Large Anomalous Unidirectional Magnetoresistance in a Single Ferromagnetic Layer

Abstract

Unidirectional magnetoresistance (UMR) in a ferromagnetic bilayer due to the spin Hall effect (SHE) provides a facile means of probing in-plane magnetization to avoid complex magnetic tunnel junctions. However, the UMR signal is very weak and usually requires a lock-in amplifier for detection, even in a bilayer involving $\mathrm{Ta}$ or $\mathrm{Pt}$ with a large spin Hall angle (SHA). Here, we report a type of UMR, termed the anomalous UMR (AUMR), in a single $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ layer without any adjacent SHE layers, where the UMR signal is about 10 times larger than that in $\mathrm{Ta}/\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ structures and can be detected by using conventional dc multimeters in the absence of lock-in amplifiers. We further demonstrate that the extracted AUMR, by excluding thermal contributions, shows reversal signs for the $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ and $\mathrm{Ni}\text{\ensuremath{-}}\mathrm{Fe}$ single layers with opposite SHAs, indicating that the AUMR may originate from the self-generated spin accumulation interacting with magnetization through the giant-magnetoresistance-like mechanism. These results suggest that the AUMR contributes UMR signals larger than the interfacial spin Hall UMR in the $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$-based systems, providing a convenient and reliable approach to detect in-plane magnetization for two-terminal spintronic devices.