 . Monte Carlo simulations of this model qualitatively reproduce anomalies observed in these materials like a discontinuous giant volume change and anomalous thermal expansion behavior.     

Anisotropy in elastic modulus of hydrogel containing magnetic particles

We derive the analytic expression of elastic modulus for a gel containing magnetic particles with a magnetic dipole moment. The obtained elastic modulus is anisotropic and the modulus increases and decreases with the density of magnetic particle when the direction of strain is perpendicular and parallel to the direction of magnetization, respectively. This behavior is qualitatively in good agreement with previous experimental data [T. Mitsumata, et al., Macromol. Rapid Commun. 23, (2002) 178].     

Magnetic properties of chalcogenide spinel CuCr2Se4 nanocrystals

The magnetic properties of chalcogenide spinel CuCr₂Se₄ nanocrystals have been studied as a function of crystallite size (15-30 nm). A solution-based method is used for the facile synthesis of the nanocrystals with good size control. They have close to cubic morphology with a narrow size distribution and exhibit superparamagnetic behavior at room temperature. The Curie temperature and saturation magnetization of the nanocrystals are lower as compared with the bulk and decrease with decreasing nanocrystal size. A similar trend is observed in the paramagnetic state for the Curie-Weiss temperature and effective magnetic moment. The low temperature magnetization behavior can be qualitatively explained by spin glass dynamics.     

Thermal entanglement in a two-spin-qutrit system under a nonuniform external magnetic field

The thermal entanglement in a two-spin-qutrit system with two spins coupled by exchange interaction under a magnetic field in an arbitrary direction is investigated. Negativity, the measurement of entanglement, is calculated. We find that for any temperature the evolvement of negativity is symmetric with respect to magnetic field. The behavior of negativity is presented for four different cases. The results show that for different temperature, different magnetic field give maximum entanglement. Both the parallel and antiparallel magnetic field cases are investigated qualitatively (not quantitatively) in detail, we find that the entanglement may be enhanced under an antiparallel magnetic field.     

窄缝中气泡在磁性液体中的上升与变形

对窄缝中气泡在磁性液体中的上升与变形进行了可视化研究。定性分析了纳米磁性颗粒引发的黏度效应及表面活性剂分子依附作用对气泡上升速度与形状的影响。试验工质为体积浓度6.33%的水基Fe_3O_4磁性液体,同时对比了质量浓度25%的四甲基氢氧化铵水溶液、质量分数30%和50%的蔗糖溶液以及水中气泡的上升运动。窄缝间隙分别为1 mm和2 mm,气泡由底部不同直径的圆孔产生。试验结果表明:由于活性剂分子的存在,磁性液体气泡上升过程中由扁椭圆形渐转变为上圆下平的冠状,而其略大于水的黏度使气泡在1 mm窄缝中保持直线上升运动,但窄缝间隙增大到2 mm后,磁性液体中气泡的运动轨迹仍会发生振荡。     

Crossover behavior of disordered interacting two-dimensional electron systems in a parallel magnetic field

The crossover behavior of disordered interacting two-dimensional electron systems in a parallel magnetic field is analyzed. Using the so-called crossover one-loop renormalization group equations for the resistance and electron-electron interaction amplitudes, experimentally observed transformation of the temperature dependence of the resistance from a reentrant (nonmonotonic) behavior in relatively weak fields to an insulating-type behavior in stronger fields is qualitatively explained. The text was submitted by the authors in English.     

Effect of electric current on the magnetic and magnetoelastic properties of amorphous ferromagnetic alloys

The magnetic and magnetoelastic parameters of Fe₆₄Co₂₁B₁₅ and Fe_81.5B_13.5Si₃C₂ amorphous ferromagnetic alloys treated by direct electric current in air are studied as functions of the applied magnetic field and current density. The samples of the alloy have the form of narrow strips with different lengths. It is found that the magnetoelastic parameters of the dc-treated alloys depend on the magnetic field in a qualitatively different way. From the behavior of the magnetic and magnetoelastic parameters of the alloys in the magnetic field a model of magnetization nonuniform distribution in amorphous ferromagnetic alloys subjected to dc treatment is proposed.     

On the theory of the symmetrical double SQUID

A calculation is presented which describes the behavior of a symmetrical double SQUID, i.e., an RF-SQUID in which a symmetric DC-SQUID is used instead of the weak link. It is shown that in the case of a symmetrical double SQUID, in contrast to a traditional RF-SQUID, a variation of the input signal applied magnetic flux to be measured qualitatively transforms the behavior of the system, giving rise to much higher output signals.     

Non-Langevin high-temperature magnetization of nanoparticles in a weak magnetic field

Experimental evidence and theoretical substantiation are presented for the asymptotic behavior of high-temperature magnetization of an ensemble of nanoparticles in a weak magnetic field, which was predicted earlier and which differs qualitatively from the “Langevin” limit for ideal superparamagnetic particles. It is shown that the physical reason for the new asymptotic behavior is the temperature-independent “positive” tilt of the uniform magnetization vector at local energy minima in the direction of the field; this asymptotic behavior is associated with the nonstandard thermodynamics of single-domain particles, which depends on the ratio of characteristic frequencies of regular precession and random diffusion of this vector. An alternative approach is proposed for describing the magnetic dynamics of an ensemble of nanoparticles in a magnetic field, and the precession orbits of the magnetization vector are considered as stochastic states of each particle, whereas each state is characterized by the trajectory-averaged value of magnetization.     

