Response of an isolated magnetic grain suspended in a liquid to a rotating field

We have studied the response of an isolated uniaxial magnetic grain suspended in a liquid to an applied fieldh rotating with frequencyω. In the presence of an applied static field (H?h), at low frequencies (i.e. for fast relaxation), the easy axis followsh, while at high frequencies the behavior is similar to that of a bulk sample. In zero static field, the response of a ferromagnetic grain is more complicated; there exists a critical frequencyω _e below which a steady state is reached, with the easy axis followingh. Forω>ω _e the mechanical behavior depends crucially on the initial conditions. Finally, a superparamagnetic grain has a (different) critical frequencyω _e, below which it reacts similarly to the ferromagnetic particle, while forω>ω _e it does not follow steadily the rotating field, but can only oscillate about its initial position.     

Energy Spectrum of Bound-Spinons in the Quantum Ising Spin-Chain Ferromagnet

We study the excitation energy spectrum in the S=1/2 ferromagnetic Ising spin chain with the easy axis z in a magnetic field h={h _x ,0,h _z }. According to Wu and McCoy’s scenario of weak confinement, the fermionic spinon excitations (kinks), being free at h _z =0 in the ordered phase, are coupled into bosonic bound states at arbitrary small h _z >0. We calculate the energy spectrum of such excitations in the leading order in small h _z , using different perturbative methods developed for the similar problem in the Ising Field Theory.     

Phase diagram of antiferromagnetic cobalt fluoride

The H-T phase diagram of antiferromagnetic cobalt fluoride in an external magnetic field H perpendicular to the easy magnetization axis A is completed and used to construct a phase diagram in the variables H _z and H _y . In this diagram, the lines corresponding to second-order phase transitions (between an angular phase and a phase with antiferromagnetic vector I ⊥ A) begin and end in fields of a spin-flip transition (i.e., in an exchange field). A peculiarity of these lines of phase transitions is that each of them has two tricritical points at which this line of second-order phase transitions transforms into a line of first-order phase transitions. A critical angle between the direction of the external magnetic field and the basal plane within which the first-order phase transition takes place is determined.     

Properties of the Ising magnet confined in a corner geometry

The properties of Ising square lattices with nearest neighbor ferromagnetic exchange confined in a corner geometry, are studied by means of Monte Carlo simulations. Free boundary conditions at which boundary magnetic fields ±h are applied, i.e., at the two boundary rows ending at the lower left corner a field +h acts, while at the two boundary rows ending at the upper right corner a field −h acts. For temperatures T less than the critical temperature T_c of the bulk, this boundary condition leads to the formation of two domains with opposite orientation of the magnetization direction, separated by an interface which for T larger than the filling transition temperature T_f(h) runs from the upper left corner to the lower right corner, while for T<T_f(h) this interface is localized either close to the lower left corner or close to the upper right corner. It is shown that for T=T_f(h) the magnetization profile m(z) in the z-direction normal to the interface simply is linear and the interfacial width scales as w∝L, while for T>T_f(h) it scales as . The distribution P(?) of the interface position ? (measured along the z-direction from the corners) decays exponentially for T<T_f(h) from either corner, is essentially flat for T=T_f(h), and is a Gaussian centered at the middle of the diagonal for T>T_f(h). Unlike the findings for critical wetting in the thin film geometry of the Ising model, the Monte Carlo results for corner wetting are in very good agreement with the theoretical predictions.     

Etude par effet Mössbauer de la briartite (Cu2FeGeS4)

Mössbauer experiments on Cu₂FeGeS₄ have shown that the ground orbital level of the Fe²⁺ ions is |L_z=0>, which corresponds to the existence of an easy plane of magnetization perpendicular to the tetragonal axis 0z; in the antiferromagnetic phase the magnetic moments actually lie inside this plane. The tetragonal splitting of the ground orbital doublet ₃ is found to be about 1430 cm^?1, and the Néel temperature is 12,3±0,3 K. A. self consistent calculation of molecular field is used to explain the order of magnitude of the hyperfine field observed at low temperature (H_hf=167±2 kOe at 4,2 K).     

Ground-state phase diagrams of Ising metamagnet in a mixed longitudinal and transverse magnetic field

The ground-state magnetic properties of a two-sublattice Ising metamagnet in a mixed longitudinal and transverse magnetic field are studied within the effective-field theory. A parameter j₂=J₂/J₁ is introduced, which reflects the strength ratio of spin coupling between adjacent planes and in each plane. In addition to the second-order transition lines, the first-order transition lines are also presented, since the ground-state energy can be calculated numerically. The ground-state phase diagrams in h_x–h_z are presented. The results show that when j₂<0 the phase transition of the system is always first-order for h_x<2.751, and when −1000?j₂<0 it is always second-order for h_x>4.36. For the given h_x (0<h_x<14.71), the longitudinal critical magnetic field increases as j₂ decreases. The reentrant phenomenon occurs in the range of j₂<−11.89, h_x>14.71. There is no fourth-order critical point in the phase diagrams given by using EFT as found by using mean field theory (MFT).     

