Abstract
Unidirectional Spin Hall magnetoresistance (USMR) is a non-linear phenomenon recently observed in ferromagnet (FM)/nonmagnetic metal (NM) bilayer structures. Two very different mechanisms of USMR have been proposed; one relies on the current-direction-dependence of electron-magnon scattering in a FM layer, and the other on the current-direction-dependence of the spin accumulation at the FM/NM interface. In this study, we investigate the USMR in epitaxial Cr/Fe bilayers finding that the USMR is significantly enhanced when the Fe magnetization is aligned to a particular crystallographic direction where the magnon magnetoresistance (MMR) by the electron-magnon scattering becomes stronger. This highlights the importance of the electron-magnon scattering for the understanding of USMR in Cr/Fe bilayers. Our result also suggests a route to enhance the efficiency of magnon generation in the magnonic devices. Lastly, we discuss the Ising-type spin exchange as a possible origin of the crystallographic direction dependences of the USMR and the MMR.
Highlights
Unidirectional Spin Hall magnetoresistance (USMR) is a non-linear phenomenon recently observed in ferromagnet (FM)/nonmagnetic metal (NM) bilayer structures
The lattice strain in the Cr/Fe films was estimated from the high-resolution transmission electron microscopy (HRTEM) images; the tensile strain of +3.43% was induced along [0−11] direction (Fig. 1d), due to the lattice mismatch of −4.5%
For conventional 3d FMs Fe, Co, and Ni, they estimated that magnons with energies up to 150 meV are involved in the electron–magnon scattering at room temperature
Summary
Unidirectional Spin Hall magnetoresistance (USMR) is a non-linear phenomenon recently observed in ferromagnet (FM)/nonmagnetic metal (NM) bilayer structures. Later experimental observations on topological insulators/FM bilayers proposed a qualitatively different mechanism[11,14], which has no analogy to the interfacial contribution of GMR This mechanism attributes the USMR to the electron–magnon scattering; the spin current may create or annihilate magnons depending on the σ direction of spin current and the resulting change in the magnon number modulates the electron–magnon scattering contribution to the resistance. The modification of the interfacial resistance (or the spin-dependent scattering) has been taken into account to explain the observed USMR in heavy metal (HM)/FM metal bilayers, such as Pt/Co, Ta/Co9 Exploiting this mechanism, a thin Cu (or Ag) interlayer is inserted into the HM/FM interface in an attempt to enhance USMR further[15]. More persuasive experimental evidence of the electron–magnon scattering mechanism of the USMR itself are still desired
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