Effects of the degree of grain alignment on the hard magnetic properties of sintered NdFeB magnets

r increases monotonously with an enhancement in the degree of grain alignment (i.e. decrease in the orientation coefficient σ), whereas the intrinsic coercivity _jH_c decreases monotonously. This dependence leads to a non-monotonic behaviour of the magnetic-inductive coercivity _bH_c and a saturation of the maximum energy product (BH)_max. Received: 28 August 1997/Accepted: 12 January 1998     

Effects of excessive grain growth on the magnetic and mechanical properties of hot-deformed NdFeB magnets

The magnetic and mechanical properties of rare-earth magnets hot-deformed at temperature range 750-950 °C have been investigated. The grains tended to grow excessively from dozens of nanometers to several microns at the temperatures above 850 °C. The alignment of grains was disrupted by the hot deformation at the high temperatures. The Nd-rich phase was extruded at the temperatures which are higher than 850 °C. The Nd-rich phase extrusion resulted in the reduction of density by 1% and the reduction of remanence from 1.42 to 0.72 T. The reduction of grain boundaries caused by flat platelet-shaped grains changing to spherical grains and the weak binding strength among large grains of Nd₂Fe₁₄B phase may be the main reasons for the low mechanical strength of hot-deformed magnets.     

Study of high-coercivity sintered NdFeB magnets

Magnetic powders for sintered NdFeB magnets have been prepared by using an advanced processing method including strip casting, hydrogen decrepitation, jet milling and rubber isotropic press. The effects of Dy, Ga and Co addition on the microstructure and magnetic properties of sintered magnets have been investigated. By adopting a suitable component ratio and adjusting proper technological parameters, we have prepared high-coercivity sintered NdFeB magnets with hard magnetic properties of _jH_c=25.6 kOe, B_r=13.2 kG and (BH)_max=39.9 MGOe. The temperature coefficient of coercivity of the magnets (between 20 and 150 °C) is –0.53%/°C. The magnetic properties at high temperature satisfy the needs of permanent magnet motors.     

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     

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     

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.     

添加Nb在快淬NdFeB永磁体中的作用研究

系统研究了Nb元素的添加对快淬(Nd,Dy)_11.5Fe_82.4-nNb_nB_6.1(n=0,0.5,1,1.5,2)永磁体磁性能、温度特性及显微组织的影响. 结果表明：少量Nb元素的添加可以在不显著影响剩磁的情况下较大幅度提高磁体的内禀矫顽力，降低磁通不可逆损失. Nb在NdFeB磁体中的作用是使晶粒细小化、均匀化、规则化，提高交换耦合钉扎场H_p，减小材料内部的散磁场，显著降低磁通不可逆损失，改善NdFeB磁体在高温下的使用性能.     

The coercive field of sintered and melt-spun NdFeB magnets

The magnetic hysteresis loops have been investigated in the temperature range between 4.2 and 575 K for aligned sintered permanent magnets of nominal composition Nd₁₅Fe₇₇B₈ and for isotropic melt-spun ribbons of composition Nd₁₅Fe₇₆B₉. The measured temperature and field dependence of the coercive field is analysed within the framework of theoretical results for nucleation fields of the ideal Nd₂Fe₁₄B matrix and, of disturbed surface regions of Nd₂Fe₁₄B grains. Furthermore the pinning of domain walls at thin soft magnetic grain boundary phases is considered for the high temperature range. It is concluded that for both types of NdFeB magnets the relevant magnetic hardening mechanisms at lower temperatures are nucleation processes in magnetically inhomogeneous regions whereas at higher temperatures the pinning of domain walls at grain boundaries predominates. The critical temperature where the change of nucleation hardening to pinning hardening occurs depends sensitivity on the crystal anisotropy, the grain boundary microstructure and the macroscopic grain- and multi phase arrangements.     

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r2Fe14B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Pr2Fe14B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.     

Comparison of the Kerr effect and X-ray pole figure methods for determining crystallographic texture in PrFeB magnets

In this study, magnetic domains in Pr₁₆Fe₇₆B₈ sintered magnets have been observed by Kerr effect and a histogram of the angular distribution of domain orientations has been used to determine the magnetic texture (cos Φ). The degree of easy-axis alignment of Pr₂Fe₁₄B matrix grains in these magnets has been also determined by X-ray pole figure analysis using the (0 0 4) reflection. The (0 0 4) pole figure measurements were carried out by the Schultz's reflection method. The (0 0 4) normalized intensity data has been fitted for a Gaussian distribution and the degree of crystal alignment, cos Θ, has been calculated using the Stoner–Wohlfarth model. Comparison of these methods has been carried out. It has been shown that in magnets with medium and high degrees of crystallographic alignment, the pole figure values are higher than that obtained by the Kerr effect method. Conversely, in magnets with low degrees of alignment, cos Θ is lower than cos Φ.     

