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利用扫描速率法研究纳米晶复合永磁合金Nd_8.5Fe₇₆Co₅Zr₃B_6.5Dy₁, Nd_9.5Fe₇₅Co₅Zr₃B_6.5Nb₁和Nd_9.5Fe_75.4Co₅Zr₃B_6.5Ga_0.6的磁黏滞行为, 计算了合金的扰动场及磁交换长度, 分析了其交互作用、微观结构和磁性能之间的关系. 结果表明, 3种合金的扰动场分别为4.80, 4.87和5.09 kA/m; 磁交换长度差别不大, 分别为4.53, 4.41和4.20 nm. Nd_9.5Fe₇₅Co₅Zr₃B_6.5Nb₁合金的交互作用最强, 主要是因为合金中的晶粒尺寸细小(约为15 nm)且分布均匀. 3种合金均呈单一的硬磁特征, 其中Nd_9.5Fe_75.4Co₅Zr₃B_6.5Ga_0.6合金磁化反转的一致性最好, 使得其剩磁较高, 最大磁能积较大.

A well known feature of ferromagnetic materials is the time dependent behavior of the magnetic polarization, i.e. magnetic viscosity, which arises from thermal activation over energy barriers. It is found that magnetic parameters, such as the fluctuation field (H_f) and the exchange interaction length (l_ex), have a close relationship with the microstructure of the materials. Therefore, investigation on magnetic viscosity is helpful to understand the coercivity mechanism of ferromagnetic materials. In this work, ingots with nominal composition Nd_8.5Fe₇₆Co₅Zr₃B_6.5Dy₁, Nd_9.5Fe₇₅Co₅Zr₃B_6.5Nb₁ and Nd_9.5Fe_75.4Co₅Zr₃B_6.5Ga_0.6 were prepared by arc-melting pure metals Nd, Fe, Co, Zr, Dy, Nb, Ga and Fe-B alloy in an argon atmosphere. A small portion of an ingot weighing about 5 g was re-melted in a quartz nozzle and ejected onto a rotating copper wheel in a range of 10~30 m/s. The annealing treatment was carried out at 690~710 ℃ for 4~5 min. Vibrating sample magnetometer (VSM), XRD and TEM were used to study magnetic viscosity behavior and exchange interaction for Nd₂Fe₁₄B/α-Fe nanocomposite permanent alloys. Furthermore, the relationship among exchange interaction, microstructure and magnetic property was discussed. For the nanocomposite Nd_8.5Fe₇₆Co₅Zr₃B_6.5Dy₁, Nd_9.5Fe₇₅Co₅Zr₃B_6.5Nb₁ and Nd_9.5Fe_75.4Co₅Zr₃B_6.5Ga_0.6 alloys, H_f and l_ex were obtaind by sweep rate measurement. The H_f were 4.80, 4.87 and 5.09 kA/m, and l_ex were 4.53, 4.41 and 4.20 nm for permanent Nd_8.5Fe₇₆Co₅Zr₃B_6.5Dy₁, Nd_9.5Fe₇₅Co₅Zr₃B_6.5Nb₁ and Nd_9.5Fe_75.4Co₅Zr₃B_6.5Ga_0.6 alloys, respectively. It suggested that the l_ex had a minor change. The Nd_9.5Fe₇₅Co₅Zr₃B_6.5Nb₁ alloy had the strongest exchange interaction among three alloys in this work. It is due to a refined microstructure and uniform distribution of grains. Furthermore, the behavior of the irreversible susceptibility (χ_irr) as a function of applied magnetic field (H) was investigated. A single sharp peak could be seen near coercive field in the χ_irr-H curve in three alloys, suggesting that the magnetization reversal was a uniform reversal process. The Nd_9.5Fe_75.4Co₅Zr₃B_6.5Ga_0.6 alloy exhibited a sharper and narrower peak, indicating a more rapid change in magnetization and a strong interaction between adjacent magnetic phases. Since exchange interaction of neighboring grains favors the nucleation of reversed domains, remanence enhancement is generally achieved at the expense of coercivity. Among three alloys, Nd_9.5Fe_75.4Co₅Zr₃B_6.5Ga_0.6 alloy showed the optimum magnetic properties, that is, the coercivity H_c=687.56 kA/m, the remanence B_r=0.92 T, the maximum magnetic energy product (BH)_max=120.88 kJ/m³. It was mainly due to consisting of well-coupled grains with near perfect alignment of the easy magnetization direction, which improved the remanence and maximum energy product.