Electrical- and magneto-transport across the thermo-elastic martensitic transformation in anti-site-disordered off-stoichiometric Co-Fe-Ti-Si Heusler alloy thin films
In this work, we systematically investigate the effect of Anti-site Disorder (ASD) on electrical resistivity $\rho(T)$ and transverse magnetoresistance $MR_{\perp}$ in off-stoichiometric Co-Fe-Ti-Si (CFTS) thin films across the thermo-elastic martensitic phase transformation (MPT). The CFTS films with A2 ASD exhibit a negative temperature coefficient of resistivity (n-TCR) and an upturn below $\sim 30\,$K. In sharp contrast, the partially L2$_1$-ordered films are metallic in nature, characterized by a resistivity minimum at low temperatures ($T_{\min} \cong 30\,$K) and a positive TCR for $T > T_{\min}$. The change in the sign of TCR finds a straightforward explanation in terms of the competition between the quantum corrections (weak localization, electron-diffuson scattering) and the ballistic scattering mechanisms (electron-magnon and electron-phonon). We find that, stronger the atomic ASD, more prominent the quantum corrections and the weaker the scattering of $e-m$ and $e-p$ scattering. All the CFTS films exhibit a distinct thermal hysteresis and a significant drop in resistivity, symptomatic of a MPT, near the characteristic temperatures: martensite-end $T_{Me} \cong 300\,$K and austenite-begin $T_{\mathrm{Ab}} \cong 325\,$K. Regardless of the strength of ASD, in the martensite phase the anti-symmetric (ASMR) component of $\mathrm{MR}_{\perp}(H)$ dominates over the symmetric (SMR) counterpart, whereas the reverse is true (i.e. SMR $\gg$ ASMR) for the austenite phase at temperatures $T_{\mathrm{Ab}} \cong 325\,$K $\le T \le 375\,$K, where $\mathrm{MR}_{\perp}$ increases very sharply with temperature as the austenite phase grows rapidly at the expense of the martensite phase. The present results assert that the CFTS Heusler alloy thin films are promising candidates for shape-memory devices and for spintronic applications such as spin valves.