Variation du champ seuil avec une tension uniaxiale dans FeCl2 A 4,2°K

In FeCl₂, at low temperature, the Fe²⁺ ions present an antiferromagnetic order characterised by an easy magnetization axis perpendicular to the ion-containing planes. There exists a value h_s of a magnetic field parallel to the axis, which transforms the system from an antiferromagnetic order into a saturated paramagnetic order. There we present the variations of h_s at 4·2°K as well as the deformations of the sample, when we impose upon it a strain parallel to the axis without allowing its lateral dilatation. These results, when added to those already obtained with χ_i in similar experiments enable us to determine the variations of the crystalline anisotropy and anti-ferromagnetic exchange (between Fe²⁺ ions belonging to two consecutive planes) in terms of the deformations. Our results concerning h_s are different from the ones obtained when exerting a hydrostatic strain. We compare the two cases by using the relations between elastic constants of FeCl₂ obtained at 300°K and recorded in the Appendix.     

铁磁/反铁磁双层膜中冷却场对交换偏置场的影响

用铁磁畴壁模型研究了非补偿界面铁磁/反铁磁双层膜中冷却场（包括大小及其方向）对交换偏置场h_E的影响.结果表明：当冷却场的方向与反铁磁层磁易轴一致时,h_E大小与冷却场大小无关.当冷却场的方向偏离磁易轴时,h_E的大小随偏离角度的增大有缓慢的改变,但当冷却场的方向偏离到临界角度γ_c处,h_E的大小发生突变,其γ_c的大小随冷却场的增大而增大.特别是当冷却场的偏离角度大于γ_c后,h_E出现由负转正的现象,其转变点还与冷却场的大小有关.另外,h_E与铁磁层原子层数N_F的关系会发生由h_E∝N^-1_F向h_E∝N^-λ_F的转变,其中λ>1.其发生转变的条件与N_F、冷却场大小和方向密切相关. 关键词： 铁磁/反铁磁双层膜 交换偏置 冷却场     

Transfer processes in multi-pulse NMR spin-locking and the spin-echo formation mechanism

The polarizations of spins in the local field h_z? S(h_z, t) (the layer polarization) are calculated. In terms of layer polarizations, the solid-echo formation mechanism and the transfer processes in multi-pulse NMR spin-locking are explained.     

Dynamics of the random field Ising model

Dynamics of the kinetic Ising model in the presence of static random fields is investigated using a self-consistent method. It is shown that if the interface fluctuations of the low temperature phase are small the system at low temperatures stays in a state without long range order. For this state the spin correlation function 〈S_q(t)S_?q(O)> averaged over all configurations of random fields decays exponentially in time with a single wavevector dependent relaxation time which is finite at the transition temperature T₀ and remains very long below T₀. In the mean field approximation the correlation time at the magnetic Bragg peak and at T₀ scales with the magnitude of the random field as τ∞h^?z_h with z_h = 1 for d = 2 and $\text{z}{}_{\mathrm{h}}\text{=}\text{4}\text{3}$ for d = 3, respectively.     

Quantum Monte Carlo Investigation of the magnetic properties of weakly interacting antiferromagnetic chains with an alternating exchange interaction with spin S = 1/2

An approximation dependence of the spontaneous magnetic moment at a site, σ/σ(0) ? 1 = and the antiferromagnet-singlet state phase boundary, J ₂/J ₁ = 0.52(3)δ, are determined by the quantum Monte Carlo method in the self-consistent sublattice molecular field approximation for weakly inter-acting (J ₂) antiferromagnetic chains with spin S = 1/2 and alternating exchange interaction (J ₁ ± δ). The Néél temperature and a number of critical temperatures which could be related with the filling energy of two singlets (ΔS ^z = 0) and one triplet (ΔS ^z = 1) spin bands, each of which is split by the sublattice field (h ^{x, y} ≠ h ^z into two subbands, are determined on the basis of the computed correlation radii of the two-and four-spin correlation function, the squared total spin 〈 (S ^z)²〉 with respect to the longitudinal components, the dimerization parameter, and the correlation functions between the nearest neighbors with respect to longitudinal and transverse spin components. On the basis of the Monte Carlo calculations, the critical temperatures and possible energy gaps at the band center are determined for the antiferromagnets CuWO₄ and Bi₂CuO₄ and for the singlet compounds (VO)₂P₂O₇ and CuGeO₃, agreeing satisfactorily with existing results, and new effects are also predicted.     

