Disorder-Assisted Spin Filtering at Metal-Ferromagnet Interfaces: An Alternative Route to Anisotropic Magnetoresistance.

We demonstrate that anisotropic magnetoresistance (AMR) in metal-ferromagnet bilayers can arise entirely from interfacial scattering, without invoking bulk spin Hall or inverse spin Hall effects. Using a minimal boundary-value formulation of the Boltzmann equation with interfacial exchange and Rashba spin-orbit coupling, we identify a resonant spin-filtering mechanism whereby one spin projection becomes immune to backscattering at a clean interface. This mechanism quantitatively reproduces the magnitude (Δρ/ρ∼10^{-4}-10^{-3}), thickness dependence, and angular symmetry conventionally attributed to spin Hall magnetoresistance in Pt/YIG. Crucially, the maximal AMR scales linearly in the smaller of the interfacial exchange or spin-orbit coupling: a parametric behavior forbidden in any spin-Hall-based theory. The effect is intrinsically sensitive to interface quality, charge transfer, and disorder, providing clear signatures that can be experimentally tested. Our results establish interfacial spin filtering as an essential and previously overlooked origin of AMR in metal-ferromagnet heterostructures, calling for a fundamental reassessment of magnetotransport mechanisms in spin-orbit-coupled metals.