NdFeB纳米复合永磁材料的交换耦合相互作用和有效各向异性

以Nd₂Fe₁₄B/αFe为例，采用立方体晶粒结构模型，研究了纳米复合永磁材料中不同磁性晶粒间的交换耦合相互作用和有效各向异性.纳米复合永磁材料的有效各向异性Keff等于软、硬磁性相各向异性的统计平均值，每个晶粒的各向异性由晶粒表面交换耦合部分和晶粒内部未交换耦合部分的各向异性共同确定.计算结果表明，软、硬磁性相晶粒尺寸分布显著地影响有效各向异性Keff的值.当软、硬磁性晶粒尺寸D相同时，Keff随晶粒尺寸和硬磁性相体积分数的降低而减小, 当D<20nm 时，K 关键词： 纳米复合永磁材料 交换耦合相互作用 有效各向异性 晶粒尺寸     

纳米复合永磁材料的有效各向异性与矫顽力

研究了纳米Nd2Fe14B/α-Fe复合永磁材料中晶粒交换耦合相互作用对有效各向异性的影响和变化规律.结果表明：晶粒之间的交换耦合作用使材料的有效各向异性Keff随晶粒尺寸的减小而下降、随软磁性相成分的增加而降低. 当晶粒尺寸 减小到4nm时，Keff值减小为其各自通常各向异性常数值的1/3—1/4.有效各向异性的变化特点与矫顽力的变化规律基本相同.纳米复合永磁材料矫顽力的降低主要由于有效各向异性的减小而引起. 关键词： 纳米复合永磁材料 交换耦合相互作用 有效各向异性 矫顽力     

晶粒间界相对纳米Nd_2Fe_(14)B各向异性和矫顽力的影响

本文采用立方晶粒结构模型研究了晶粒间界相对纳米硬磁材料各向异性和矫顽力的影响.结果表明:晶粒间界相减弱了交换耦合作用.当晶粒尺寸D为定值时,随着晶粒间界相厚度d的增加,晶粒平均各向异性单调增加,而材料的有效各向异性Keff、矫顽力单调减小.我们计算的矫顽力随晶粒尺寸的变化与相关理论和实验结果基本一致.     

Studies on the magnetic properties for single-phase nanocrystalline FePt magnets with different orientation degrees

The single-phase nanocrystalline FePt magnets composed of 343 irregular-shaped grains are built. The demagnetization curves are simulated by micromagnetic finite element method. The remanence, coercivity and maximum energy product of the magnets decrease with deteriorating grain alignment. The characteristics of variation of magnetic properties with the degree of orientation are closely related to the average grain size of nanocrystalline magnets. The contribution of intergrain exchange coupling (IGEC) to remanence enhancement is associated to the degree of orientation, and decreases with improved grain alignment. With decreasing grain size, coercivity increases for anisotropic nanocrystalline magnets, which is completely different from that of isotropic nanocrystalline magnets.     

晶粒易轴取向度对纳米晶永磁Pr2Fe14B磁性的影响

构造了立方和不规则形状晶粒的各向异性纳米晶单相Pr₂Fe₁₄B磁体 .利用微磁学的有限元法，模拟计算了样品的磁滞回线.计算结果表明，随着磁体晶粒易轴取向度的变差， 磁体的剩磁、矫顽力均随之下降.不同晶粒尺寸的纳米晶单相Pr₂Fe_{14B磁体，其磁 性能随取向度的变化快慢不同，原因在于磁体中的晶间交换作用 (IGEC) 的强弱不同.随着 晶粒取向度的提高，纳米晶单相磁体的矫顽力逐渐增加，这完全不同于烧结磁体. 关键词： 纳米晶磁体 矫顽力 剩磁}     

Morphological and microstructural study of L10-ordered FePt and L10-FePt/Fe ultrathin films grown by UHV e-beam evaporation technique

Ultrathin films of L1₀-ordered FePt alloy with different thickness were grown by UHV e-beam evaporation technique. In this ultrathin regime, the increase of the thickness induces a strong improvement of the magnetic properties followed by a decrease of the grain size. Starting from these hard layers, FePt/Fe/FePt trilayers with different thickness of the outermost layer have been grown. The samples show a single-phase magnetic behaviour and a strong perpendicular anisotropy thanks to the exchange-coupling that established at the soft/hard interfaces. By increasing the thickness of the outer layer, a strong reduction of the switching field distribution has been obtained with the appearance of a predominant exchange-type interaction among the magnetic grains. The morphology is characterized by well separated and elongated islands, while the magnetic pattern shows finely dispersed bubble domains.     

