双模随机晶场对纳米管上Blume-Capel模型磁化强度和相变的影响

近年来, 磁性纳米管的物理性质和相关应用得到了人们的广泛关注. 利用有效场理论研究了纳米管上双模随机晶场中Blume-Capel模型的磁化强度和相变性质, 得到了系统的磁化强度与温度和随机晶场的关系及其相图. 结果表明: 系统在稀释晶场、交错晶场和同向晶场中会表现出不同的磁学性质和相变行为; 稀释晶场和交错晶场会抑制系统的磁化强度, 导致其基态饱和值小于1, 而同向晶场则不会; 随着随机晶场参量的变化, 系统存在多个相变温度, 并呈现出三临界现象和重入现象.     

Entanglement control in one-dimensional s= 1/2 random XY spin chain

The entanglement in one-dimensional random XY spin systems where the impurities of exchange couplings and the external magnetic fields are considered as random variables is investigated by solving the different spin-spin correlation functions and the average magnetization per spin. The entanglement dynamics near particular locations of the system is also studied when the exchange couplings （or the external magnetic fields） satisfy three different distributions （the Gaussian distribution, double-Gaussian distribution, and bimodal distribution）. We find that the entanglement can be controlled by varying the strength of external magnetic field and the distributions of impurities. Moreover, the entanglement of some nearest-neighbouring qubits can be increased for certain parameter values of the three different distributions.     

Microkinetics of the Ising-Glauber model in the binary approximation

Phase transitions in ferromagnets described by the Ising model are investigated in terms of the solution of a hierarchy of microscopic equations for unary and binary distribution functions. A dynamical procedure for making the equations self-consistent is developed. For cubic crystals, an equation of state relating the long-range order parameter to temperature and magnetic field is obtained in analytic form. The temperature dependence of the magnetic susceptibility and of the specific heat are calculated. A stability criterion is obtained for the stationary states of the system is obtained. The dynamics of magnetization reversal by constant and ac external fields are investigated. Fiz. Tverd. Tela (St. Petersburg) 40, 519–523 (March 1998)     

DFT study on magnetic interaction in an orbitally degenerate Ti3+ dimer complex

The magnetic interaction in an orbitally degenerate transition metal dimer complex is investigated using a typical example of a d(1)-d(1) dimer complex, the Ti2Cl9(3-) cluster. The local orbital functions are defined by linear combinations of the molecular orbital functions which are calculated by density functional theory (DFT). In the DFT calculation, the Perdew-Burke-Ernzerhof (PBE) functional and hybrid PBE0 functional are utilized. The matrix elements of the effective Hamiltonian of the d-electrons are evaluated by the DFT calculation except for one parameter which is determined by comparing the zero-temperature magnetic susceptibility in the direction along the c axis χ(∥) with the experimental result. By the calculation with the PBE0 functional, the zero-temperature magnetic susceptibility in the perpendicular direction χ(⊥) and the temperature dependence of the susceptibilities in both directions agree with the experiment. On the other hand, by the calculation with the PBE functional, χ(⊥) is smaller than the experimental values because the on-site potential is underestimated.     

Fiber magnetic-field sensor based on nanoparticle magnetic fluid and Fresnel reflection

A simple fiber sensor for magnetic field measurement based on nanoparticle Fe(3)O(4) magnetic fluid and relative Fresnel reflection is presented. The sensor includes only a light source, three couplers, two photodetectors, and two fiber sensing ends. Magnetic fields at different concentrations of magnetic fluid are measured. Magnetic fluid with high concentration can be used for the measurement of weak magnetic fields, while low concentration fluid is used for the measurement of strong magnetic fields. The temperature dependence of the sensor is also addressed.     

