Anomalous and topological Hall effects in epitaxial thin films of the noncollinear antiferromagnet Mn_3Sn

摘要

Noncollinear antiferromagnets with a D0_19 (space group = 194, P6_3/mmc) hexagonal structure have garnered much attention for their potential applications in topological spintronics. Here, we report the deposition of continuous epitaxial thin films of such a material, Mn_3Sn, and characterize their crystal structure using a combination of x-ray diffraction and transmission electron microscopy. Growth of MnjSn films with both (0001) c-axis orientation and (4043) texture is achieved. In the latter case, the thin films exhibit a small uncompensated Mn moment in the basal plane, quantified via magnetometry and x-ray magnetic circular dichroism experiments. This cannot account for the large anomalous Hall effect simultaneously observed in these films, even at room temperature, with magnitude σ_(xy)(μ_0H =0T) = 21 Ω~(-1) cm~(-1) and coercive field μ_0H_c = 1.3 T. We attribute the origin of this anomalous Hall effect to momentum-space Berry curvature arising from the symmetry-breaking inverse triangular spin structure of Mn_3Sn. Upon cooling through the transition to a glassy ferromagnetic state at around 50 K, a peak in the Hall resistivity close to the coercive field emerges. This indicates the onset of a topological Hall effect contribution, arising from a nonzero scalar spin chirality that generates a real-space Berry phase. We demonstrate that the polarity of this topological Hall effect, and hence the chiral nature of the noncoplanar magnetic structure driving it, can be controlled using different field-cooling conditions.