The physics of amorphous and nanocrystalline hard magnetic materials

The magnetic properties of amorphous and nanocrystalline hard magnetic materials are summarized. The reduction of the “effective” anisotropy field due to exchange coupling in nanocrystalline materials is demonstrated. This leads experimentally as well as theoretically to a remanence enhancement and to a reduced coercivity. Also the domain structure shows the effect of exchange coupling. Nd–Fe–Al is taken as an example of a new “amorphous” hard magnetic material. For magnetostrictive materials the possibility of reducing the anisotropy in nanocrystalline samples without loosening the high magnetostriction is discussed.     

多层膜［Co85Cr15/Pt］20的磁性、垂直磁记录特性和微结构的关系

采用磁控溅射法制备了性能优良的以Pt为缓冲层的［Co85₈₅Cr15_{15/Pt］2020 多层膜，研究了溅射气压对［Co8585Cr1515/Pt］2020多层膜微结构和磁性的 影响.研究结果表明，Ar溅射气压对［Co8585Cr1515/Pt］2020多层膜的微结构 、垂直磁各向异性和矫顽力有重要的影响 关键词： 溅射气压 多层膜 垂直磁各向异性 有效磁各向异性常数}     

纳米晶永磁材料晶间交换耦合作用的模拟计算研究

利用微磁学有限元法，模拟计算了单相和复相各向同性纳米晶磁体的起始磁化曲线、退磁曲线和回复曲线.验证了用δm(H)曲线的正峰值来衡量纳米晶磁体晶间交换耦合作用的有效性.计算 结果表明，纳米晶单相和复相磁体的晶间交换耦合作用都随晶粒尺寸的增加而降低，当晶粒尺寸过大时 复相磁体表现出两相行为，其δm(H)曲线出现了两个正峰值.分析表明，外场较小的正峰值是软磁相与硬磁相晶粒之间交换耦合作用的结果，而外场较大的正峰值是硬磁相晶粒之间交换耦合作用的结果. 关键词： 纳米晶永磁 晶间交换耦合 δm(H)曲线     

Microstructure and magnetic characteristics of nanocrystalline Ni0.5Zn0.5 ferrite synthesized by a spraying-coprecipitation method

Nanocrystalline Ni0.5Zn0.5 ferrite with average grain sizes ranging from 10 to 100 nm is prepared by using a spraying-coprecipitation method. The results indicate that the nanocrystalline Ni0.5Zn0.5 ferrite is ferromagnetic without the superparamagnetic phenomenon observed at room temperature. Specific saturation magnetization of nanocrystalline Nio.sZno.5 ferrite increases from 40.2 to 75.6 emu/g as grain size increases from 11 to 94nm. Coercivity of nanocrystalline Ni0.5Zn0.5 ferrite increases monotonically when d 〈 62 nm.The relationship between the coercivity and the mean grain size is well fitted into a relation Hc - d^3. A theoretically evaluated value of the critical grain size is 141nm larger than the experimental value 62nm for nanocrystalline Ni0.5Zn0.5 ferrite. The magnetic behaviour of nanocrystalline Ni0.5Zn0.5 ferrite may be explained by using the random anisotropy theory.     

Soft Magnetic Materials in a Bulk Form Prepared by Powder Compaction

The amorphous, nanocrystalline and polycrystalline ferromagnetic alloys are known as materials with excellent soft magnetic properties. These attractive magnetic properties are challenge for researchers to extend investigation of these materials with the aim to broaden their technical exploitation. The shape in which amorphous, nanocrystalline and polycrystalline materials are usually prepared, is in many cases not suitable shape for application, therefore it is logical to attempt to prepare such material in a more “bulk” form, for example in the form of a cylinder or a ring, that would be more convenient for industrial applications. One of the ways to prepare material in bulk form is to compact the powder. There is rational assumption that the non-magnetostrictive alloys (amorphous Co-Fe-Si-B, nanocrystalline Fe-Nb-Cu-Si-B, and polycrystalline Ni-Fe) may be suitable for the preparation of bulk samples by high-pressure compression, because mechanical stress does not induce magnetic anisotropy in ferromagnetic material during preparation process.We observed that milling of ribbons prepared by rapid quenching method leads to the increase of coercivity, which is caused by the increase of the fraction of magnetization vector rotation in the magnetization processes (the fraction of domain wall motion decreases). After long milling the powder particles become single-domain and can be magnetized by the magnetization vector rotation only, exhibiting maximum value of coercivity.Consolidation of powder with high value of coercivity leads to the “magnetic contact” between powder particles resulting in the decrease of coercivity to the value comparable with that for as-spun ribbons.     