A Mössbauer spectroscopy study of antiferromagnetic iron arachidate Langmuir-Blodgett films

Langmuir-Blodgett multilayers of ferric arachidate (abbreviated: FeA) on silicon wafers have been investigated by means of conversion electron Mössbauer spectroscopy (CEMS) and absorption Mössbauer spectroscopy at different temperatures between room temperature and 4.2 K, without and with external magnetic fields up to 5 T. The films show a quasicrystalline structure with ac-axis orientation perpendicular to the plane and a random distribution of orientations in the plane of the films. All Mössbauer spectra show Fe³⁺. At low temperatures, antiferromagnetic ordering has been observed. The lines in both the room temperature and the low-temperature spectra are significantly broadened, which is due to a distribution of electric field gradients and hyperfine fields. The reason for this is a slightly disordered iron environment. The orientation and the distribution function of the internal magnetic fields have been determined.     

Distribution function for random interaction fields in disordered magnets: Spin and macrospin glass

Systems with an arbitrary dependence of exchange integral on the distance between atoms which are randomly scattered in an amorphous substance are investigated by averaging over random fields of interaction in the framework of the Ising model. This method is also used for describing long-term magnetization relaxation in a system of single-domain particles scattered in a nonmagnetic matrix. Random field distribution functions are obtained for the dipole-dipole and the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions. Long-term relaxation in macrospin glasses is investigated.     

Calculation of the thermal conductivity and the specific heat of the lattice for a spin-boson system at low temperatures

The thermal conductivity of the lattice is calculated for a spin-boson system using the method of inverse linear response theory. The spin-boson Hamiltonian consists of coupled two-level systems (TLS) represented by pseudospin operators, acoustical phonons and an interaction between both systems. The lattice specific heat is determined in the framework of perturbation theory with respect to spin-boson coupling. For the calculation of both quantities results for spin-correlation functions derived within self-consistent random phase approximation are applied. At low temperatures the influence of spin-boson coupling and of interacting TLS on the temperature dependence of the lattice thermal conductivity and of the lattice specific heat is discussed and compared with corresponding experimental investigations.     

A Compensation Temperature Study of Bond-Diluted Mixed Ising Ferrimagnetic Spin System with Different Transverse Fields

The magnetic properties of bond-diluted nearest-neighbor interaction mixed spin-1/2 and spin-1 Ising ferrimagnetic spin system with different transverse fields are investigated within the framework of the finite cluster approximation (FCA). Particular emphasis is given to the square lattice with coordination number 2 = 4 for which magnetizations are obtained. The interactions Jij are assumed to be independent random variable with distribution P(J_ij) = pδ(J_ij-J) + (1-p)δ(J_ij), where J < 0. If bond concentration p varies in the certain ranges, we find that the compensation temperature is obtained for the values of the different transverse fields Ω_1/2 and Ω₁ in a restricted region. We obtain the values of the critical different transverse fields and critical bond concentration in this paper.     

Random field effects on the phase diagrams of spin-1/2 Ising model on a honeycomb lattice

The Ising model with quenched random magnetic fields is examined for single Gaussian, bimodal and double Gaussian random field distributions by introducing an effective field approximation that takes into account the correlations between different spins that emerge when expanding the identities. Random field distribution shape dependence of the phase diagrams, magnetization and internal energy is investigated for a honeycomb lattice with a coordination number q=3. The conditions for the occurrence of reentrant behavior and tricritical points on the system are also discussed in detail.     

Investigation of the Disordered Antiferromagnetic Ising Model with the Effects of Random Magnetic Fields

Computer simulation of phase transitions is made by the Monte Carlo method using a three-dimensional disordered antiferromagnetic Ising model in the external magnetic field. It is found that in the case where the spin concentration in a system is lower than a threshold one, the effects of random magnetic fields destroy the second-order phase transition and lead to the first-order phase transition into a new phase state of the system characterized by a ground spin-glassy state and metastable energy states at finite temperatures. The dependence of the threshold concentration on the external magnetic field is revealed.     

