Large cryogenic magnetocaloric effect in the blocking state of GdAl2/Al2O3 nanocapsules
Large cryogenic magnetocaloric effect in the blocking state of GdAl2/Al2O3 nanocapsules
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DOI：
10.1103/physrevb.76.144404
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发表时间：
2007-10-01
影响因子：
3.7
通讯作者：
Zhang, Z. D.
中科院分区：
文献类型：
--
作者：
Ma, S.;Li, W. F.;Zhang, Z. D.
采用改进电弧放电技术对GdxAl100-x （x=50、60、70、80和90）合金进行蒸发，制备了以GdAl2为晶心、Al2O3为非晶壳的纳米胶囊。采用x射线衍射、能谱、x射线光电子能谱和高分辨率透射电子显微镜对GdAl2/Al2O3纳米胶囊的形貌、平均尺寸、晶格常数和表面特性进行了详细的研究。详细分析了纳米胶囊的形成机理。讨论了不同结构的纳米胶囊的居里温度和各向异性常数的差异。在180 ~ 5 K范围内，GdAl2/Al2O3纳米胶囊的磁熵变化随温度T的降低而不断增大，当温度趋于5 K时，磁熵变化迅速增强。在7 ~ 1 T的磁场范围内，电弧放电制备的Gd70Al30、Gd80Al20和Gd90Al10纳米胶囊在7.5 K时的最大熵变δ S分别达到18.02、18.71和31.01 J kg(-1) K-1。低温下大熵变的出现是由于纳米胶囊具有较低的各向异性能垒和较高的磁矩密度。从超顺磁性和磁热学的角度讨论了磁熵变化与温度倒数（1/T）之间的线性关系。
The nanocapsules with crystalline cores of GdAl2 compound and shells of amorphous Al2O3 were prepared by evaporating GdxAl100-x (x=50, 60, 70, 80, and 90) alloys using a modified arc-discharge technique. The morphologies, average sizes, lattice constants, and surface characteristics of GdAl2/Al2O3 nanocapsules were studied in detail by means of x-ray diffraction, energy dispersive spectroscopy, x-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The formation mechanism of the nanocapsules was analyzed in detail. The differences in Curie temperatures and anisotropy constants of these nanocapsules were discussed with respect to their different structural characteristics. From 180 to 5 K, the magnetic entropy change of the GdAl2/Al2O3 nanocapsules continuously increases with decreasing temperature T and rapidly enhances when the temperature tends to 5 K. The largest entropy change -Delta S at 7.5 K can respectively reach 18.02, 18.71, and 31.01 J kg(-1) K-1 by varying the magnetic field from 7 to 1 T for the nanocapsules synthesized by arc-discharging Gd70Al30, Gd80Al20, and Gd90Al10 alloys. The appearance of a large entropy change at low temperatures was ascribed to a lower anisotropy energy barrier and a high magnetic-moment density of the nanocapsules. The linear relation between the magnetic entropy change and the reciprocal of the temperature (1/T) was discussed in terms of superparamagnetism and magnetocaloric theory.