Anisotropic magnetoresistance and anomalous Nernst effect in antiferromagnetic MnTe thin films*

The NiAs-type MnTe, an antiferromagnetic semiconductor, is theoretically predicted to exhibit exotic magneto-transport properties. For spintronic applications, the easy-plane magnetic anisotropy offers a unique advantage of having three possible Neel vector orientations for multi-state memory. However, the electrical detection and manipulation of the antiferromagnetic order parameter have not been explored. Here, we report a magneto-transport study carried out in MnTe thin films which are epitaxially grown on GaP  substrates using molecular beam epitaxy. The Hall bar devices are fabricated on a 75 nm thick MnTe film by photolithography and Ar plasma dry etching. The four-terminal DC resistance vs. temperature curve shows a well-defined peak at 290 K, indicating the Neel temperature of the MnTe film. By performing the longitudinal resistance Rxx with a rotating in-plane magnetic field of fixed magnitude, we obtain Rxx oscillations as a function of the azimuthal angle. Even with a current density as small as ~2×10⁵ A/m², we observe a transition of the oscillation period from π to 2π as the field strength exceeds ~ 5 T. At low fields, the π-oscillation is characteristic of the anisotropic magnetoresistance due to the canted magnetic moment along the magnetic field direction.  At high fields, the 2π oscillation indicates the anomalous Nernst effect under a small vertical temperature gradient produced by the Joule heating. In the intermediate field range, the Rxx displays a complex pattern, which is attributed to the role of in-plane anisotropy. Discussions about the underlying physical mechanism of the observed phenomena will be presented.