On the mean field theory bound on the magnetization

The mean field bound on magnetization is proved for a class of one-component ferromagnetic systems and forD components systems with arbitraryD.     

Micromagnetic simulations of reversal magnetization in cerium-containing magnets

Single-grain models with different cerium contents or structural parameters have been introduced to investigate the reversal magnetization behaviors in cerium-containing magnets. All the micromagnetic simulations are carried out via the object oriented micromagnetic framework(OOMMF). As for single(Nd,Ce)_2 Fe_(14)B type grain, the coercivity decreases monotonously with the increase of the cerium content. Four types of grain structure have been compared: single(Nd,Ce)_2 Fe_(14)B type, core((Nd,Ce)_2 Fe_(14)B)-shell(Nd_2 Fe_(14)B) type with 2 nm thick shell, core(Ce_2 Fe_(14)B)-shell(Nd_2 Fe_(14)B) type, and core(Nd_2 Fe_(14)B)-shell(Ce_2 Fe_(14)B) type. It is found that core((Nd,Ce)_2 Fe_(14)B)-shell(Nd_2 Fe_(14)B)type grain with 2 nm thick shell always presents the largest coercivity under the same total cerium content. Furthermore,the relationship between the coercivity and the shell thickness t in core((Nd,Ce)_2 Fe_(14)B)-shell(Nd_2 Fe_(14)B) type grain has been studied. When the total cerium content is kept at 20.51 at.%, the analyzed results show that as t varies from 1 nm to 7 nm, the coercivity gradually ascends at the beginning, then quickly descends after reaching the maximum value when t = 5 nm. From the perspective of the positions of nucleation points, the reasons why t affects the coercivity are discussed in detail.     

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.     

Optimizing magnetization orientation of permanent magnets for maximal gradient force

The force exercised on a permanent magnet (PM) in a nonuniform field (gradient force) is dependent on the magnetization orientation of the magnet. In this paper, it is shown theoretically that the gradient force is greatest when the magnetization through the magnet, or at least at its surface, is collinear with the external field. The formulae for calculating the force between an axis-symmetric optimal magnet and a coaxial axis-symmetric coil are presented. Using the finite element method (FEM), calculations of the magnetic field distribution of an optimal cylindrical magnet and some its approximations are performed. The forces between these magnets and a pancake coil are computed and compared. For a system consisting of a magnet with a height of 1 unit and a diameter of 2 units and magnetization invariant in field and an annular pancake coil with a diameter of 2.4 units, a thickness of 0.2 units, an inner diameter of 0.4 units and a distance from the magnet of 0.2 units, the force on the optimal magnet was 1.44 times greater than the force on an axially magnetized magnet of the same size and magnetization magnitude. The optimal magnetization may be approximated by magnetization inclined at a constant angle to the axis and by a combination of axially and radially magnetized sections. With magnetization at a constant angle to the axis in the axis plane, the force was greatest when the angle was about 45°, being 1.38-fold compared to the force on an axially magnetized magnet. When the magnet was composed of an axially magnetized cylindrical core and a radially magnetized outer ring, the force was greatest when the volume of the core was approximately equal to the volume of the ring, being 1.26-fold compared to the force on an axially magnetized magnet. The optimal magnet and its approximations also provided a reduced stray field. A short review of methods of the fabrication of permanent magnets (PMs) with a continuous variation of the magnetization orientation and with radial magnetization orientation is given.The results of this study can be used to design linear electromagnetic (micro)actuators.     

On the theory of the spontaneous magnetization of thin films. Part I

Conclusion From (3) it is seen that the quantum-mechanical theory of thin films leads to quite analogous results as for massive specimens. It is obvious that we should arrive at the same results as in (3) if from the very beginning we solved the whole problem by means of the theory of molecular fields putting the molecular fields of the individual atomic planes proportional to their magnetization. We shall therefore make use of this fact in the future by working with the method of molecular fields which permits of a somewhat more flexible procedure than the quantum-mechanical solution. At the same time, however, there appear in the equations constants of the molecular fields which are not more closely defined and which, as we know, are proportional to the exchange interaction energy between neighbours but with the present state of the theory this is not a disadvantage in comparison with the quantum-mechanical procedure, since the exchange integrals have not yet been calculated.A comparison of (3) with experiment will be left to the second part of this paper, where we shall compare the results of measurement with the equivalent equations in the molecular-field form.This paper was read at the international conference on magnetic phenomena in Moscow, 1956.     

Spin nematics and magnetization plateau transition in anisotropic kagome magnets

We study S=1 kagome antiferromagnets with an isotropic Heisenberg exchange J and strong easy-axis single-ion anisotropy D. For D>J, the low-energy physics can be described by an effective S=1/2 XXZ model with antiferromagnetic Jz approximately J and ferromagnetic J perpendicular approximately J2/D. Exploiting this connection, we argue that nontrivial ordering into a "spin-nematic" occurs whenever D dominates over J, and discuss its experimental signatures. We also study a magnetic field induced transition to a magnetization plateau state at magnetization 1/3 which breaks lattice translation symmetry due to ordering of the Sz and occupies a lobe in the B/Jz-Jz/J perpendicular phase diagram.     