具有Dzyaloshinskii-Moriya相互作用的一维随机量子XY模型中的纠缠特性

研究了一维随机量子XY自旋链中中心两量子位的纠缠特性,在该系统中引入了自旋间的交换耦合杂质、磁杂质和Dzyaloshinskii-Moriya相互作用,并且杂质满足高斯分布关系.通过数值计算,求出了自旋的关联函数和平均磁化强度,给出了Concurrence的解析表达式.结果表明：高斯分布和Dzyaloshinskii-Moriya相互作用对两量子位的纠缠有重要的影响,选择合适的交换耦合、外界磁场和Dzyaloshinskii-Moriya相互作用参数,可以控制和提高中心两量子位的纠缠. 关键词： 纠缠 XY模型')" href="#">随机量子XY模型 高斯分布 Dzyaloshinskii-Moriya相互作用     

Spectrum of relaxation times for ising spin clusters in random fields

Summary Exact results on the single-spin-flip Glauber dynamics of six-coupled random field Ising spins with the coordination number of four are presented. Two distributions of random fields (RF), binary (BD) and Gaussian (GD) ones, are investigated. The effects of the static magnetic field are discussed. In the zero-magnetic-field case, the number of diverging relaxation times is equal to the number of energy minima minus one. This rule breaks in the presence of a magnetic field. The longest relaxation times in the absence of the field verify the Arrhenius law with the energy barrier determined by the energy needed to invert the ground-state spin configuration. At low temperature, according to the Arrhenius law, the spectrum of relaxation times shows a two-peaked distribution on a logarithmic scale. In the GD case of RF, the energy barrier distribution is continuous, while it is quasi-discrete in the BD case.     

Local magnetic states and hyperfine interactions in Fe/Ti magnetic superlattices

Multilayer Fe/Ti films are synthesized by deposition in a Penning discharge. Measurements are made of thhe static hysteresis loops and Mössbauer spectra on Fe⁵⁷ nuclei. The hyperfine magnetic field distribution functions are calculated. It is established that the spontaneous magnetization of Fe/Ti magnetic superlattices undergoes very strong oscillations as a function of the Ti layer thickness. Three groups of peaks are noted in the hyperfine field distribution functions, corresponding to three nonequivalent states of the Fe ions, in one of which these ions do not have a characteristic magnetic moment. These results also agree with measurements of the temperature dependence of the magnetization in weak magnetic fields. For some Ti interlayer thicknesses the saturation magnetization scaled to the Fe content is much higher than the saturation magnetization of bulk Fe.     

Effect of atomic environment on the Fe hyperfine structure and the thermal stability of magnetic order in L10 ordered Fe-Pt alloys

The ⁵⁷Fe Mössbauer effect measurements were made for the L1₀ ordered Fe-Pt alloys with 39-62 at% Pt and the effect of local atomic environment on the hyperfine structure was investigated. Furthermore, the thermal stability of magnetic order was investigated for the alloys with high Pt concentration. From the analyses of the observed Mössbauer spectra, we found that dipole-field-like anisotropic transferred hyperfine fields are mainly responsible for the large difference in hyperfine field between Fe-site and Pt-site in the Fe-rich alloys. In the Pt-rich region far from stoichiometry, the existence of many Fe-sites occupied by excess Pt atoms causes a distribution of exchange fields. Therefore, the iron atoms in different local environments may have their several hyperfine fields with different temperature dependence. The anomalous temperature dependence of the averaged hyperfine field and line broadening observed for the 61, 62 at% Pt alloys can be understood from the co-existence of various sub-spectra with different temperature dependence. As a result, the thermal stability of magnetic order is largely reduced as the Pt concentration exceeds 60 at%.     

Influence of the inhomogeneity of local magnetic parameters on the curves of magnetization in an ensemble of Fe<Subscript>3</Subscript>C ferromagnetic nanoparticles encapsulated in carbon nanotubes

Methods have been proposed and tested for analyzing local magnetic parameters in a system of single-domain ferromagnetic nanoparticles using their magnetization curves. The magnetic inhomogeneity in ensembles of Fe₃C nanoparticles encapsulated in carbon nanotubes has been investigated. It has been established that the Fe₃C nanoparticles encapsulated in carbon nanotubes are characterized by two-modal distribution functions of the local magnetic anisotropy energy. The particle distribution over the blocking temperature is reconstructed from the experimental temperature dependence of the coercive force. The allowance made for the inhomogeneity of the local magnetic parameters of the Fe₃C nanoparticles, which were studied by the proposed methods, explains the discrepancy between the magnetic anisotropy energy determined by the method of the magnetization approaching saturation and the magnetic anisotropy energy estimated from the coercive force of single-domain nanoparticles.     

