Permanent magnets based on 4f-3d compounds

This paper briefly traces the 20-year development history of the magnets that are based on intermetallic compounds of rare-earth and iron-group transition metals. Some fundamental properties that make the 4f-3d alloys useful for permanent magnets are reviewed. The wide variety of practical magnet types, methods for their production and some economic aspects are discussed. Prospects for further development and improvement of these magnets are analyzed in terms of various application requirements.     

Permanent sextupole magnets

Compact but very strong sextupole permanent magnets have been assembled for use in a newly developed cold atomic beam polarized ion source. The sextupole magnetic field characteristics have been found to be satisfactory.     

Permanent magnets for microwave devices

The magnetron, invented in 1939, created the need for a microwave tube magnet which evolved from the early horn magnet to the U magnet, the E magnet, and finally the bowl magnet. A more recent innovation, the self-shielded cylindrical magnet, is now used on many crossed-field amplifiers. The klystron and the traveling wave tube created the need for permanent magnets with gaps longer than their diameters. The field straightener and the periodic permanent magnet facilitated the use of permanent magnets in focusing these linear beam tubes. The operation of magnetrons, crossed field amplifiers, klystrons, traveling wave tubes, and microwave ferrite devices, and the permanent magnets which are used in conjunction with these devices are described. Magnetic circuit design criteria, design methods, and the magnetization and stabilization of permanent magnets are discussed. The requirements of microwave devices have provided much of the incentive for the great advances in permanent magnet materials made in the last decade. The recent development of rare-earth-cobalt powders which exhibit the highest magnetocrystalline anisotropy ever recorded are of great interest to the microwave device designer.     

The T-criterion for engineering materials based on strength anisotropy

Summary In most engineering materials the yield limits in tension and compression are in general different. Their ratio,R= _0C / _0T , characterizing the strength anisotropy of the material, was found to influence significantly the modes of both plane stress and plane strain fractures. The theoretical analysis presented in this paper introduced the necessary modifications of theT-criterion of fracture in order to cover the effect of the strength anisotropy, called the strength-differential effect (SDE). According to the statement of the proposed fracture criterion, the maximum value of the ratio of strain energy density components,T _R =T _V /T _D , when calculated along the elastic-plastic boundary around the tip of a crack indicates the angle of initial crack path. Crack onset is characterized by a critical value of the SED. For the determination of the elastic-plastic boundary the most general form of a failure criterion, that is the paraboloid failure condition was used.With 12 Figures     

Permanent magnet materials

Permanent magnets are finding ever-increasing uses as magnet technology develops and materials improve. Permanent magnet materials are evaluated in terms of the geometry and configuration of the application for which they are intended. The origin of the properties of most modern permanent magnet materials is interpreted in terms of fine-particle theory. The fine-particle structures are produced either synthetically or by a solid-state precipitation reaction in an alloy. The preparation and properties of commercial magnets are described. The Alnico alloys are based on magnetically heat-treated iron-nickel-aluminum-cobalt alloys and account for the great majority of present-day production. Ferrites are finding increasing use. Elongated single-domain fine-particle magnets and other materials are used in smaller quantities. A number of interesting, not yet commercial, materials deriving their properties primarily from crystal anisotropy are described.     

Coating cracks in materials with general anisotropy

The problem of a straight crack lying in the coating of a semi-infinite substrate is considered. The most general case of anisotropy is assumed for both materials (coating and substrate). The solution of Atkinson and Eftaxiopoulos [2] for a dislocation in an anisotropic bimaterial is modified to solve the fundamental problem of a dislocation in the coating, by including a corrective solution that accounts for the additional boundary conditions at the free surface. The coating crack is modelled as a continuous distributions. The resulting system of singular integral equations is solved by applying an inversion theorem and then using numerical integration. The stress intensity factors for modes I, II and III can be directly obtained from the dislocation densities. Numerical results are presented to compare the present analysis with existing solutions for some particular geometries and material models. For the general anisotropic case, results are presented showing the influence of a number of geometric and material parameters.     

