The nanocapsules with crystalline cores of GdAl 2 compound and shells of amorphous Al 2 O 3 were prepared by evaporating Gd x Al 100−x ͑x = 50, 60, 70, 80, and 90͒ alloys using a modified arc-discharge technique. The morphologies, average sizes, lattice constants, and surface characteristics of GdAl 2 /Al 2 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 to 5 K, the magnetic entropy change of the GdAl 2 /Al 2 O 3 nanocapsules continuously increases with decreasing temperature T and rapidly enhances when the temperature tends to 5 K. The largest entropy change −⌬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 Gd 70 Al 30 , Gd 80 Al 20 , and Gd 90 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/T͒ was discussed in terms of superparamagnetism and magnetocaloric theory.

Magnetic and magnetocaloric effects of (Mn1−xFex)5Ge3 compounds are studied systematically. The maximum of magnetic entropy changes of 8.01 J/kg K under an external field change of 5 T is obtained for (Mn0.9Fe0.1)5Ge3, which is the largest value in Mn5Ge3-based solid solutions. Moreover, the Fe substitution increases the refrigeration capacity (RC) value greatly. The largest RC value of 237 J/kg in (Mn0.8Fe0.2)5Ge3 even compares favorably to that of many well-known magnetic refrigeration materials. Thus the Fe-containing (Mn1−xFex)5Ge3 compounds are much-improved magnetic refrigerants for the application of room-temperature magnetic refrigeration. The increase of the RC value is probably resulted from the formation of magnetic nanostructure.

In Fe0.8Mn1.5As compound, an external magnetic field induces a metamagnetic transition from an antiferromagnetic phase to a ferrimagnetic phase above Ts=285K, leading to large magnetocaloric effects around room temperature. Instead of showing inverse magnetocaloric effects, the sign of the entropy change ΔSM in the compound is unexpectedly negative, revealing a different mechanism. The maximum value of ΔSM is 6.2J∕kgK at 287.5K for a magnetic field change of 5T. The study on systems with antiferromagnetism-related metamagnetic transitions may open an important field in searching good materials for room-temperature magnetic refrigeration.

The structure and formation of nanoparticles without encapsulation of the intermetallic compound DyCo 2 were investigated by using x-ray diffraction and high-resolution transmission electron microscopy. The DyCo 2 nanoparticles are stable in air without any shell protection. A large magnetic-entropy change of 13.2 J kg −1 K −1 was found at 7.5 K in an applied-field change from 1 to 7 T, which is ascribed to the large magnetic moment density and the weak interaction energy in the nanoparticles. Such oxidation-resistant rare-earth transition-metal compound nanoparticles with large cryogenic magnetocaloric effect are useful for refrigeration applications at low temperatures.

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In Fe 0.75 Mn 1.35 As compound, a metamagnetic transition from an antiferromagnetic phase to a ferrimagnetic phase can be induced above its phase transition temperature T s = 165 K by an external magnetic field, which leads to large magnetocaloric effects around T s . The sign of the magnetic entropy change DS M in the Fe 0.75 Mn 1.35 As compound is negative, not as expected as an inverse magnetocaloric effect, and the maximum value of DS M is 4.2 J/kg K at 167.5 K for a magnetic field change of 5 T. Although it induces an irreversible lattice expansion, the cycling of a magnetic field does not induce an irreversible change in the magnetic transitions and magnetocaloric behaviors. The antiferromagnetism-related metamagnetic transitions with a large magnetic entropy change may provide with an opportunity in searching novel materials for magnetic refrigeration.