纳米复合永磁材料中软磁性相交换硬化的研究

本文就纳米复合永磁材料中软磁相被交换硬化问题,从一维模型和三维模拟计算进行了分析研究. 一维和三维各向异性样品研究表明,在相同微结构下,当硬磁相的各向异性降低时,除矫顽力降低外,在磁矩全部反转之前退磁曲线是一样的. 因此,硬磁相各向异性的降低不会导致最大磁能积(BH)_max增大和剩磁增加. 对于三维各向同性样品的模拟计算表明,降低硬磁相的各向异性会使剩磁和(BH)_max都明显降低. 因此,增强硬磁相的各向异性并增大硬磁相晶粒尺寸是提高 关键词： 纳米复合永磁 矫顽力 剩磁 磁能积     

Interface mediated control of microstructure and magnetic properties of FePt-C thin films

The microstructure and magnetic properties of FePt-C film deposited on the MgO/CrRu underlayer are studied in lieu of carbon doping in FePt and RF biasing assisted etching of the MgO underlayer. Increasing amounts of carbon doping in the FePt deteriorates the chemical ordering as a result of reduced kinetic energy of FePt adatoms due to frequent collision with carbon adatoms. This leads to a reduction in the magnetocrystalline anisotropy energy and hence the coercivity. RF etching of the MgO/CrRu underlayer before deposition of FePt helps to reduce the grain size and intergranular exchange interaction without adversely affecting the chemical ordering. Reduced intergranular exchange interaction leads to an increase in the coercivity.     

交换耦合相互作用对纳米晶粒各向异性的影响

研究纳米磁性材料晶粒间的交换耦合相互作用,提出适合各种耦合条件晶粒界面及表面处各向异性的表达式.结果表明:软、硬磁性相晶粒间的交换耦合相互作用降低了硬磁性相晶粒的各向异性,提高了软磁性相晶粒的各向异性.软硬磁性晶粒的平均各向异性先增加后减小,在某个晶粒尺寸值达到最大值,这种变化趋势与相关的理论及实验结果一致.硬硬磁性晶粒的平均各向异性随晶粒尺寸的变化趋势与相关的实验结果也一致.     

Effects of grain size distribution on coercivity and permeability of ferromagnets

Grain size dependence of coercivity and permeability (GSDCP) theory is extended to include grain size distribution in ferromagnets. It is found that the experimental data do not agree with the GSDCP theory on the transition location of different grain size ranges (The GSDCP theory has three different grain size ranges for different magnetization processes.). Correspondingly, including the grain size distribution the GSDCP theory fits the experimental data very well. These results prove that the grain size distribution indeed affects the magnetic properties of nanocrystalline ferromagnets.     

Phase distribution and computed magnetic properties of high-remanent composite magnets

Numerical micromagnetic calculations using finite-element techniques allow a quantitative treatment of the correlation between the microstructure and the basic magnetic properties of two-phase permanent magnets such as the remanence, the coercive field and the maximum energy product. For the investigation of (A) the role of the amount of the soft magnetic phase, and (B) the effect of grain shape, realistic three-dimensional grain arrangements have been used. The numerical results show that both short-range exchange and long-range magnetostatic interactions determine the magnetic properties. The optimal microstructure of an isotropic nanocrystalline permanent magnet was found to consist of soft magnetic particles with a large spontaneous magnetization embedded between hard magnetic grains. Exchange interactions than enhance the remanence of isotropic, composite magnets of Nd₂Fe₁₄B and -Fe by about 60%. Because of exchange hardening the soft magnetic phase can be increased up to 50% without a significant loss of coercivity. A uniform grain structure suppresses strong demagnetizing fields and this increases coercivity by 30% as compared with irregular shaped particles.     

