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

TL;DR

This study addresses how anti-site disorder (ASD) in off-stoichiometric Co-Fe-Ti-Si (CFTS) Heusler thin films regulates electrical transport and magnetoresistance across thermo-elastic martensitic transformations. By growing 100 nm films with controlled ASD (A2 vs partially ordered L2$_1$) and performing comprehensive $\rho(T)$ and $MR_{\perp}$ measurements across the martensite and austenite phases, the authors reveal a clear ASD-dependent shift from semiconducting-like to metallic transport, with a resistivity minimum near $T_{min} \approx 30$ K and a low-temperature upturn in highly disordered samples. A transport model incorporating quantum corrections (weak localization, electron-diffuson) and ballistic scattering (electron-magnon, electron-phonon) fits the data and shows stronger ASD enhances quantum corrections while suppressing $e$-$m$ and $e$-$p$ scattering; electron-electron interactions are negligible. Magnetically, $MR_{\perp}$ is antisymmetric and small in the martensite phase, but becomes symmetric and larger in the austenite phase, with a clear transition around $T_{Me} \approx 300$ K and $T_{Ab} \approx 325$ K, indicating ASMR-dominated martensite and SMR-dominated austenite, respectively. These results position CFTS thin films as promising materials for ferromagnetic shape-memory devices and spintronic applications such as spin valves, where ASD engineering tunes transport and magneto-transport properties across MPT.

Abstract

In this work, we systematically investigate the effect of Anti-site Disorder (ASD) on electrical resistivity $ρ(T)$ 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.

Figures (10)

• Figure 1: (a) Temperature-dependent zero-field resistivity for the disordered TS200 and TS350 CFTS films in the cooling cycle (blue open circles) and the heating cycle (red dots). The insets highlight the regions where thermal hysteresis occurs due to the coexistence of martensite and austenite phases. (b) Zero-field resistivity (red open circles) with theoretical fits (black solid curves) in the temperature range $5$--$105\,$K, based on Eq. (2) with $\beta_{\mathrm{e\text{-}m}} = \alpha_{\mathrm{e\text{-}p}} = 0$, for the TS200 and TS350 films. The insets provide an enlarged view of the resistivity data together with the theoretical fit in the low-temperature region.
• Figure 2: (a) Temperature-dependent zero-field resistivity for the ordered TS450, TS500, and TS550 CFTS films in both the cooling cycle (blue open circles) and the heating cycle (red dots). The insets highlight the regions where thermal hysteresis occurs due to the coexistence of martensite and austenite phases.(b) Zero-field resistivity (red open circles) with theoretical fits (black solid curves) in the temperature range $5$--$250\,$K, based on Eq. (2) for the TS450, TS500, and TS550 films. The insets show an enlarged view of the resistivity data, along with the theoretical fits, in the low-temperature region.
• Figure 3: $\mathrm{MR}_{\perp}$ as a function of temperature, calculated from $\rho(T)$ at $H = 0$ and $H = 90\,\mathrm{kOe}$ for the CFTS films TS200, TS450, and TS550. The insets display the raw $\rho(T)$ data at $H = 0$ and $H = 90\,\mathrm{kOe}$.
• Figure 4: $\mathrm{MR}_{\perp}(H)$ isotherms for (a) the completely site-disordered TS200 CFTS film and (b) the optimally site-ordered TS500 CFTS film.
• Figure 5: Individual symmetric (blue symbols) and anti-symmetric (green symbols) components of the $\mathrm{MR}_{\perp}(H)$ data (red open circles) for (a) the completely site-disordered TS200 CFTS film at $10\,$K, $250\,$K, and $350\,$K, and (b) the optimally site-ordered TS500 CFTS film at $10\,$K, $300\,$K, and $350\,$K.