NdFeB纳米晶双相复合永磁材料研究进展

NdFeB纳米晶双相复合永磁材料,由于其潜在的优异磁性能和商业价值,成为当今材料领域研究的热点。本文就近年来NdFeB纳米晶双相复合永磁材料的发展状况,从该类合金的交换耦合作用、分类以及提高磁性能的方法三个方面为重点,作简要的评述。     

Liquid crystal alignment in electro-responsive nanostructured thermosetting materials based on block copolymer dispersed liquid crystal

Novel well-defined nanostructured thermosetting systems were prepared by modification of a diglicydylether of bisphenol-A epoxy resin (DGEBA) with 10 or 15?wt% amphiphilic poly(styrene-b-ethylene oxide) block copolymer (PSEO) and 30 or 40?wt% low molecular weight liquid crystal 4'-(hexyl)-4-biphenyl-carbonitrile (HBC) using m-xylylenediamine (MXDA) as a curing agent. The competition between well-defined nanostructured materials and the ability for alignment of the liquid crystal phase in the materials obtained has been studied by atomic and electrostatic force microscopy, AFM and EFM, respectively. Based on our knowledge, this is the first time that addition of an adequate amount (10?wt%) of a block copolymer to 40?wt% HBC-(DGEBA/MXDA) leads to a well-organized nanostructured thermosetting system (between a hexagonal and worm-like ordered structure), which is also electro-responsive with high rate contrast. This behavior was confirmed using electrostatic force microscopy (EFM), by means of the response of the HBC liquid crystal phase to the voltage applied to the EFM tip. In contrast, though materials containing 15?wt% PSEO and 30?wt% HBC also form a well-defined nanostructured thermosetting system, they do not show such a high contrast between the uncharged and charged surface.     

The influence of crystal anisotropy on mechanical behaviour

Anisotropy of crystal structure leads to complications in mechanical behaviour. Robert Cahn, 50 years ago, made valuable contributions through determination of crystallographic features of plastic deformation in large crystals in polycrystalline -uranium. This research area has become increasingly linked with the effects of internal and external stresses on many materials in polycrystalline form comprised of grains with anisotropic crystal structure. The extent of irreversibility of deformation when such materials are subjected to thermal cycles leads to the significance of crystallographic textures but major effects on mechanical behaviour are often apparent where grains are randomly aligned without preferred crystal orientation when small external stresses are imposed. The importance of these features, their main characteristics and their analysis are briefly reviewed     

On crystal alignment in polycrystalline YBCO materials based on TEM observation

Previously, we have observed that parallel lattice fringes show up regularly in MTG samples, indicative of good alignment of unit cells in strips of size 100 nm × a few times 100 nm. We have prepared films ( 80 nm) for TEM observation using YBCO material fabricated by the conventional solid-state reaction method in order to study the crystal texture in a length scale from a few 100 nm down to 3 Å. Parallel lattice fringes are observed to stretch across a cross-section 0.3×0.5 m². Cross lattice fringes intersecting at 90° and 45° are observed as rare events. This result suggests that the degree of alignment in the crystal texture is higher than expected inside a grain of size a few m.     

Nanomagnets with high shape anisotropy and strong crystalline anisotropy: perspectives on magnetic force microscopy

We report on a new approach for magnetic imaging, highly sensitive even in the presence of external, strong magnetic fields. Based on FIB-assisted fabricated high-aspect-ratio rare-earth nanomagnets, we produce groundbreaking magnetic force tips with hard magnetic character where we combine a high aspect ratio (shape anisotropy) together with strong crystalline anisotropy (rare-earth-based alloys). Rare-earth hard nanomagnets are then FIB-integrated to silicon microcantilevers as highly sharpened tips for high-field magnetic imaging applications. Force resolution and domain reversing and recovery capabilities are at least one order of magnitude better than for conventional magnetic tips. This work opens new, pioneering research fields on the surface magnetization process of nanostructures based either on relatively hard magnetic materials-used in magnetic storage media-or on materials like superparamagnetic particles, ferro/antiferromagnetic structures or paramagnetic materials.     

Permanent magnetic materials with low reversible temperature coefficient

Permanent magnets made from SmCo5exhibit negative reversible change in magnetization with increasing temperature, typically of the order of 0.04% per deg C between -100 and +200°C. For certain special applications in precision instruments such as gyros and accelerometers, it is highly desirable to improve this property. Earlier studies have shown that ternaries of the composition RxSm1-xCo5(where R = Gd, Ho, Er, or Dy and x is nearly 0.4) exhibit improved temperature compensation. A systematic investigation of the temperature coefficients of magnetization of a number of quaternaries in the temperature range 100-400 K has been undertaken. The effect of variation of the cobalt concentration on the temperature compensation of one typical system has also been examined. The results indicate that good temperature compensated magnets can be synthesized with composition Sm0.6Gd0.3DY0.1Co5(α = 0.0056 at 200-300 K; 0.002 at 300-350 K and ∼0 at 350-400 K). Slight variation of cobalt concentration does not have significant effects on the temperature compensation. Lattice constants, saturation magnetization, and the theoretical energy products of a number of quaternaries are also reported.     