Current density effect on magnetic properties of nanocrystalline electroplated Ni80Fe20/Cu composite wires

In developing high-sensitivity micro sensors for very weak magnetic field, extremely high permeability magnetic material is essential for the sensing element. In this study, the effect of plating current density in nanocrystalline electrodeposition of permalloy on the crystal grain size and consequently on the soft magnetic properties of the deposited layer of Ni₈₀Fe₂₀/Cu composite wire is investigated. It is found that the coercivity of the deposited Ni₈₀Fe₂₀ increases and the MI effect ratio of the Ni₈₀Fe₂₀/Cu wire decreases with increasing current density in the lower range of current density (0.6–2 A/dm²) while the opposite trend is observed as the current density range increases in a higher range (2–8 A/dm²). It seems that increasing plating current density has the effect of decreasing the crystal grain size of the plated material, resulting in lower coercivity of the plated material. This effect, due to decreased grain size, is dominating in the higher range of plating current density. However, it also has the effect of increasing residual stresses in the plated material, which is dominating in the lower range of plating current density, resulting in higher coercivity of the plated materials.     

Studies on high-moment soft magnetic FeCo/Co thin films

The dependences of soft magnetic properties and microstructures of the sputtered FeCo (=FeFeCo薄膜 溅射条件 软磁性 高饱和磁化强度FeCo film, sputtering conditions, high saturation magnetization, soft magnetic properties2005-10-263/7/2006 12:00:00 AMThe dependences of soft magnetic properties and microstructures of the sputtered FeCo （=Fe65Co35） films on Co underlayer thickness tCo, FeCo thickness tFeCo, substrate temperature Ts and taxget-substrate spacing dT-s are studied. FeCo single layer generally shows a high coercivity with no obvious magnetic anisotropy. Excellent soft magnetic properties with saturation magnetization μ0Ms of 2.35 T and hard axis coercivity Hch of 0.25 kA/m in FeCo films can be achieved by introducing a Co underlayer. It is shown that sandwiching a Co underlayer causes a change in orientation and reduction in grain size from 70 nm to about 10 nm in the FeCo layer. The magnetic softness can be explained by the Hoffmann＇s ripple theory due to the effect of grain size. The magnetic anisotropy can be controlled by changing dT-S, and a maximum of 14.3 kA/m for anisotropic field Hk is obtained with dT-S=18.0 cm.     

Effect of microstructure on the coercivity of HDDR Nd-Fe-B permanent magnetic alloy

The effect of the grain boundary microstructure on the anisotropy and coercivity was investigated in an HDDR Nd-Fe-B permanent magnetic alloy. Considering the special microstructure of its magnetic powder grain, an anisotropic theoretical model influenced simultaneously by the structure defect at the grain boundary and the exchange coupling interaction was put forward. The variations of the structure defect factors based on the nucleation and pinning mechanism with 2r ₀/lex (where r ₀ and lex are the defect thickness and the length of exchange coupling, respectively) were calculated. The results show that the coercivity mechanism of an HDDR Nd-Fe-B permanent magnetic alloy is greatly related to its microstructure defect at the grain boundary. For a fixed lex, when 2r ₀/lex < 1.67, the coercivity is controlled by the pinning mechanism; when 2r ₀/lex > 1.67, it is determined by the nucleation mechanism. The coercivity reaches the maximum when 2r ₀/lex = 1.67. The calculation result is consistent well with the experimental result given by Morimoto et al. Supported by the National Natural Science Foundation of China (Grant No. 50671055)     

Improved magnetic properties and fracture strength of NdFeB by dehydrogenation

Effects of the dehydrogenation of the hydrogen decrepitated (HD) powders on the magnetic properties and the fracture strength of sintered NdFeB magnets were studied. It was found that the lattice parameters and the crystal phase of NdFeB changed significantly with the various hydrogen contents of the resultant HD powders due to the different degrees of dehydrogenation. The magnetic properties and fracture strength increased with decreasing hydrogen content, reaching the maximum increases of 200% for both intrinsic coercivity and bending strength, which can be ascribed to the improved microstructure of the sintered NdFeB magnets. The hydrogen remaining in the HD powders diffused out and affected drastically the grain and grain boundaries by the hydrogen out-take channel during the subsequent sintering process.