Angular dependence of the coercive force of textured rare earth permanent magnets

On the basis of model calculations experimental results on the angular dependence of the coercivity H_c and the remanence coercivity H_R of hard magnetic materials of the type SmCo₅, Sm₂(Co, Fe, Cu, Zr)₁₅ and Nd₂Fe₁₄B are discussed. In the model coherent rotation as well as incoherent magnetization jumps (e.g. 180°-Bloch walls) are included. The texture is described by an axial symmetric distribution of the easy axes with only oneparameter. For Sm₂(Co, Fe, Cu, Zr)₁₅ the model explains irreversible (H_R (θ)-curves) as well as reversible (H_R(θ)−H_c(θ)) magnetization processesin good agreement with the experiments, whereas stronger deviations exist for SmCo₅ and Nd₂Fe₁₄B, especially in the H_c(θ)-curves. These deviations should be caused by other reversible magnetization processes     

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.     

A finite block spin model for permanent magnets

We propose a finite block spin phenomenology for permanent magnets in which we consider an average domain as a block spin. The permanent ferromagnetism arises in two ways: (1) the ferromagnetism that occurs inside a big block spin, i.e. the intrablock ferromagnetism, and the ferromagnetism between small block spins (SBSs) in a big block spin (BBS) which can be induced by collective ferromagnetic pairing between two SBSs mediated by temperature-irreversible bosonic strains; and (2) the ferromagnetism between BBSs, i.e. the interblock ferromagnetism. The coercivity originates from temperature-irreversible strains treated as external phonons.     

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.     

On the theory of the magnetization of dimerized magnets

To describe a quantum phase transition in a dimerized antiferromagnet, the formalism of the Landau theory of phase transitions has been applied. The energy of the ground state is minimized by varying the parameter of the mixing of the wavefunctions with different multiplicities. The critical behavior of the parameter of the wavefunction is responsible for the critical behavior of the observables.     

Theory of thermal remagnetization of permanent magnets

A self-consistent mean-field theory explaining the thermal remagnetization (TR) of polycrystalline permanent magnets is given. The influence of the environment of a grain is treated by an inclusion approximation, relating the field inside the grain to the local field outside by means of an internal demagnetization factor n. For the switching fields and the fluctuations of the local fields around the mean field, Gaussian distributions of widths σ_s, and σ_f, respectively, are assumed. The isothermal hysteresis curve, the recoil curves, and the TR dependent on the model parameters n, σ_s, and σ_f are calculated. Furthermore, the influence of the initial temperature and the strong dependence of the TR on the demagnetization factor of the sample are studied, and it is shown that for reasonable parameter sets TR effects up to 100% are possible. The theoretical results correspond well with the experimental situation.     

Investigation of parameters of undulators with permanent magnets

Investigations of a hybrid type undulator for the terahertz FEL with ferrite magnets were carried out. The study began with an undulator simulation in FEMLAB environment and measurements on the created model. The work presents the results of study of the dependence of magnetic field on the height of the magnetic elements and study of the magnetic induction depending on the ratio of the width of magnetic elements to the distance between them. It is shown that the hybrid undulator scheme allows selecting the optimum parameters of the undulator. The obtained results can be used for choosing the optimum parameters of undulators of the terahertz-range FEL.     

Thermal hysteresis of permanent magnets of alnico-type alloys

A study is made of the thermal magnetic hysteresis of permanent magnets of the alloy YuNDK25BA in the residually magnetized state with different operating points after two cycles (+20° to 400° to 20°C) and (+20° to –195° to 20°C). The results are interpreted with the model developed earlier by the present authors for thermalhysteretic magnetization reversal in dispersion-hardened alloys. The results can be useful in evaluating the thermal stability of magnets and systems.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizkia, No. 7, pp. 35–38, July, 1979.     