Anisotropic powder from sintered NdFeB magnets by the HDDR processing route

Sintered NdFeB-based scrap magnets were recovered and processed using the HD and HDDR routes. The effects of varying the HDDR processing temperature were investigated (over the range 835-930 °C). The disproportion was carried out with a pressure ramp to a maximum of 1000 mbar hydrogen pressure with a 1 h hold time at each step and the optimum recombination conditions were set at 100 mbar with a 20 min hold time. Anisotropic NdFeB powder was produced in all cases with the best magnetic properties achieved at a processing temperature of 880 °C, producing powder with a remanence of 1.10(±0.02) T and an intrinsic coercivity of 800 (±16) kA m⁻¹ and giving a (BH)_max of 129(±2.5) kJ m⁻³.     

Electronic control of spin alignment in pi-conjugated molecular magnets

Intramolecular spin alignment in pi-conjugated molecules is studied theoretically in a model of a Peierls-Hubbard chain coupled with two localized spins. By means of the exact diagonalization technique, we demonstrate that a spin singlet (S=0) to quartet (S=3/2) transition can be induced by electronic doping, depending on the chain length, the positions of the localized spins, and the sign of the electron-spin coupling. The calculated results provide a theoretical basis for understanding the mechanism of spin alignment recently observed in a diradical donor molecule.     

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}     

Effect of exchange-coupling interaction on anisotropy of grain in nanoscaled magnets

The effect of inter-grain exchange-coupling interaction on the anisotropy of grain in nanoscaled magnets has been investigated by putting forward an expression of anisotropy at grain boundary, , which is suitable for different coupling conditions, and expresses well the coherency between soft and hard grains. The average anisotropy of grain 〈K^ij〉 has been calculated based on and the theory of partial exchange-coupling interaction. It has been found that the average anisotropy of hard or soft grain, 〈K^hh〉 or 〈K^ss〉, increases with increasing grain size D monotonously when hard-hard or soft-soft grains couple. When soft-hard grains touch each other, with increasing D, the variation of average anisotropy of soft-hard grain 〈K〉 depends on the anisotropy at grain interface , which denotes the affection degree of hard grain on the anisotropy of soft grain. Compared with other results, it is more reasonable that ranges from to . The variations of anisotropy with D we calculated are consistent with those of coercivities given by other authors when is fixed in a certain range.     

Effects of powder flowability on the alignment degree and magnetic properties for NdFeB sintermagnets

The magnetic powders for sintered NdFeB magnets have been prepared by using the strip casting (SC), hydrogen decrepitation (HD) and jet milling (JM) techniques. The effects of powder flowability and addition of a lubricant on the alignment degree and the hard magnetic properties of sintered magnets have been studied. The results show that the main factor affecting powder flowability is the aggregation of magnetic particles for powders in a loose state, but it is the friction between the powder particles for powders that are in a compact state. The addition of a lubricant with suitable dose can slightly prevent the congregating of powders, obviously decrease the friction between the powder particles, improve the powder flowability, and increase the alignment degree, remanence and energy product density of sintered magnets. Mixing a suitable dose of lubricant and adopting rubber isostatic pressing (RIP) with a pulse magnetic field, we have succeeded in producing the sintered NdFeB magnet with high hard magnetic properties of B_r=14.57 KG, _jH_c=14.43 KOe, (BH)_max=51.3 MGOe.     

Determination of the crystallographic texture of sintered PrFeB magnets based on X-ray diffraction patterns

An alternative method for determining the degree of crystallographic alignment (cos Θ) of the magnetic Pr₂Fe₁₄B phase (Φ) is proposed. The method is based on the relative X-ray diffracted intensities of the Φ planes of sintered magnets with and without texture. The degree of crystallographic alignment is also determined by X-ray pole figures using the (0 0 4) reflection and considered as a standard reference for comparison. The method is applied to Pr₁₆Fe₇₆B₈ magnets with 0.51cos Θ0.97. The difference between the crystallographic alignments determined by these two methods is 3% within the experimental error. The advantages and limitations of using X-ray diffraction patterns to quantitatively evaluate the texture of sintered magnets are also discussed.     

Correlation functions of pure and diluted Ising magnets in the mean-field approximation

A mean-field method, which is a variant of the fixed-scale renormalization group transformation and is applied to both pure and diluted magnets, has been considered. It has been shown that, for pure magnets, the method is equivalent to the Bethe approximation. This method has been used to calculate the magnetization and correlation functions of both pure and bond-diluted Ising magnets.