Phase transformation and magnetocaloric effect of Co-doped Mn–Ni–In melt-spun ribbons

Abstract

Ribbon-shaped magnetocaloric materials are favorable to achieve high heat-transfer efficiencies due to their large specific surface area. In this work, Mn50Ni41−xIn9Cox (0 ≤ x ≤ 4) ribbons were prepared using a melt-spinning technique, and the corresponding phase transformation and magnetocaloric properties were studied. The large temperature gradient during melt-spinning caused the initial austenite in the ribbons to form typical columnar-shaped grains with a strong ⟨001⟩A preferred orientation perpendicular to the ribbon plane. After cooling, the ribbons undergo martensitic transformation from cubic austenite to monoclinic eight-layered modulated (8 M) martensite. High angle annular dark field-scanning transmission electron microscopy observations indicate that martensite lattice modulation is inhomogeneous at atomic scales. Co substitution for Ni not only strongly influences the phase transformation temperatures but also greatly enhances ferromagnetic coupling. As a result, an enlarged magnetization difference across the martensitic transformation under a field change of 5 T in the Mn50Ni38In9Co3 ribbon induces a large magnetic entropy change up to 12.1 J kg−1 K–1 and a refrigeration capacity of 197 J kg–1 around room temperature. In addition, a wide operational temperature region up to 31 K is obtained in the Mn50Ni37In9Co4 ribbon due to the enhanced sensitivity of the transformation temperature shift under a magnetic field.