Symmetry of transversal conductivity signal in antiferromagnetic Mn5Si3

Abstract

The Hall effect family is of fundamental importance among the magneto-transport phenomena, as it represents a way to dissipationless transport. [1] Hall effects manifest as a transverse component to conductivity, perpendicular to the direction of the electrical current. Apart from the long-known ordinary Hall effect that is present in all conductive materials, the anomalous Hall effect was a long time thought to be exclusive to ferromagnetic systems. In the last years, the Hall effect family has been explored in materials with zero net magnetic moment. Both anomalous Hall effect [2] and topological Hall effect were detected [3] also in non-collinear antiferromagnets (AFM). Very recently, a novel type of spontaneous Hall effect was reported also in the collinear phases of antiferromagnetic Mn5Si3 [4] and RuO2 [5]. All the above-mentioned Hall effects rely on detection of transversal voltage which is expected to be odd in magnetic field. Any even contribution to the transversal voltage is often considered as an artefact related to misalignment of the Hall cross and, consequently, only antisymmetrized transversal voltage data are presented. However, it has been theoretically shown [6] that for certain low-symmetry magnetic systems, even components would be present in the transverse voltage data.In our presentation, we aim to experimentally demonstrate the presence of the even contribution to transversal resistance in a low-symmetry antiferromagnetic phase of Mn5Si3 indicating that the symmetry of the signal could serve as a probe of the degree of spin symmetry of the material. The ratio between odd and even contributions in various samples will be shown and the role of the crystal quality will be discussedMn5Si3 is a compensated antiferromagnet that can order both collinearly and non-collinearly in different temperature regimes, with the two phase transitions at TN1 ≈ 70 K and TN2 ≈ 200 K in thin-film layers [4]. Our samples are 20 nm thick epilayers, prepared by molecular beam epitaxy on Si(111) substrate, which results in the c-axis of Mn5Si3 to be oriented perpendicular to the sample plane. The layers were patterned to Hall bar devices with a channel width of 10 µm using standard laser lithography and Ar+ milling. Both longitudinal (ρxx) and transverse (ρxy) resistivities were measured simultaneously as a function of external magnetic field Bz, applied in the out-of-plane direction.The longitudinal and transverse signals measured at two different temperatures corresponding to the collinear (180 K) and non-collinear (40 K) antiferromagnetic phase of Mn5Si3, are shown in Fig. 1(a) and (b), respectively. Note that all the curves presented in Fig. 1 were shifted for clarity, and the ordinary Hall resistance was removed from ρxy by subtracting the signal linear in Bz.The longitudinal signal Δρxx (see Fig.1(a)) corresponds to the magnetoresistance (MR). The strong magnetoresistance observed at 40 K is significantly suppressed in the collinear phase at 180 K in accord with the expected behaviour [5]. In contrast, the transverse resistivity persists even in the collinear phase (see Fig 1(b)), with a clear hysteretic behaviour and coercive field of 2 T. This spontaneous Hall response has recently been attributed to a novel antiferromagnetic Zeeman effect [4].As for the case of any Hall effect, generated transverse voltage is expected to be odd in the inducing magnetic field. However, the data in Fig. 1 (b) clearly contain both odd and even component. This feature is highlighted in Fig. 2, where the corresponding hysteresis curves are split into symmetric (a) and antisymmetric (b) parts. Notably, the symmetrical component that reflects signals even Bz is comparable in magnitude to the antisymmetrical one at 40 K, but disappears entirely at 180 K. To exclude the possible origin in MR-related artefacts, we compare the shapes of the ρxx and even ρxy by subtraction the normalized curves, i.e. Δρ(Bz) = ρxx(Bz)/ ρxx(0) - Δρ(sym)xy(Bz)/ ρ(sym)xy(0). The resulting non-trivial curve, shown in Fig. 2(c), demonstrates the difference in shapes of MR and even transverse voltage. The comparison is, however, possible only in the low-temperature regime, as the transverse detected at 180 K do not display any sizable even component.This observation is in agreement with the assumed lowering of the symmetry in the non-collinear phase of Mn5Si3. As shown in [6], the shape of the linear response tensor can be extended to deal with more complex situations, such as the low magnetic symmetry of the Mn5Si3. The transversal voltage does not need to show purely antisymmetric elements. **