Disorder-assisted Spin-Filtering at Metal/Ferromagnet Interfaces: An Alternative Route to Anisotropic Magnetoresistance

Abstract

We introduce a minimal interface-scattering mechanism that produces a sizable anisotropic magnetoresistance (AMR) in metal/ferromagnet bilayers (e.g., Pt/YIG) without invoking bulk spin or orbital Hall currents. In a $δ$-layer model with interfacial exchange and Rashba spin-orbit coupling, charge transfer at a high-quality interface creates a spin-selective phase condition (interfacial spin filtering) that suppresses backscattering for one spin projection while enhancing momentum relaxation for the other. The resulting resistance anisotropy peaks at an optimal metal thickness of a few nanometers, quantitatively reproducing the thickness and angular dependences typically attributed to spin Hall magnetoresistance (SMR), as well as its characteristic magnitude. Remarkably, the maximal AMR scales linearly with the smaller of the two coupling strengths - exchange or spin-orbit, highlighting a mechanism fundamentally distinct from SMR. Our scattering formulation maps onto Boltzmann boundary conditions and predicts other clear discriminants from SMR, including strong sensitivity to interfacial charge transfer and disorder.

Figures (4)

• Figure 1: Schematic of spin filtering for charge flow parallel to a metal/ferromagnet interface. Interfacial disorder strongly relaxes the momentum of one spin projection, whereas the other experiences nearly specular reflection (minimal momentum loss). The effect is maximized when one of the spin channels acquires interface scattering phase equal $\pi$.
• Figure 2: The AMR constant $\mathrm{\Delta}\rho/\rho$ as a function of the charge transfer parameter $u_0$ for different values of the interface exchange and Rashba couplings, $\gamma$ and $\lambda$. Both figures correspond to a sufficiently narrow metal film with $W/\ell=0.5$. Large AMR signal of both signs is observed for $2u_0\simeq -\sqrt{U_0/E_\textrm{F}}$ and $\gamma\simeq \lambda$.
• Figure 3: AMR constant as a function of film thickness $W/\ell$ for three values of interface exchange parameter $\gamma$ and for $E_\textrm{F}/U_0=0.8$, $\lambda = 0.01$, $2u_0=-\sqrt{U_0/E_\textrm{F}}$.
• Figure 4: Spatial dependence of the spin current density $\sigma_y v_z$ for vanishing exchange parameter $\gamma=0$, $u_0=-0.4$, $E_\textrm{F}/U_0=0.8$, and film thickness $W/\ell=8$.