Structural ordering driven anisotropic magnetoresistance, anomalous Hall resistance, and its topological overtones in full-Heusler Co2MnSi thin films

Abstract

We report the evolution of crystallographic structure, magnetic ordering, and electronic transport in thin films of full-Heusler alloy Co2MnSi deposited on (001) MgO with annealing temperatures (TA). By increasing the TA from 300 °C to 600 °C, the film goes from a disordered nanocrystalline phase to B2 ordered and finally to the L21 ordered alloy. The saturation magnetic moment improves with structural ordering and approaches the Slater-Pauling value of ≈5.0μB per formula unit for TA=600 °C. At this stage, the films are soft magnets with coercive and saturation fields as low as ≈7 mT and 350 mT, respectively. Remarkable effects of improved structural order are also seen in longitudinal resistivity (ρxx) and residual resistivity ratio. A model based upon electronic transparency of grain boundaries illucidates the transition from a state of negative dρ/dT to positive dρ/dT with improved structural order. The Hall resistivity (ρxy) derives contribution from the normal scattering of charge carriers in external magnetic field, the anomalous effect originating from built-in magnetization, and a small but distinct topological Hall effect in the disordered phase. The carrier concentration (n) and mobility (μ) have been extracted from the high field ρxy data. The highly ordered films are characterized by n and μ of 1.19 × 1029 m−3 and 0.4 cm2V−1s−1 at room temperature. The dependence of ρxy on ρxx indicates the dominance of skew scattering in our films, which shows a monotonic drop on raising the TA. The topological Hall effect is analyzed for the films annealed at 300 °C. We find maximum topological contribution to Hall resistivity around 0.5 T while it approach to zero with increasing magnetic field. The anisotropic magnetoresistance changes its sign from positive to negative as we go from as deposited to 600 °C annealed film, suggesting a gradual increase in the half-metallic character.