Coercivity of precipitation hardened cobalt rare earth 17:2 permanent magnets

The influence of the microstructure on the coercivity has been investigated by means of transmission electron microscopy. It is shown that a thin coherent (CoCu)₅Sm-cell boumdary phase, separating cells of 17:2-crystal structure, acts as a pinning centre for magnetic domain walls. The attractive interaction force is interpreted in terms of a micromagnetic theory for domain wall pinning. The coercive force is determined by the domain wall energy gradient and by the magnetoelastic coupling energy between domain wall stresses and lattice deformation strains. The calculated coercive force due to the lattice mismatch, originated by the cellular coherent precipitation structure, is comparable to the experimentally obtained values.     

Biological permanent magnets

Hyperfine Interactions - Precoated galvanized steel sheets were submitted to Prohesion test (PT) and to outdoor marine exposure test (OT). The corrosion products were different in both cases....     

Neodymium-iron-boron permanent magnets

Permanent magnet research and technology have been propelled into a new era by the rare earth-iron-boron materials, R₂Fe₁₄B. Energy products surpassing all previous values have been attained in magnets based on Nd₂Fe₁₄B, the prototypical compound. In this review we place Nd-Fe-B in the historical context of permanent magnet evolution, summarize the intrinsic properties of the R₂Fe₁₄B phases, and discuss the properties of practical Nd-Fe-B magnets produced by the two methods in present commercial use.     

Mn-based permanent magnets

Mn-based intermetallic compounds have attracted much attention due to their fascinating structural and physical properties, especially their interesting hard magnetic properties. In this paper, we have summarized the magnetic and structural properties of Mn-based intermetallic compounds(Mn X, where X = Al, Bi, and Ga). Various methods for synthesizing single phases of MnAl, MnBi, and Mnx Ga were developed in our lab. A very high saturation magnetization of 125 emu/g,coercivity of 5 kOe, and maximum energy product(BH)max of 3.1 MG·Oe were achieved at room temperature for the pure τ-Mn–Al magnetic phase without carbon doping and the extrusion process. Low temperature phase(LTP) MnBi with a purity above 95 wt.% can be synthesized. An abnormal temperature coefficient of the coercivity was observed for the LTP MnBi magnet. Its coercivity increased with temperature from 100 K to 540 K, reached a maximum of 2.5 T at about540 K, and then decreased slowly to 1.8 T at 610 K. The positive temperature coefficient of the coercivity is related to the evolution of the structure and magnetocrystalline anisotropy field of the LTP MnBi phase with temperature. The LTP MnBi bonded magnets show maximum energy products(BH)max of 8.9 MG·Oe(70 kJ/m~3) and 5.0 MG·Oe(40 k J/m~3) at room temperature and 400 K, respectively. Ferrimagnetic Mnx Ga phases with L10 structures(x 2.0) and D022 structures(x 2.0) were obtained. All of the above structures can be described by a D0_(22) supercell model in which 2 a-Ga and 2 b-Mn are simultaneously substituted. The tetragonal D0_(22) phases of the Mnx Ga show high coercivities ranging from 7.2 kOe for low Mn content x = 1.8 to 18.2 kOe for high Mn content x = 3 at room temperature. The Mn_(1.2) Ga sample exhibits a room temperature magnetization value of 80 emu/g. The hard magnetic properties of coercivity_iH_c = 3.5 kOe, remanence Mr = 43.6 emu/g, and(BH)max = 2.5 MG·Oe were obtained at room temperature. Based on the above studies, we believe that Mn-based magnetic materials could be promising candidates for rare earth free permanent magnets exhibiting a high Curie temperature, high magnetocrystalline anisotropy, and very high coercivity.     

Rare earth-cobalt permanent magnets

This paper reviews the historical background and the development of rare earth-cobalt-based permanent magnets from basic science studies on rare earth-transition metal alloys in the 1960's to today's broad spectrum of commercial magnet types and their applications. It puts the RE-Co magnets in perspective relative to older magnet types and also traces the path to the subsequent development of the related Nd-Fe-B magnets. The treatment is qualitative, with emphasis on the relationship between fundamental properties of the compounds and the interaction between microstructure and magnetic domain walls that makes high coercivity and the exceptional hard magnetic properties of the rare-earth magnets possible. The various kinds of RE-Co magnets in production and use today, some of their engineering properties, and economic aspects governing their applicability, cost and availability are also discussed. Many references provide a guide to the special literature regarding the physics, metallurgy, manufacture, product selection and properties of rare earth-cobalt magnets.     

Magnetization processes in Nd-Fe-B permanent magnets

The hysteresis loop of Nd₁₅Fe₇₇B₈ can be decomposed in hysteresis loops of two magnetic phases; one with a comparatively low coercivity and the other with a high coercivity. The volume ratio of these phases depends on the temperature and the angle between the external field and pseudo c-axis.     