Collapse of static magnetization of type II anisotropic superconductors

The effect of suppression of static magnetization (magnetization collapse) of an anisotropic hard superconductor by an alternating magnetic field is studied experimentally and theoretically. The measurements are performed in constant and alternating magnetic fields H and h(t) which are parallel to the sample surface and are mutually orthogonal. Superconducting samples of the Y-123 system are cut from a grain-oriented piece in such a way that the c axis lies in the plane of the plate. It has been established that the amplitude of the alternating field for which the complete suppression of the magnetization occurs significantly depends on the field orientation relative to the crystallographic axes of the sample. The dynamics of the magnetization collapse is studied, and it is established that, in some cases, the magnetic moment varies nonmonotonically with the growth in amplitude h₀ of the alternating field. A peculiar effect of transition (induced by field h(t)) of the superconductor from the paramagnetic into the diamagnetic state is discovered. The results of observations are explained in the framework of the model of critical state generalized to the anisotropic case.     

Electrodynamic features of anisotropic hard superconductors

The low-frequency electromagnetic response of superconducting plates characterized by strong anisotropy of current-carrying capacity in the plane of the sample is studied experimentally and theoretically. Measurements are made on polycrystalline textured plates of the Y-123 system with the c axis lying in the plane of the sample and on a single crystal with a single preferred direction of twinning boundaries. It is shown that the shape of the curves describing the dependence of the relative losses q on the ac field amplitude h ₀ is quite sensitive to the orientation of vector h ₀ in the sample plane. As in the case of isotropic samples, the q(h ₀) dependence is characterized by a single size peak if vector h ₀ is oriented along one of the principal symmetry directions of the anisotropic critical current density. If h ₀ deviates considerably from principal directions, two size peaks are observed on the q(h ₀) curve. A detailed analysis of the evolution of the q(h ₀) curves upon a rotation of vector h ₀ in the sample plane is carried out.     

Dynamic phase transitions and dynamic phase diagrams in the kinetic spin-5/2 Blume-Capel model in an oscillating external magnetic field: Effective-field theory and the Glauber-type stochastic dynamics approach

Using an effective field theory with correlations, we study a kinetic spin-5/2 Blume-Capel model with bilinear exchange interaction and single-ion crystal field on a square lattice. The effective-field dynamic equation is derived by employing the Glauber transition rates. First, the phases in the kinetic system are obtained by solving this dynamic equation. Then, the thermal behavior of the dynamic magnetization, the hysteresis loop area and correlation are investigated in order to characterize the nature of the dynamic transitions and to obtain dynamic phase transition temperatures. Finally, we present the phase diagrams in two planes, namely (T/zJ, h₀/zJ) and (T/zJ, D/zJ), where T absolute temperature, h₀, the amplitude of the oscillating field, D, crystal field interaction or single-ion anisotropy constant and z denotes the nearest-neighbor sites of the central site. The phase diagrams exhibit four fundamental phases and ten mixed phases which are composed of binary, ternary and tetrad combination of fundamental phases, depending on the crystal field interaction parameter. Moreover, the phase diagrams contain a dynamic tricritical point (T), a double critical end point (B), a multicritical point (A) and zero-temperature critical point (Z).     

Kinetic theory of runaway breakdown in inhomogeneous thundercloud electric field

Kinetic theory of runaway breakdown in inhomogeneous thunderstorm electric field is developed. The kinetic equation, boundary and initial conditions are formulated. Spectrum of X-ray emission generated by runaway electrons is calculated. It has quite a specific form: a sharp maximum at energies 50–60 keV and a rapid fall both to the smaller and higher energies. The intensity of the emission is growing effectively with the relation E_m/E_c (of maximal electric field E_m to critical field E_c) and falling down sharply with the distance near the point z where electric field E(z) is equal to critical field E_c.     

First passage time of multiple Brownian particles on networks with applications

In this article, we study some theoretical and technological problems with relation to multiple Brownian particles on networks. We are especially interested in the behavior of the first arriving Brownian particle when all the Brownian particles start out from the source s simultaneously and head to the destination h randomly. We analyze the first passage time (FPT) Y_sh(z) and the mean first passage time (MFPT) 〈Y_sh(z)〉 of multiple Brownian particles on complex networks. Equations of Y_sh(z) and 〈Y_sh(z)〉 are obtained. On a variety of commonly encountered networks, we observe first passage properties of multiple Brownian particles from different aspects. We find that 〈Y_sh(z)〉 drops substantially when particle number z increases at the first stage, and converges to d_sh, the distance between the source and the destination when z→∞. The distribution of FPT Prob{Y_sh(z)=t},t=0,1,2,… is also analyzed in these networks. The distribution curve peaks up towards t=d_sh when z increases. Consequently, if particle number z is set appropriately large, the first arriving Brownian particle will go along the shortest or near shortest paths between the source and the destination with high probability. Simulations confirm our analysis. Based on theoretical studies, we also investigate some practical problems using multiple Brownian particles, such as communication on P2P networks, optimal routing in small world networks, phenomenon of asymmetry in scale-free networks, information spreading in social networks, pervasion of viruses on the Internet, and so on. Our analytic and experimental results on multiple Brownian particles provide useful evidence for further understanding and properly tackling these problems.     

