Sign reversal of unidirectional magnetoresistance in monocrystalline Fe/Pt bilayers

The discovery of unidirectional magnetoresistance (UMR) in ferromagnet/heavy metal heterostructures provides one possibility to detect the magnetization direction with a simple two-terminal geometry. We investigated the temperature and thickness dependence of UMRin single-crystalline Fe/Pt bilayers and discovered an obvious sign reversal when increasing the Fe thickness at low temperature. Meanwhile, the same phenomenon is observed when increasing temperature for thick Fe samples. All the UMR mechanisms are quantitatively analyzed and the UMR contribution induced by the thermal effect shows a thickness-dependent sign change, thus the sign reversal of UMR is attributed to the competition between the contributions on UMR from the thermal effect and the spin accumulation induced by the spin Hall effect in the Pt layer. Our results emphasize the thermal contribution to the UMR in ferromagnet/heavy metal heterostructures and suggest a promising way to tune the UMR, which will promote its application in information storage devices.