Electrical-and magneto-transport across the thermo-elastic martensitic transformation in anti-site-disordered off-stoichiometric Co-Fe-Ti-Si Heusler alloy thin films

Anti-site disorder (ASD), present in Heusler alloy thin films, is known to significantly affect their physical properties, but a complete understanding of the actual role of ASD is lacking. In this work, we systematically investigate the effect of ASD on electrical resistivity ρ(T ) and transverse magnetoresistance MR ⊥ in off-stoichiometric Co-Fe-Ti-Si (CFTS) thin films across the thermoelastic martensitic phase transformation (MPT). The CFTS films with A2 ASD exhibit a negative temperature coefficient of resistivity (n-TCR) and an upturn below ∼ 30 K. In sharp contrast, the partially L21-ordered films are metallic in nature, characterized by a resistivity minimum at low temperatures (Tmin ∼ = 30 K) and a positive TCR for T > Tmin. 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 TMe ∼ = 300 K and austenite-begin TAb ∼ = 325 K. Regardless of the strength of ASD, in the martensite phase the anti-symmetric (ASMR) component of MR ⊥ (H) dominates over the symmetric (SMR) counterpart, whereas the reverse is true (SMR ≫ ASMR) for the austenite phase at temperatures TAb ∼ = 325 K ≤ T ≤ 375 K, where MR⊥ 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.