Influence of the Film Thickness and the Composition On the Martensitic Transition in Ni-Mn-Sn Thin Films

Two sample series of Ni-Mn-Sn thin films have been prepared in order to investigate the influence of the film thickness and composition on the martensitic transition. Epitaxial thin films with thickness ranging from 10 nm to 200 nm have been deposited on heated MgO substrates by co-sputtering from elemental targets.
The investigated compositions are Ni_51.6Mn_32.9Sn_15.5 (series A) and Ni_51.6Mn_34.9Sn_13.5(series B).

The films are investigated by temperature dependent resistivity, magnetization and X-ray diffraction measurements.
For both series, we find that the martensite start temperature decreases with decreasing film thickness because of size scale effects and the influence of the rigid substrate. Also the resistivity change decreases with decreasing film thickness due to a thickness independent amount of residual austenite.

The major differences between series A and B are higher transition temperatures in series B and a higher hysteresis width in series A. Also series A shows a higher critical film thickness below which the martensitic transition is suppressed and a larger amount of residual austenite.

The difference in the transition temperatures is mainly due to a higher valence electron density in series B. The larger hysteresis width, critical film thickness and amount of residual austenite, however, are the results of a better sample quality close to the substrate and a better phase compatibility between martensite and austenite in series B.