Structural characterization of NiMnX (X:In,Sn) thin film alloys

Ferromagnetic Ni-Mn-X (Sn, In, and Sb) alloys are predicted as potential candidates for energy efficient Magnetocaloric Effect (MCE) technologies. The MCE is the basis of magnetic refrigeration and it is expected to leads to a groundbreaking progress on conventional refrigeration methods. In our research, NiMnX (Sn, In) thin film alloys were for first time fabricated by co-sputter deposition method. The Mn losses due to the high Mn vapor pressure produce a deviation from the desired Ni50Mn37X13 (Sn, In) composition, which are partially compensated by increasing power of the Mn target gun. A systematic study of thin film co-sputter fabrication was divided into three stages; i) a NiMnX (Sn, In) target was evaporated at different temperatures in order to study the grain size temperature dependence, ii) the nominal chemical composition (Ni50Mn37X13) was reach by controlling the power value applied to the Mn target, iii) the dependence of the phase transformation temperature with the grain size of the thin film alloys was studied. Grain size can be controlled by modifying the substrate temperature (Ts). The crystal structure is highly dependent on composition and 10-14M monoclinic crystal structure can be only found for when the Sn and In values range from 10 to 13 %. Austenite-martensite transformation, and therefore magnetocaloric effect, can only be founded for this compositional range. Our results confirm that nominal composition, Ni50Mn37Sn13, can be reached in thin films for given co-sputter parameter. As expected NiMnSn alloys in the shape of thin films only exhibit austenite-martensite transformations in a narrow compositional range. Further studies are required to confirm the potentiality of this material as a potential candidate for magnetic cooling technology.