Microstructural and magnetic properties of epitaxial Ni₅₀Mn_37/35Sn_13/15 Heusler alloy thin films grown by pulsed laser deposition

Ni₅₀Mn₃₇Sn₁₃ and Ni₅₀Mn₃₅Sn₁₅ epitaxial alloy thin films with two different compositions are grown on c plane sapphire substrate by pulsed laser deposition. The epitaxial relationship between untransformed L2₁and sapphire is α-Al₂O₃ [011¯0] ‖ L2₁ [011]. The Curie temperatures of the films are 316 and 334 K, respectively. The microstructure of the films consists of untransformed austenite (L2₁) and three different martensite with L1₀, 4O and 5M structures. The relative volume fractions of four predominant phases are ∼50, 40, 5 and 5%, respectively at room temperature for both the films. The epitaxial L2₁ phase grows under tensile strain on sapphire (0 0 0 1) substrate and favours the formation of predominantly L1₀ over other martensite phases below the transition temperature. The interface between various untransformed and transformed phases are found to be coherent. The overall experimental magnetization of the thin films is slightly less than the theoretical values. Local HREELS spectra of Mn and Ni L_3,2 absorption edges confirm the presence of some non-magnetic phases in the structure particularly in 4O and 5M form of martensite might account for the observed differences with the theoretical value.