Enhanced magneto-caloric effect upon Co substitution in Ni-Mn-Sn thin films

Highlights: • Stable ferromagnetic martensite phase is obtained in all films at room temperature. • Very high Curie temperature (T_C) is reported for the first time in Ni-Mn-Sn films. • Large T_C shift is observed with Co substitution in Ni-Mn-Sn films. • Large increase in ΔS_m and RC across T_C with Co substitution is reported for 360 nm film. Mn rich Ni-Mn-Sn and Ni-Mn-Co-Sn alloy films were deposited on Si (1 0 0) substrate by dc magnetron sputtering from Ni₅₀Mn₃₇Sn₁₃ alloy target at 1.8 Pa Ar pressure with 70 W dc power. Co was introduced by placing Co chips on the Ni-Mn-Sn target. As-deposited films were vacuum annealed at 823 K for 1 h. X-ray diffraction patterns of the films revealed modulated 14 M structure of the martensite phase at room temperature. Magnetic entropy change (ΔS_m) across the Curie temperature of the ferromagnetic films was estimated from initial isothermal magnetization curves using Maxwell's equation. ΔS_m and refrigeration capacity (RC) of Ni-Mn-Sn and Ni-Mn-Co-Sn films increased with increasing film thickness. Upon Co substitution in Mn/Sn site(s), ΔS_m and RC increased more remarkably. The change is more prominent in the case of 360 nm films, wherein a 3.8-fold increase in ΔS_M and 8.9-fold increase in RC was observed. Introduction of Co increased the magnetic moment and broadened the magnetic transition. These factors increased ΔS_m and RC in Co substituted Ni-Mn-Sn alloy. Since T_C shifted to higher temperatures with Co substitution, operating temperature of these magnetic refrigerants also shifted to higher temperature. This study indicates the possibility of developing high temperature cooling devices and waste energy harvesters using these films.