Evolution of ferromagnetism and metallicity in RF magnetron sputtered Mn–Ni–Sn Heusler alloy thin films

This study investigates the evolution of ferromagnetism and metallicity of Mn–Ni–Sn-based Heusler alloy thin films, approximately 40 nm thick. These films were synthesized via UHV RF magnetron sputtering using a Mn₂Ni_1.6Sn_0.4 target on (001)-oriented SrTiO₃ substrates. The deposition temperatures were set at 500 °C, 600 °C, 700 °C, and 800 °C, followed by a 6-h annealing process at the corresponding temperatures. At growth temperatures up to 700 °C, the films displayed B2 or L2₁-type structures or a combination thereof. However, at 800 °C, the Heusler phase decomposed, resulting in the formation of impurity phases such as Ni₃Sn or Mn₃Sn. The surface morphology changed from continuous to discontinuous with increasing growth temperature. All films exhibited Curie temperatures above room temperature. As the growth temperature increased from 500 °C to 700 °C, spontaneous magnetization increased substantially, and the bifurcation between zero-field-cooled (ZFC) and field-cooled warming (FCW) curves decreased. Films deposited at 500 °C, 600 °C, and 700 °C adhered to Bloch's law below 143 K (T ≪ T_C), while those deposited at 800 °C showed deviation from this. The increase in the residual resistivity ratio (RRR) values from 1.17 for the film grown at 500 °C to 1.64 for films grown at 700 °C, respectively, underscores the evolution of the metallic behavior. The phase-decomposed film showed an exceptionally high RRR = 956.59.