Large cryogenic magnetocaloric effect in the blocking state of GdAl2/Al2O3 nanocapsules

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.