Study of magnetic properties of La2/3Sr1/3MnO3 nanotubes by Monte Carlo simulation

Magnetic properties of a single nanotube whose walls are constituted by nanograins of La_2/3Sr_1/3MnO₃ are studied by means of Monte Carlo (MC) simulation. The system is considered as composed by ferromagnetic grains which couple via dipolar interaction. The grain size distribution is obtained from experimental measurements and the inter-grain distance distribution is obtained from a study of distance distribution among magnetic particles distributed in the tube walls. We show the magnetization behavior for a tube of 700 nm of diameter and 3.5 μm length. We discuss the simulation for different temperatures and external fields. As a main result, we show that the distribution of inter-granular distances has a unique behavior for tubular structures of similar diameter/length aspect ratio, independent of their sizes. This scaling relation allows us to perform the simulations using a tube of smaller dimensions. We succeed in explaining the magnetization curves, finding that dipolar interaction is necessary to explain the experimental behavior and that the grains behave as having magnetic dead layers.     

A behavior model of a magnetorheological fluid in direct shear mode

A model that considers the mutual interaction between particles and the external field in a magnetorheological (MR) fluid is proposed. The model predicts the magnetization, interaction energy density, and shear stress against an imposed strain along the planar direction under a uniform magnetic field. Using the Lekner summation method, slowly convergent magnetization functions are transformed into rapidly convergent ones. By applying the proposed model to an application suitable for commercial clutches and dampers working in direct shear mode, it is revealed that the mean magnetization vector and the associated functional quantities of the MR fluid behave harmonically with respect to the imposed shear strain.     

Initial magnetization curve and hardening mechanism in rapidly quenched NdFeB magnets

Rapidly quenched NdFeB alloys were prepared with varying grain sizes well below and above the critical size for single domain particles. The contributions of the single and multi-domain particles to the initial magnetization curve are analyzed. By changing the quenching speed, i.e., the average grain size, the shape of the initial magnetization curve changes characteristically. The volume fraction of single domain grains is determined from the initial magnetization curves. From scanning electron micrographs the grain size distribution is evaluated and the critical size for single domain particles in the bulk material is deduced from a quantitative analysis of the magnetization curves. It reaches twice the value of the theoretical value for isolated spherical particles. A low temperature treatment following the thermal demagnetization lowers the initial susceptibility in underquenched ribbons as well as in an MQIII magnet. This effect reflects the irreversibility of the transition from the multi to the single domain particle state during the cooling. The temperature dependence of the single domain particle size is deduced from the initial magnetization curves of low temperature treated samples. It is shown that these experimental results are consistently explained assuming the nucleation mechanism to apply for rapidly quenched NdFeB magnets. The results are compared to the behaviour of hard ferrites.     

Some comments on magnetization reversal in uniaxial ferrite magnets

The interpretation of Hadjipanayis et al. of their experimental results on magnetization reversal in ferrite magnets is critically analyzed on the basis of the present knowledge of the mechanisms governing magnetization, magnetization reversal and coercivity, both in single and polydomain particles.     

Domain formation in films of magnetic nanoparticles with a random distribution of anisotropy axes

A theoretical model describing the magnetization distribution in a system of closely packed ferromagnetic grains with a random distribution of easy magnetization axes is constructed. It is demonstrated that, in this system, the domain structure with domains characterized by a random distribution of magnetization axes is formed even if the magnetostatic energy is negligible and can be ignored. The domain size increases linearly with an increase in the ratio of the exchange energy of the interaction between grains to the anisotropy energy of a single grain. The inclusion of the magnetostatic energy only insignificantly changes the domain size but leads to the formation of a vortex magnetization distribution inside the domains. The behavior of the system is numerically simulated by the Monte Carlo method. The results of the simulation confirm the conclusions drawn from the theoretical model.     

Spin-polarized ground state for interacting electrons in two dimensions

We study numerically the ground state magnetization for clusters of interacting electrons in two dimensions in the regime where the single particle wave functions are localized by disorder. It is found that the Coulomb interaction leads to a spontaneous ground state magnetization. For a constant electronic density, the total spin increases linearly with the number of particles, suggesting a ferromagnetic ground state in the thermodynamic limit. The magnetization is suppressed when the single particle states become delocalized.     

外磁场中单畴反铁磁颗粒的宏观量子效应

用瞬子方法研究了外加磁场对单畴双轴反铁磁颗粒宏观量子效应的影响. 当外磁场沿易磁化方向时,简并基态中的一个能量抬高,变为亚稳态,其隧穿衰变率随外磁场增大;当外磁场沿中间磁化方向时,能级的隧穿劈裂随外磁场的变化而振荡.从而提出一种观察单畴反铁磁颗粒中宏观量子相干和隧穿现象的实验方法.     

