Disordered spin gapless semiconducting CoFeCrGa Heusler alloy thin films on Si (100): experiment and theory

Abstract

Spin gapless semiconductors (SGSs) are an intriguing class of quantum materials that bridge the gap between half-metallic ferromagnets and semiconductors. The presence of a semiconducting bandgap for one spin channel and zero band gap for other spin channels, together with the possibility of four different band structure configurations, makes them one of the most desirable candidates to be used in tunable spin transport based spintronics devices. Here, we have performed various structural, magnetic and transport measurements on an optimized CoFeCrGa (CFCG) Heusler alloy thin film (∼50 nm) grown over a Si(100) substrate using an industry-viable magnetron sputtering technique. The grown film showed B2-ordering under the given set of X-ray diffraction measurement conditions with a saturation magnetization (M_s) of 1.86μ_B per f.u. (at 5 K) and a Curie temperature of ∼595 K. Nearly linearly varying longitudinal resistivity with a negative temperature coefficient was observed. A fitted longitudinal conductivity curve through a “two-carrier model” shows a slight band overlap in the gapless channel for one spin channel and a small energy gap (ΔE) of 167 meV for other spin channels. A negative and linear out-of-plane magnetoresistance response was observed in these films. The temperature dependent anomalous Hall effect measurement gives nearly temperature independent carrier concentration (and/or) mobility with an anomalous Hall conductivity of 91.35 S cm⁻¹ at 5 K. The first principles calculations have also been performed for bulk and (220) CFCG surfaces to correlate the various structural, electronics and magnetic properties of the optimized CFCG Heusler alloy thin film. The DFT derived results, viz. lattice parameter and M_S exhibit a good match with the experimentally observed results. All these properties collectively imply that the grown film possesses disordered-SGS like behaviour. It is remarkable to note that CFCG films with the (022) surface possess a very high electronic spin polarization of 91%. The results of the study suggest that CFCG is a potential candidate to be used in spintronics-based devices such as spin-injectors.