Magnetic forces between arrays of cylindrical permanent magnets

Permanent magnet arrays are often employed in a broad range of applications: actuators, sensors, drug targeting and delivery systems, fabrication of self-assembled particles, just to name a few. An estimate of the magnetic forces in play between arrays is required to control devices and fabrication procedures. Here, we introduce analytical expressions for calculating the attraction force between two arrays of cylindrical permanent magnets and compare the predictions with experimental data obtained from force measurements with NdFeB magnets. We show that the difference between predicted and measured force values is less than 10%.     

Microstructures and magnetic properties of Fe---Pt permanent magnets

We have investigated the magnetic properties of Fe---38.5Pt, Fe---39.5Pt and Fe---50.0Pt (at%) alloys after various heat treatment conditions using a vibrating sample magnetometer, and correlated these properties with the microstructures of the alloys by transmission electron microscopy. The Fe---50Pt alloy shows poor magnetic hardness regardless of the heat treatment conditions. The magnetic hardness of the Fe---39.5Pt alloy shows a maximum value after annealing for 10 h at 873 K, while it monotonically decreases after annealing at 1073 K. The alloy with the highest coercivity was composed of a single phase γ₁ with an average domain size of approximately 10 nm. The electron diffraction results indicate that the alloy is frustrated with accumulated stress, induced by a cubic → tetragonal transformation which occurs without twinning. On the other hand, when stress is relieved by twin formation after prolonged aging, the coercivity decreases. By annealing at 1073 K, the well known polytwin structure evolves. However, only poor hard magnetic properties are observed when this polytwin structure appears. Hence, the highest coercivity is attributed to the formation of nanoscale L1₀ ordered antiphase domains which is expected to be a highly anisotropic single domain magnetic particle.     

Structure and phases of low-neodymium NdFeB permanent magnets

Structures and phase compositions of two low-neodymium magnetically hard materials, differing by the way of preparation — centrifugal atomization and melt spinning — were compared using Mössbauer spectroscopy, X-ray diffraction and measurements of thermomagnetic curves. Better hard magnetic characteristics of the melt-spun material are explained on the basis of the differences in the content of surface and/or interface Fe(Nd,B) phases. Their remarkable presence in the centrifugally atomized material lowers the content of Fe₃B, Fe₂B, α-Fe, and Nd₂Fe₁₄B phases that are responsible for the magnetic quality of the material. There are only subtle differences in the phase compositions of both materials after thermomagnetic measurement, where the α-Fe phase prevails as a product of the thermal decomposition.     

Mössbauer study of permanent magnets Fe-Pt

The Mössbauer effect measurements of⁵⁷Fe have been performed for Fe-Pt alloys with 36–50 at. % Pt. The spectra are composed of three kinds of sextets for the alloys with 36–40 at.% Pt which have excellent permanent magnet properties in the imperfectly ordered state of a CuAuI-type structure. The magnetic hyperfine field of⁵⁷Fe is about 25% larger at Pt-sites than at the normal Fe-sites. One of the sextets may be caused by local atomic disorder. The local atomic disorder would be responsible for high coercivity of these alloys.     

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。     

Resin-bonded permanent magnetic films with out-of-plane magnetization for MEMS applications

Resin-bonded permanent magnets with out-of-plain direction of magnetization and improved magnetic properties for magnetic MEMS actuator have been created. The material investigated consists of magnetically anisotropic strontium ferrite particles embedded into epoxy resin matrix upto a volume loading of 80%. Intrinsic coercivity H_ci of 6000 Oe (480 kA/m), residual magnetic flux density B_r up to 4000 G (0.4 T) and maximum energy product (BH)_max of 3.0 MG Oe (23.6 kJ/m³) have been attained due to magnetic-field-induced alignment of the ferrite particles during curing process.     

Optimal design method for magnetization directions of a permanent magnet array

In many magnetic systems, the permanent magnet (PM) pattern has a great influence on their performance. This study proposes a systematic optimization method for designing discrete magnetization directions. While previous works have been mostly dependent on researchers’ intuition, the developed method is systematic and can be applied to a two-dimensional PM-type eddy current brake model. The effectiveness of the method is confirmed, where the design’s aim is to maximize the braking force on a moving conductor. The sensitivity analysis is accomplished by the adjoint variable method and the sequential linear programming is used as an optimizer. Several optimization results for various conditions through the proposed design method are compared to each other and the optimal magnet configuration for an eddy current brake is suggested.     