New micromagnetic states of magnetically soft nanoparticles with a nearly cubic shape

The ground state of nonellipsoidal particles can be inhomogeneous due to the effect of a demagnetizing field. The approach proposed here for studying such particles is based on the combination of symmetry analysis and perturbation theory. The general formulation of this approach, which makes it possible to analyze weakly inhomogeneous states for particles with a complex shape, is considered. The ground state of cubic particles of magnetically soft materials is calculated analytically, and the effect of small strains of cubic particles on the magnetization distribution in the particles is investigated. It is shown for the example of magnetically soft cubic particles that even a small deviation of the particle shape from symmetrical may result in the realization of a special magnetic state in such particles, in which the symmetry in the magnetization distribution is lower than the particle symmetry. A change in the parameters of a particle can substantially modify its magnetic properties and may even induce a phase transition to a state with a different symmetry.     

Novel combustion route of synthesis and characterization of nanocrystalline mixed ferrites of Ni-Zn

A novel combustion method of synthesis has been employed in this study for the preparation of nanoparticles of Ni-Zn ferrites. The preparation method is simple yet effective and its novelty lies in the direct mixing of reactants and the fuel. The structural and morphological studies on the nanoparticles of Ni-Zn ferrites have been carried out using X-ray diffractometer (XRD) and scanning electron microscope (SEM). The values of grain size of the ferrites obtained using the Scherrer's formula are in the range between 10 and 20 nm. The mean value of X-ray density of the Ni-Zn ferrites is around 5343 Kg/m³, which is more than the one experimentally observed for their bulk counterparts. The distribution of cations has been proposed theoretically for each concentration of Ni-Zn ferrite with reference to their respective experimental lattice constant values. Room-temperature magnetic measurements are carried out using vibrating sample magnetometer (VSM) with a view to understand the impact of the nano-regime on the magnetic parameters. The observed values of magnetization are in the range from 4 to 26 emu/g which is lower than that of bulk particles of Ni-Zn ferrite.     

Monte Carlo calculation of the magnetization behavior and the magnetocaloric effect of interacting particles

The magnetization behavior and the magnetic entropy change of a system made up of ferromagnetically interacting particles are calculated by using Monte Carlo simulation. The effect of the magnetic anisotropy of particles and the dipolar–dipolar interaction between particles on the magnetization and the magnetic entropy change of the system are discussed. It is found that there is no spontaneous magnetization, both the magnetic anisotropy of particles and the dipolar–dipolar interaction between particles restrains the system's magnetizing in the external magnetic field. The magnetic entropy change decreases with the increase in temperature in the system without the dipolar–dipolar interaction; however, the dipolar–dipolar interaction between particles makes the magnetic entropy change of the system have maximum value at low temperatures.     

Magnetization reversal processes in layered high-temperature superconductors with ferromagnetic impurities

The self-consistent interaction of a vortex system of a high-temperature superconductor and ferromagnetic impurities, including single impurities and their clusters, has been considered in the model of a layered high-temperature superconductor. For different temperatures and concentrations of ferromagnetic impurities, the magnetization reversal loops have been calculated by the Monte Carlo method taking into account an ensemble of ferromagnetic particles with different orientations of their easy magnetization axes with respect to the direction of an external magnetic field and for different magnetic anisotropy energies. It has been demonstrated that there is a nonlinear interaction of the high-temperature superconductor with ferromagnetic impurities, in which the initially thermodynamically reversible character of the magnetization reversal of the ferromagnetic ensemble can become irreversible. For a periodic lattice of clusters of ferromagnetic impurities, the magnetization curves of the high-temperature superconductor have been calculated for different sizes and configurations of the clusters. It has been revealed that, when extended defects are oriented parallel to the direction of the entrance of vortices in the sample, the length of the defects does not affect the remanent magnetization. It has been shown that the inclusion of the interaction between the magnetic moments inside the impurity cluster leads to a decrease in the magnetization reversal loop, the coercivity, and, accordingly, the energy loss due to magnetization reversal.     

Magnetostatic interactions in cylindrical nanostructures with non-uniform magnetization

Cylindrical magnetic nanostructures, like nanowires or nanotubes, should be used for the new generation of magnetic devices. Therefore, the investigation of inter-element interaction is an intense area of research. In this paper we investigated cylindrical nanostructures with non-uniform magnetization field. We focus on particles with a periodic magnetization function and using Fourier series we reduced the problem to a single integral expression. Analytical expressions for both, the self and the interaction magnetostatic energy, are given. These expressions are used to analyze multisegmented tubes, as a function of the number of segments and the distance between particles.     

