Microstructure, Magnetism and Electrical Transport Properties of Mn-Ni-Sn based Heusler Alloy Thin Films Grown Using RF Magnetron Sputtering: Consequences of Annealing Conditions

Abstract

The study investigates the effects of annealing on the structural, morphological, magnetic, and transport properties of Mn-Ni-Sn-based Heusler alloy thin films grown by UHV RF Magnetron sputtering. A commercial target with the nominal composition Mn₂ Ni_1.6Sn_0.4 was used, and the films were deposited on (001) oriented SrTiO₃ substrates. Thin films were deposited at 500 °C, 600 °C, 700 °C, and 800 °C and in situ annealing was done at the respective deposition temperatures for 6 hours. X-ray reflectivity indicated a deposition rate of »4 nm/min. The films exhibited B2 or L2₁-type structures, or a mixture of both, depending on the annealing temperature. At the highest growth temperature (800 °C), additional diffraction maxima between 40-45° were likely due to Ni₃Sn or Mn₃ Sn impurity phases, suggesting thermally activated decomposition. Surface microstructures consisting of dark and bright regions evolved from continuous to discontinuous morphology with the increase of the growth temperature. The bifurcation between zero field-cooled (ZFC) and field-cooled warming (FCW) curves decreased, and the magnetic moment increased with deposition temperatures up to 700 °C. The Curie temperature for all films was above room temperature. Films grown at 500 °C, 600 °C, and 700 °C followed the Bloch law below 143 K. However, the film grown at 800 °C, followed this law between 14 K and 75 K. Films grown up to 700 °C behaved like a local magnetic moment system, which is crucial for spin polarization in Heusler systems. Phase degeneration at 800 °C destroyed the half-metallic behavior. All films showed metallic behavior with different resistivity and temperature dependence. Residual Resistivity Ratio (RRR) values were 1.17, 1.51, and 1.64 for films grown at 500 °C, 600 °C, and 700 °C, respectively. The phase degenerated film showed the steepest decline in resistivity, with an exceptionally high RRR of approximately 956.59.