Departamento de Física, Universidad de Oviedo, Calvo Sotelo, s/n, 33007 Oviedo, Spain.
J Phys Condens Matter. 2010 Jun 2;22(21):216005. doi: 10.1088/0953-8984/22/21/216005. Epub 2010 May 5.
Nanocrystalline Nd(2)Fe(17) powders have been obtained by means of high-energy ball milling from nearly single-phase bulk alloys produced by arc melting and high temperature homogenization annealing. The rhombohedral Th(2)Zn(17)-type crystal structure of the bulk alloy remains unaltered after the milling process, with almost unchanged values for the cell parameters. However, the severe mechanical processing induces drastic microstructural changes. A decrease of the mean crystalline size down to around 10 nm is observed, giving rise to a considerable augmentation of the disordered inter-grain boundaries. This modification of the microstructure affects the magnetic behaviour of the milled powders, although the magnetic structure remains collinear ferromagnetic. While a unique ferro-to-paramagnetic transition temperature, T(C) = 339 ± 2 K, is observed in the bulk alloy, the nanocrystalline samples exhibit a more likely distribution of T(C) values. The latter seems to be responsible for the significant broadening of the temperature range in which magneto-caloric effect is observed, and the lowering of the maximum value of the magnetic entropy change.
采用高能球磨法从电弧熔炼和高温均相退火制备的近乎单相块体合金中获得了纳米晶 Nd(2)Fe(17)粉末。块体合金的菱方 Th(2)Zn(17)型晶体结构在球磨过程中保持不变，晶胞参数几乎保持不变。然而，剧烈的机械加工会引起剧烈的微观结构变化。观察到平均晶粒尺寸减小到约 10nm，无序晶界的数量显著增加。这种微观结构的变化会影响球磨粉末的磁性能，尽管磁结构仍然是共线铁磁的。在块体合金中观察到独特的铁磁-顺磁转变温度 T(C)=339±2K，而纳米晶样品则表现出 T(C)值的更可能分布。后者似乎是导致观察到磁热效应的温度范围显著变宽以及磁熵变化的最大值降低的原因。