Stochastic resonance in low-disperse magnets: Mechanism of subbarrier magnetization reversal

The phenomenon of stochastic resonance is studied theoretically in a system of single-domain particles with an “easy-axis”-type magnetic anisotropy at temperatures close to absolute zero, for which the tunneling of the magnetic moment vector between stable states dominates. Calculations are made on the basis of the master equation in the model of discrete orientations in the quasi-adiabatic approximation. The dynamic magnetic susceptibility components of the system under the action of a weak rf field are calculated. The critical temperature at which a transition from the above-the-barrier mechanism of magnetization reversal to the subbarrier mechanism occurs is estimated.     

Spin wave theory of strongly anisotropic magnets

A strong anisotropy gives rise to a non-spherical precession of the spins with different amplitudes in the x and y directions. The highly anharmonic exchange interaction thereby becomes effectively anisotropic. The possibility of detecting a genuine two-ion anisotropy is discussed, and comments are given on the interpretation of the spin wave data in Tb and Er.     

高温超导俘获场磁体脉冲充磁仿真模型研究

在对俘获场磁体的脉冲充磁过程进行三维仿真时,现有的数值模拟方法有待具体分析比较,同时由于计算时间长和收敛困难,难以实现对大规模带材堆叠的模拟。针对以上问题,系统分析了H模型、H均一化模型和T-A模型的三维脉冲充磁仿真,对比了它们在77 K时不同堆叠层数下的俘获场、磁化损耗和计算时间,并基于以上模型提出了分组合并的简化模型。结果表明H均一化模型和T-A模型的计算误差随层数的增加而减小,验证了分组合并模型的有效性,并成功对上百层的堆叠进行了仿真。因此,分组合并模型解决了三维大规模堆叠有限元仿真困难的问题。     

Quantum beats of magnetization generated by a standing acoustic wave in molecular magnets

We have shown that macroscopic quantum beats of the magnetization can be produced by the standing acoustic wave in molecular magnets. Under the impulsive drive we have demonstrated complete control over the beat structure by adjusting the time interval between the initial state and the application of the sound wave. The optimal condition for the pronounced beat is discussed.     

The magnetization behavior of nanocrystalline permanent magnets based on the Stoner–Wohlfarth Model

The magnetization behavior in nanocrystalline permanent magnets has been investigated using mean field Stoner–Wohlfarth model. The model is comparatively simple but allows the numerical treatment of extended nanostructures. The predominant intergrain exchange coupling is expressed by a mean field constant N_m in spite of no explicit relationship between N_m and the microstructure. By this model, the experimental phenomena, such as the remanence enhancement, the shape of demagnetization curve, the irreversible magnetization, the exchange bias field and their respective temperature dependence, can be well explained. The N_m dependence of magnetic properties has been extensively discussed in the present paper.     

Anisotropy and magnetization process in soft magnets: Principles,experiments, applications

Understanding and controlling the anisotropy energy and its effects has proved vital to the development of soft magnetic materials and their applications. Indeed, acting on composition and structure and working out specific annealing treatments, a large variety of anisotropy-governed behaviors under DC and AC excitation can be obtained. These are discussed in the present paper, together with special problems arising in the characterization of anisotropic soft magnets and a few significant applications. It is stressed how features like J$J$–H$H$ loop shape, energy losses, and magnetoresistance effects can be controlled, in crystalline and amorphous materials, by the methods of induced anisotropy. The high-frequency behavior of these materials can be strongly affected by the anisotropy field via resonant absorption of energy. This calls for tradeoff between the values of permeability and resonance frequency.     

On the Onsager-Yang-Value of the spontaneous magnetization

We show that the value of the spontaneous magnetization for the two-dimensional Ising model computed by Onsager is indeed, the appropriate derivative of the free energy with respect to the magnetic field. The argument is based on a simple application of the duality transformation.Lavoro eseguito nell'ambito del G.N.F.M.     

On the magnetization curves of bubble lattices

The paper presents magnetization curves of bubble lattices in garnet samples of composition (YSmLuCa)₃(FeGe)₅O₁₂, in which we observed a discontinuity in the magnetization at a certain negative value of the external magnetic field applied normal to the sample surface. This points to an anomaly in the behaviour of the existing honeycomb domain structure. The proposed theoretical interpretation of this behaviour is based on the interaction of closely spaced pairs of Bloch walls.     

On the self-reversal of the magnetization of nickel

The effect of longitudinal and circular permanent and alternating (f=50 Hz) magnetic fields on the torsional and ordinary magnetization hysteresis loop of poiycrystalline nickel wire specimens at room temperature is elucidated, described, and interpreted.Translated from Izvestiya VUZ. Fizika, No. 12, pp. 119–123, December, 1973.