Influence of annealing temperatures on the magnetostructural transition and magnetocaloric effect of Ni₄₀Co₁₀Mn₄₀Sn₁₀powders

The phase structure, the magnetostructural transition behavior and the associated magnetocaloric effect have been investigated in Ni₄₀Co₁₀Mn₄₀Sn₁₀bulk and powders, mainly focusing on the effects of annealing temperatures ranging from 573 K to 773 K. A non-modulated martensitic structure was detected in the bulk and as-ground powders, but a seven-layered orthorhombic structure was observed after annealing. The magnetostructural transition in the bulk alloy from weak magnetic martensite to ferromagnetic austenite was also confirmed in the as-ground and annealed powders. Compared with the bulk alloy, the transition temperatures of the as-ground powders decreased markedly; the transition temperature hysteresis broadened, and the magnetization change across the transition was severely reduced. With increasing annealing temperatures, the parameters of the magnetostructural transition gradually improved. When the powders were annealed at 773 K, the magnetostructural transition behaved the most similar to the bulk alloy. Consequently, the large magnetic entropy change of 27 J/(Kg K) and the effective cooling capacity of 171.3 J/Kg that were obtained under the magnetic field of 5 T were very close to the bulk alloy. The internal stress and surface effects were considered to understand the observed phenomena.