Dynamic susceptibility of a spin ice near the critical point

We consider spin ice magnets (primarily, Dy₂Ti₂O₇ in the vicinity of their critical point on the (H, T) plane. We find that the longitudinal susceptibility diverges at the critical point, leading to the behavior qualitatively similar to the one which would result from non-zero conductance of magnetic charges. We show that dynamics of critical fluctuations belongs to the universality class of easy-axis ferroelectric and calculate logarithmic corrections (within two-loop approximation) to the mean-field critical behavior.     

Magnetic susceptibility of noninteracting fermions in a confined geometry

A model system of an ideal gas of neutral fermions in a confined geometry of different symmetry and size is theoretically examined. The behavior of these systems is found to exhibit qualitatively new features such as the oscillations in magnetic susceptibility with changing geometry size and particle density, indicating that the geometric confinement substantially affects the thermodynamic properties of the system.     

BCC Fe surface and cluster magnetism using a magnetic potential

A semi-empirical potential for ferromagnetic bcc iron that also models the co-linear magnetic moment of each atom, has been applied to clusters and surfaces. Surface energies of low index facets, surface relaxation and magnetic moment are calculated and compared to various other empirical potentials, recent density functional theory (DFT) results and experimental data. Quantitatively the resulting surface magnetic moments enhancements are underestimated, however qualitatively the general behavior of the magnetic moment as a function of surface or cluster layer is comparable to that predicted by DFT calculations.     

On the AC magnetic susceptibility of spin-chains: Solitons in one-dimensional systems

It is shown that relatively simple phenomenological considerations of the motion of thermally activated solitons (kinks of various types) in single spin chains can explain and describe qualitatively and quantitatively the temperature and frequency behavior of magnetic susceptibility in ac magnetic fields. The comparison of experimental data and theoretical results allows one to estimate the number of solitons and “weak” places in correlation regions of a sample. The text was submitted by the authors in English.     

Influence of the martensitic structure transformation in LaAgxIn1?x on the magnetic properties of substituted Ce

The martensitic phase transformation has no observable effect on the magnetic susceptibility of Ce in LaAg _x In_1–x which agrees with the assumption of a crystal field splitting by 300 K of the² F _5/2 ground state of Ce³⁺ ions. The magnetic ordering temperature and the Kondo minimum move to higher temperatures together with the martensitic phase transformation when the Ag concentration is reduced. This behavior can be related qualitatively to the lowering of the 5d – e _g levels in the center of the Brillouin-zone with increasing In content.     

Influence of the martensitic structure transformation in LaAg x In1−x on the magnetic properties of substituted Ce

The martensitic phase transformation has no observable effect on the magnetic susceptibility of Ce in LaAg _x In_1–x which agrees with the assumption of a crystal field splitting by 300 K of the² F _5/2 ground state of Ce³⁺ ions. The magnetic ordering temperature and the Kondo minimum move to higher temperatures together with the martensitic phase transformation when the Ag concentration is reduced. This behavior can be related qualitatively to the lowering of the 5d – e _g levels in the center of the Brillouin-zone with increasing In content.     

Effect of grain misorientation on the stripe domains in evaporated cobalt films

X-ray diffraction and magnetic force microscopy techniques were used to investigate the structural and the static magnetic properties of vapor-deposited cobalt films with various thicknesses t ranging from 50 to 195 nm. Texture measurements revealed that as the thickness increases, the films become predominantly c-axis oriented. Magnetic stripe domains structure was only observed for the thicker films, with t=195, 173 and 125 nm, while such a magnetic configuration was expected for all the samples based on the theoretical studies. Since the layers present increasing c-axis misorientation when the thickness decreases, we assume that this effect can prevent the stripe domains formation. This behavior is qualitatively explained by a simple model which describes the stripe domains structure taking into account the role of a small misorientation of the anisotropy axis.     

Stochastic switching and dynamical freezing in nonlinear spin systems

We consider the controlled switching of individual spins in a nonlinear, interacting spin chain by means of external magnetic fields. We show analytically and by full numerical simulations that stochastic switching is achievable when the driving fields are such that the underlying semi-classical dynamics is chaotic. On the basis of random matrix theory and the geometry of quantum evolution we confirm the quantum case to follow qualitatively the semi-classical behavior.     

Circular-to-linear and linear-to-circular conversion of optical polarization by semiconductor quantum dots

We report circular-to-linear and linear-to-circular conversion of optical polarization by semiconductor quantum dots. The polarization conversion occurs under continuous wave excitation in the absence of any magnetic field. The effect originates from quantum interference of linearly and circularly polarized photon states, induced by the natural anisotropic shape of the self-assembled dots. The behavior can be qualitatively explained in terms of a pseudospin formalism.