Within magnetic materials, anisotropic magnetoresistance (AMR) measures the dependence of the resistivity on the angle between the high magnetic susceptibility direction of the magnetic order and the current direction. When analyzed using spectral decomposition, the AMR of most magnetic materials is composed of even harmonics the most common of which are 2-fold (C2) and 4-fold (C4) arising from s-d scattering and crystal field effects respectively. The generation of odd harmonics in the spectral decomposition of the AMR are observed in magnetic films when the magnetic film is combined with another non-magnetic material to form a heterostructure. The observed unidirectional AMR is a result of the explicit breaking of the spatial inversion symmetry at the interface of the engineered heterostructure. Alternatively, the spatial inversion symmetry may be broken internally within the magnetic film under the application of a magnetic field leading to the observation of signatures of unidirectional AMR in the spectral decomposition without the presence of the non-magnetic material. In this work, we observe odd spectral harmonics (C1 and C3) associated with the presence of unidirectional AMR in 20 nm thin-films of antiferromagnetic FeRh measured at 20K, as seen in Fig. 1(A). In Fig. 1(B), the presence of the odd spectral harmonics is confirmed using a tight-binding representation of the tetragonal FeRh lattice in the antiferromagnetic phase where the transport quantities are calculated using the non-equilibrium Green’s function formalism. Furthermore, we attribute the presence of the C1 and C3 harmonics to the magnetic moment that the Rh atoms develop when immersed in an external in-plane magnetic field. We quantify the size of the inherent unidirectional AMR component as a function of the applied magnetic field and show that the internal unidirectional AMR component is non-negligible and omnipresent in AMR measurements even in the absence of non-magnetic layers.
References J. Oh, L.Humbard, V. Humbert, AIP Advances 9, 045016 (2019) T. Huong T. Nguyen, V.Nguyen, Communications Physics volume 4, Article number: 247 (2021)