Intra-cluster exchange-coupled high-performance permanent magnets

Inert gas condensation has been used to produce Fe-rich Fe–Pt clusters imbedded in C or SiO₂. Compositions of the clusters ranged from the single-phase Fe₃Pt phase field to the single-phase FePt phase field, and included compositions in the two-phase Fe₃Pt+FePt phase field. The as-formed clusters formed in the A1 fcc structure for all compositions, and after proper heat treatment transformed to the Fe₃Pt and/or FePt phases, depending on composition. Because the clusters were well isolated, the scale of the phases was limited by the cluster size. This intracluster structuring on such a fine scale ensured that the soft Fe₃Pt and hard FePt phases were fully magnetically exchange-coupled with each other, which allowed greater soft phase fractions comparing with previous work. Energy products of the two-phase clusters with 50% Fe₃Pt exceeded 25 MGOe, compared to 11.8 MGOe for the single-phase FePt clusters. Micromagnetic simulations revealed remarkable similarities with the experimental results with respect to the relationship between both coercivity and energy product as a function of cluster composition.     

Faraday isolator using permanent magnets

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Undulator with permanent ferrimagnetic magnets

Undulator for terahertz FEL is created of ferrite materials. The length of the undulator period is 9 cm and the number of periods is 27. By means of selection and redistribution of magnetic elements it was succeeded to reduce the spread in amplitudes of the magnetic field down to 7%. Additional windings in magnetic elements were used to compensate for the residual spread. The needed focusing gradient of the magnetic field is obtained by means of relative displacement, along the x-axis, of alternating poles with opposite signs of the magnetic field. The undulator parameters, including the properties of focusing in the horizontal plane, are studied.     

Magnetic properties of nanostructured spinel ferrites and nanocomposite Nd2Fe114B/α-Fe permanent magnets

This paper presents some of the important magnetic properties of the nanostructured spinel ferrites such as Ni_0.5Zn_0.5Fe₂O₄ and Mn_0.67Zn_0.33Fe₂O₄ and also that of the nanocomposite Nd₂Fe₁₄B/α-Fe permanent magnetic material. The increase in the magnetic transition temperature of Ni-Zn ferrite from 538 K in the bulk state to 592 K when the grain size is reduced to 16 nm is correlated to the enhancement in the AB superexchange interaction strength because of an increase in the magnetic ion concentration in the A-site on milling, as shown by the EXAFS and in-field Mössbauer studies. The particle size has been tailor-made by varying the concentration of the oxidant in the case of Mn-Zn ferrite. The critical particle size for the superparamagnetic limit has been found to be 25 nm with an effective magnetic anisotropy constant of 7.78 kJ m^?3 which is about an order of magnitude higher than that of the bulk ferrite. The exchange coupling is found to be strengthened in the nanocomposite magnet Nd₂Fe₁₄B/α-Fe, when the grain boundary anisotropy is removed by thermal annealing and thus facilitating the enhancement of the energy product     

Anisotropic permanent magnets with convergent texture

Various types of anisotropic permanent magnets with nonhomogeneous convergent texture of the axes of easy magnetization were prepared and investigated. These magnets make possible to substantially increase the value of external flux density in comparison with conventional homogeneously textured magnets. They can be manufactured from most of the modern hard magnetic materials possessing high coercivity and magnetocrystalline anisotropy.Results obtained on rare-earth based hard magnetic materials are presented. The external flux densities of convergently and homogeneously textured magnets are compared.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.Most of the results obtained in the development of permanent magnets with convergent texture are due to the introduction of research programs supported by the Academy of Sciences and coordinated by Dr. S. Krupika.The authors wish to thank Dr. S. Krupika for many fruitful discussions and suggestions.     

FEL undulator with permanent ferromagnetic magnets

A terahertz FEL ferromagnetic undulator has been created. The length of the undulator period is 9 cm with the number of the periods at 27. By means of the selection and redistribution of magnetic elements, the spread of the amplitudes of the magnetic field was reduced to 7%. Additional windings of the magnetic elements were used to compensate for residual spread. The required focusing gradient of the magnetic field was obtained as a result of the relative displacement of alternating poles with an opposite charge of the magnetic field along the x axis. Parameters of undulator, including the focusing properties in the horizontal plane, were investigated.     

Magnetic guns with cylindrical permanent magnets

The motion of a cylindrical permanent magnet (projectile) inside a tubular permanent magnet, with both magnets magnetized axially, illustrates nicely the physical principles behind the operation of magnetic guns. The force acting upon the projectile is expressed semi-analytically as derivative of the magnetostatic interaction energy. For comparison, the forces involved are also calculated numerically using finite elements methods. Based on the conservation of the magnetostatic and kinetic energies, the exit and asymptotic velocities are determined. The derived formulas can be used to optimize the generated forces and motion of the inner cylindrical magnet.     

Recovery mechanisms in nanostructured aluminium

Commercial purity aluminium (99.5%) has been cold rolled to a true strain of 5.5 (99.6% reduction in thickness). The material is very strong but low temperature recovery may be a limiting factor. This has been investigated by isothermal annealing treatments in the temperature range 5–100°C. Hardness tests, microstructural investigations by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were carried out to identify and characterise possible recovery mechanisms. Annihilation of zigzagged dislocations, positioned between deformation-induced lamellar boundaries of medium-to-high angles, and annihilation of dislocations in boundaries were found to be important recovery mechanisms, whereas other mechanisms, such as triple junction motion, subgrain coalescence, and boundary migration, were less important or negligible. The recovery kinetics was analysed based on hardness data, showing that the apparent activation energy for recovery at low temperatures was 60–86?kJ?mol^?1, consistent with thermally activated glide of jogged interior dislocations and the climb of dislocations in boundaries. These mechanisms are restricted by the presence of small intermetallic particles, which pin dislocations and boundaries and thereby raise the stability of the heavily deformed material.     

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.     

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