The evolution of the nanoscale dissipative structures in a distribution of defects within the isothermally irradiated f.c.c. crystal

A kinetic model for the influence of external noises such as fluctuations of the vacancies’ generation rate and inhomogeneity of irradiated f.c.c. crystal on the formation of nanoscale modulated dissipative structure in a spatial distribution of vacancies is considered. The generation rate of vacancies all over the sites and a density of their dislocation-type sinks are modelled as independent random uniform stationary fields and with certain defined parameters of fluctuation correlations – spatial and temporal ones. Such stochastic fields can induce a spatial redistribution of vacancies that can lead to their density stationary uniform field or stochastic one. By the average value and correlation functions of these fluctuations, the conditions are determined for interacting fluctuations of the vacancies’ density, under which this homogeneous random field becomes unstable in relation to the stochastic field with a spatially periodic mean distribution of vacancies’ density. For instance, with f.c.c. nickel as a model of the irradiated functional material, the temperature dependence of spatial period d(T) of the modulated dissipative structure of vacancies’ subsystem in f.c.c. crystal is numerically forecasted and analysed, taking into account the total (‘electrochemical’?+?‘strain-induced’) interaction between vacancies. Such d(T)-dependence is also determined by the kinetic characteristics of vacancies’ redistribution.     

Thermodynamic study of the solubility process of boron in silicon carbide,grown from the vapour phase

The thermodynamics of the solubility process of the acceptor impurity boron in silicon carbide is investigated. The thermodynamic analysis of equilibrium in the Si-C-B system has been carried out and the temperature dependences of the partial pressures of interacting components are determined for the temperature interval (1800–3000) K.The calculation of the Fermi level in p-SiC(B) is carried out. The heat of solution and the distribution coefficient of boron in silicon carbide are determined on the basis of the thermodynamic analysis of the Si-C-B system, the experimental temperature dependence of boron solubility and the calculation of the Fermi level and Fermi-Dirac distribution in p-SiC(B).     

Inhomogeneous electronic structure probed by spin-echo experiments in the electron doped high-Tc superconductor Pr1.85Ce0.15CuO4-y

63Cu nuclear magnetic resonance spin-echo decay rate (T-12) measurements are reported for the normal and superconducting states of a single crystal of Pr(1.85)Ce(0.15)CuO(4-y) in a magnetic field B(0)=9 T over the temperature range 2相似文献   

Concentration dependence of the wings of a dipole-broadened magnetic resonance line in magnetically diluted lattices

The singularities of the time autocorrelation functions (ACFs) of magnetically diluted spin systems with dipole–dipole interaction (DDI), which determine the high-frequency asymptotics of autocorrelation functions and the wings of a magnetic resonance line, are studied. Using the self-consistent fluctuating local field approximation, nonlinear equations are derived for autocorrelation functions averaged over the independent random arrangement of spins (magnetic atoms) in a diamagnetic lattice with different spin concentrations. The equations take into account the specificity of the dipole–dipole interaction. First, due to its axial symmetry in a strong static magnetic field, the autocorrelation functions of longitudinal and transverse spin components are described by different equations. Second, the long-range type of the dipole–dipole interaction is taken into account by separating contributions into the local field from distant and near spins. The recurrent equations are obtained for the expansion coefficients of autocorrelation functions in power series in time. From them, the numerical value of the coordinate of the nearest singularity of the autocorrelation function is found on the imaginary time axis, which is equal to the radius of convergence of these expansions. It is shown that in the strong dilution case, the logarithmic concentration dependence of the coordinate of the singularity is observed, which is caused by the presence of a cluster of near spins whose fraction is small but contribution to the modulation frequency is large. As an example a silicon crystal with different ²⁹Si concentrations in magnetic fields directed along three crystallographic axes is considered.