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.     

Analysis of pair interparticle interaction in nonideal dissipative systems

A new method is proposed for determining the interaction forces between particles in nonideal dissipative systems with isotropic pair potentials. The method is based on the solution of the inverse problem describing the motion of interacting particles by a system of Langevin equations and allows one to recover the parameters of the external confining potential without referring to a priori information on the friction coefficients of the particles. This procedure was tested by a numerical simulation of the problem in a wide range of parameters typical of experimental conditions in a laboratory dusty plasma. The results of the first experimental approbation of the method as applied to the analysis of the interaction of dust particles in a laboratory high-frequency capacitive discharge plasma are presented.     

Realization of the Open-Boundary Totally Asymmetric Simple Exclusion Process on a Ring

We propose a misanthrope process, defined on a ring, which realizes the totally asymmetric simple exclusion process with open boundaries. In the misanthrope process, particles have no exclusion interaction in contrast to those in the simple exclusion process, while the hop rates depend on both numbers of particles at departure and arrival sites. Arranging the hop rates, we can recover the simple exclusion property and moreover have condensation if the number of particles exceeds that of sites. One condensate grows at an arbitrary single site and then behaves as an external reservoir providing and absorbing particles. It is known that, under some condition, the misanthrope process has an exact solution for the steady-state probability distribution. We exploit this to investigate the present model in an analytical manner.     

Frustration mechanism of formation of a helical magnetic structure in the CuB2O4 two-subsystem antiferromagnet

A new mechanism is proposed for forming an incommensurate magnetic structure of the transverse helix type owing to the removal of frustration through intersubsystem exchange in a two-subsystem antiferromagnet. The symmetry analysis of the distribution of the Dzyaloshinskii-Moriya interaction and comparison with the experimental data on the field dependence of magnetization indicate that antisymmetric exchange and competition between symmetric exchanges cannot be responsible for forming a helical magnetic structure in CuB₂O₄.     

Magnetic properties and magneto-birefringence of magnetic fluids

Static magnetic properties of a large variety of magnetic fluids with magnetite particles is studied. A qualitative study of magnetization curves was performed to establish the influence of interactions or the presence of agglomerations in each sample. Improved equations for magneto-granulometric analysis, for ideal ferrofluids, were proposed. Better results for the mean magnetic diameter than in the case of using the known equations were obtained. A quantitative study using several models for ideal and interacting particles was performed to select the best method and dimensional distribution function for magneto-granulometric analysis as well as for accurately determining macroscopic quantities of samples (initial susceptibility, saturation magnetization, particle number density or magnetic volume fraction) and properties of nanoparticles (mean magnetic diameter, thickness of the nonmagnetic layer and particle distribution). A new model for magneto-birefringence was proposed and discussed as well as applied for diluted and concentrated ferrofluids. The Langevin behaviour of samples was investigated and compared with the investigation based on magnetic properties. Nanoparticles parameters like mean “magneto-optical” diameter, effective anisotropy constant, Shliomis diameter and the real part of the electrical permittivity of particles were accurately determined. Received 18 July 1999 and Received in final form 13 January 2000     

强磁场作用下Cu熔体中富Fe颗粒的迁移与排列

凝固界面前沿颗粒间的相互作用决定了颗粒的运动轨迹、分布和材料的性能,控制熔体中颗粒的迁移可用于材料的净化和提纯.在Cu-30%Fe合金液固两相区施加不同的强磁场条件,富Fe颗粒的分布和排列不尽相同.当无强磁场作用时,富Fe颗粒较均匀地分布在Cu熔体中;随着施加稳恒强磁场磁感应强度的增加,富Fe颗粒向远离重力方向的试样上端迁移,样品底部几乎无富Fe颗粒;而施加向下的梯度磁场作用后,富Fe颗粒沿重力方向向下迁移.结合强磁场作用下颗粒的受力情况,分析了Fe颗粒的迁移行为.不同磁场条件和不同区域的颗粒直径统计分析表明,随磁感应强度增加,Fe颗粒聚合增加,但施加梯度强磁场后颗粒的团聚又逐渐减弱,对此从影响颗粒运动的Stokes和Marangoni凝并速度进行了讨论.从能量最低的角度解释了富Fe相沿平行磁场方向的取向排列.     

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.