Magnetic elements for switching magnetization magnetic force microscopy tips

Using combination of micromagnetic calculations and magnetic force microscopy (MFM) imaging we find optimal parameters for novel magnetic tips suitable for switching magnetization MFM. Switching magnetization MFM is based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within the technique the sum of the scanned data with reversed tip magnetization depicts local atomic forces, while their difference maps the local magnetic forces. Here we propose the design and calculate the magnetic properties of tips suitable for this scanning probe technique. We find that for best performance the spin-polarized tips must exhibit low magnetic moment, low switching fields, and single-domain state at remanence. The switching field of such tips is calculated and optimum shape of the Permalloy elements for the tips is found. We show excellent correspondence between calculated and experimental results for Py elements.     

The role of dipolar interaction in nanocomposite permanent magnets

The effects of dipolar interactions on the magnetization behaviors and magnetic properties of the nanocomposite magnets have been studied by micromagnetic simulations. Numerical results show that the dipolar interaction plays an important role during the demagnetization process, especially in the magnets with large soft-phase content _vs$v_{\mathrm{s}}$. For the isotropic nanocomposites, the remanence enhancement can be controlled through adjustments of the grain size D   and _vs$v_{\mathrm{s}}$. However, the appearance of magnetic vortex state leads to a very low remanence in the magnets with large D   and _vs$v_{\mathrm{s}}$. The dependence of coercivity on D   and _vs$v_{\mathrm{s}}$ can be attributed to the exchange-induced magnetization reversal near the grain boundaries and the low nucleation field of soft phase, respectively. For the anisotropic nanocomposites, the reduced remanence _mr$m_{\mathrm{r}}$ is equal to 1.0$1.0$ for the magnets with small D   or with low _vs$v_{\mathrm{s}}$. However, _mr$m_{\mathrm{r}}$ decreases with increasing _vs$v_{\mathrm{s}}$ for the magnet with large D   due to the influence of dipolar interactions. The difference between the calculated coercivity _Hc$H_{\mathrm{c}}$ with and without considering dipolar interaction shows that the dipolar interaction plays a more important role during the magnetization reversal in the soft phase than that in the hard phase. The maximum calculated energy product of the isotropic nanocomposites is only about 40 MGOe due to the conflicting relation between remanence and coercivity, while that of the anisotropic nanocomposites is 112 MGOe. This reminds us that the alignment of hard grain is important to obtain high performance.     

添加Dy在快淬NdFeB永磁体中的作用

在之前研究Nb元素的添加对快淬(Nd,Dy)_11.5Fe_82.4-mNb_mB_6.1永磁体磁性能、温度特性及显微组织影响的基础上,进一步研究了Dy元素在Nd_11.5-nDy_nFe_81.4Nb₁B_6.1 (n＝0,0.5,1,1.5,2)永磁体中 关键词： NdFeB 磁性能 温度特性 显微组织 X-ray absorption fine structure     

Microstructure of precipitation hardened cobalt rare earth permanent magnets

A transmission electron microscope study of several precipitation hardened cobalt rare earth magnets has been undertaken. The magnets were in peak aged condition and varied in the chemical composition. The studies reveal a fine cellular microstructure. The shape and size of cells depend on the heat treatment and alloying elements. The cell interiors consist of at least two plate-shaped 17:2 phases and are surrounded by a 5:1 boundary phase. The replacement of zirconium with hafnium does not alter the cellular morphology and also leads to a high coercivity. The coercive force is strongly influenced by the shape and size of the cellular structure as well as by the lattice misfit between the 5:1 and 17:2 phases. The influence of the microstructure on the coercivity is discussed.