Spectral shifts and spectral switches of spatially and spectrally partially coherent pulsed beams in Young’s interference experiment

Taking the Gaussian Schell-model pulsed (GSMP) beam as a typical example of spatially and spectrally partially coherent pulsed beams, an analytical expression for the far-field spectrum of diffracted GSMP beams in Young’s interference experiment is derived, and used to study the spectral shifts and spectral switches of GSMP beams in the far field. Numerical calculation results are given to illustrate the dependence of spectral shifts and spectral switches on the obscuration ratio ε, temporal coherent length T_c, spatial correlation parameter β and diffraction angle α. It is shown that the critical angle α_c of the first- and second-order spectral switches moves away from the z axis with increasing T_c, but α_c of the third-order spectral switch moves towards the z axis with increasing T_c. The spectral transition height Δ decreases and spectral minimum S_min increases as T_c increases. α_c decreases, Δ increases and S_min decreases as β increases. The potential application of spectral switches of spatially and spectrally partially coherent pulsed beams in information encoding and transmission is proposed.     

Anisotropy modulated stepwise magnetization in triangular Heisenberg antiferromagnet

During the course of tuning anisotropy from Ising type to zero, the variation of magnetization (M) steps against magnetic field (h) is investigated in a triangular antiferromagnetic Heisenberg model using Monte Carlo techniques. It is revealed that the anisotropy is an essential key to induce the temperature-dependent stepwise M(h) curve observed in frustrated magnetic system, and it can be employed to modulate this steplike magnetic behavior effectively. When the anisotropy is strengthened, a ground state transition occurs from the homogeneous 120° triangular structure to the collinear partially disordered antiferromagnetic state. No M step is detected in the system without anisotropy. But if the anisotropy is nonzero, the M₀/3 step (where M₀ is the saturated M) will emerge on M(h) curve, which is due to an h-induced quasi-collinear ferrimagnetic state. This M₀/3 step can be extended by increasing the anisotropy. When the M₀/3 plateau dominates the h-range broad enough, the equidistant metastable substeps, which originates from the disorders frozen in the frustrated collinear spin structure, appear to be superposed on the M₀/3 plateau. Thus the system with a strong anisotropy presents the whole temperature evolution of stepwise M(h) curve in quantitative agreement with the experiments of Ca₃Co₂O₆.     

Magnetic properties of V5S8 single crystals

The magnetic properties of V₅S₈ single crystals have been investigated by susceptibility and torque measurements. The susceptibilities parallel and perpendicular to the magnetic easy axis show a remarkable anisotropy below the Neel temperature of 32 K and are nearly Isotropie above that temperature. The easy axis is found to be nearly along the c-axis of the monoclinic lattice, being inclined at an angle of 9.6 ± 1.0° from the c-axis toward the a-axis. The torque curves, measured up to 24.4 kOe at liquid helium temperature, deviate from the usual form of a sine function with increasing magnetic field. The analysis of these torque curves suggests that spin flopping may occur at 43 kOe, a comparatively low critical field. Using these experimental results, a localized d-electron model for a particular site of the vanadium-ion, proposed by previous investigators, is examined.     

Properties of phase transitions in easy-axis tetragonal antiferromagnets

Properties of phase transitions in a magnetic field H parallel to the easy axis have been investigated, and it has been shown, in particular, that the nature of the transition of the magnetic subsystem from the antiferromagnetic phase to the angular phase depends on the “sign” of the Dzyaloshinskii interaction. The conditions for orientation of the antiferromagnetism vector l in the basis plane in fields larger than the threshold field have been determined. It is shown that the transition from the angular phase to the state where the resulting magnetic moment m is parallel to the easy axis takes place in the field corresponding to a spin-flip transition. From an analysis of the configuration of the magnetic subsystem for arbitrary orientation of the external magnetic field, it follows that the values of the critical angle (ψ _cr ) for which a first-order phase transition takes place satisfy the condition in the case in which the anisotropy constant f in the basis plane is of the order of the first anisotropy constant b. Usually so that the tricritical point in the phase diagram H _y ,H _z satisfies the condition H _y ∼H _z . Fiz. Tverd. Tela (St. Petersburg) 41, 2044–2046 (November 1999)