Unconventional magnetoresistance and spin gapless semiconductor like behavior across the Martensitic transformation in off-stoichiometric Co-Fe-Ti-Si Heusler alloy thin films

In this work, the effect of anti-site disorder on magnetic, electrical resistivity, transverse magnetoresistance MR, and anomalous Hall resistivity of off-stoichiometric CFTS Heusler alloy thin films, with a particular focus on martensitic phase transformation and spin gapless semiconductor SGS-like behavior, is investigated. These thin films were grown on Si (100) substrate at different substrate temperatures, TS, ranging from 200 C to 550 C using magnetron sputtering, enabling control over the degree of anti-site atomic ordering from disordered A2 to ordered L21. All the films, irrespective of their disorder, exhibit a distinct thermal hysteresis and significant drop in resistivity, crossover from asymmetric to symmetric magnetoresistance, and a sharp increase in MR around 300 K, confirming the occurrence of a thermo-elastic martensitic phase transformation. Detailed analysis of resistivity data indicates that for TS200 and TS350 films, a SGS based two channel model describes the conductivity in the martensite phase, whereas TS450, TS500, and TS550 films, exhibit a usual metallic behavior with a resistivity minimum at low temperatures. All the CFTS films show soft ferromagnetic nature and follow the spin-wave equation up to 390 K. The saturation magnetization and Hall conductivity increase with increasing crystalline order. The scaling relation between the longitudinal resistivity and the anomalous Hall resistivity in the martensite phase revels that skew scattering is the dominating contribution in disordered films, and a change in charge carrier type from hole to electron around the martensitic transformation temperature. The asymmetric MR, persistent up to room temperature, highlights the potential of these films for spintronic applications such as spin valves.