New deep blue emitting materials based on indenopyrazine core with high thermal stability

New deep blue emitting materials 2,8-bis(3,5-diphenylphenyl)-6,6,12,12-tetraethyl-6,12-dihydrodiindeno[1,2-b:1',2'-e]pyrazine (DPP-EPY) and 2,8-bis(3',5'-diphenylbiphenyl-4-yl)-6,6,12,12-tetraethyl-6,12-dihydrodiindeno[1,2-b:1',2'-e]pyrazine (DPBP-EPY) were synthesized through introduction of m-terphenyl or triphenylbenzene bulky side groups in a new indenopyrazine core. These materials all showed high thermal stability and highly reduced intermolecular interaction. DPP-EPY and DPBP-EPY showed PL maxima of 456 nm and 460 nm in deep blue region and narrow PL spectra with full-width at half-maximum (FWHM) of 46 nm and 52 nm, respectively. As a result of making non-doped OLED devices using these synthesized materials as emitting layers, DPP-EPY showed EL spectrum of 452 nm, very narrow FWHM of 46 nm, luminance efficiency of 1.04 cd/A with current density of 10 mA/cm2 and CIE coordinate of (0.161, 0.104), creating a deep blue OLED close to the National Television System Committee (NTSC) blue standard.     

Nonuniform transformation field analysis of materials with morphological anisotropy

The effective properties of metal matrix composites with particulate reinforcement are investigated using the nonuniform transformation field analysis (NTFA) developed by Michel and Suquet [30]. In particular the effect of the particle morphology on the effective mechanical response is examined in detail. For that, an existing periodic three-dimensional mesh generation technique for particulate composites is extended to allow for anisotropic morphologies. It is shown that the effects induced by the anisotropic particles can be captured by the NTFA. Additionally, the load partitioning between reinforcement and matrix material is investigated and a good agreement to full-field computations is attained with the NTFA.     

Intergranular phase dependence of anisotropy and coercivity in nanoscaled permanent magnets

In order to understand the effect of intergranular phase on magnetic properties, we assume that the intergranular phase is nonmagnetic phase and studied the effect of intergranular phase with different thickness d on the effective anisotropy and coercivity of nanocrystalline materials. The results showed that with increasing d, the average anisotropy of grain, 〈K〉, of material increases monotonously, while the effective anisotropy, K_eff, increases firstly and decreases subsequently, and exhibits maximum value at certain d. The existence of intergranular phase weakens or prevents the exchange-coupling interaction between grains and may result the increase of K_eff, and H_c of material. The calculated dependence of coercivity on grain size is basically in accordance with the theoretical and experimental results given by other authors.     

Polarimetric investigation of materials with both linear and circular anisotropy

Abstract

We investigate light propagation through materials with both linear and circular anisotropy and find the relation of the amplitude and polarization transfer functions to the four anisotropic characteristics: linear circular birefringence, and linear and circular dichroism. We determine these four characteristics of anisotropic samples by measuring the output intensity and polarization corresponding to different input polarization azimuths and fitting the theoretical and experimental results. In our experiments we have used films of side-chain azobenzene polyesters in which optical anisotropy had been previously induced on illumination with elliptically polarized light.     

Current anisotropy influence on beam self-focusing in photorefractive materials

Abstract

We show numerically that the existing photocurrent anisotropy strongly affects the form of initially circular beam after self-focusing in photorefractive crystals. We estimate the ellipticity of the output beam in strontium barium niobate and barium titanate and compare it with existing experimental data.     

交换耦合的纳米软-硬磁性晶粒的有效各向异性常数

以α-Fe、Nd2Fe14B为例,研究了纳米晶粒尺寸对交换耦合的软-硬磁性晶粒各向异性的影响.结果表明,软-硬磁性晶粒的有效各向异性常数〈Ksh〉随软磁性晶粒尺寸Ds的增加而减小,随硬磁性晶粒尺寸Dh的增加而增大.当Ds/Dh为固定值且1/1.1≥Ds/Dh≥1/1.4时,〈Ksh〉在Ds为10～20nm之间取得一极大值.当Dh＞25nm,Ds在10nm左右时,可获得较大的〈Ksh〉.