The nanocapsules with crystalline cores of $\mathrm{Gd}{\mathrm{Al}}_2$ compound and shells of amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$ were prepared by evaporating ${\mathrm{Gd}}_x{\mathrm{Al}}_{100-x}$ ($x=50$, 60, 70, 80, and 90) alloys using a modified arc-discharge technique. The morphologies, average sizes, lattice constants, and surface characteristics of $\mathrm{Gd}{\mathrm{Al}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3$ 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 $180\text{to}5\mathrm{K}$, the magnetic entropy change of the $\mathrm{Gd}{\mathrm{Al}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3$ nanocapsules continuously increases with decreasing temperature $T$ and rapidly enhances when the temperature tends to $5\mathrm{K}$. The largest entropy change $-\Delta S$ at $7.5\mathrm{K}$ can respectively reach 18.02, 18.71, and $31.01\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ by varying the magnetic field from $7\text{to}1\mathrm{T}$ for the nanocapsules synthesized by arc-discharging ${\mathrm{Gd}}_{70}{\mathrm{Al}}_{30}$, ${\mathrm{Gd}}_{80}{\mathrm{Al}}_{20}$, and ${\mathrm{Gd}}_{90}{\mathrm{Al}}_{10}$ 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.

• Received 11 January 2007

DOI:https://doi.org/10.1103/PhysRevB.76.144404