Abstract
We have studied in-plane anisotropic magnetoresistance (AMR) in cobalt films with overlayers having designed electrically interface transparency. With an electrically opaque cobalt/overlayer interface, the AMR ratio is shown to vary in inverse proportion to the cobalt film thickness; an indication that in-plane AMR is a consequence of anisotropic scattering with both volume and interfacial contributions. The interface scattering anisotropy opposes the volume scattering contribution, causing the AMR ratio to diminish as the cobalt film thickness is reduced. An intrinsic interface effect explains the significantly reduced AMR ratio in ultra-thin films.
Highlights
Anisotropic magnetoresistance (AMR) was first observed by William Thomson[1] and, despite concerted study over almost 150 years[2], detailed understanding of the physical phenomenology underpinning anisotropic magnetoresistance (AMR) continues to emerge.[3,4,5] In ferromagnetic (FM) metals AMR is considered to be a consequence of the spin-orbit interaction (SOI) within the bulk and is manifest as a dependence of the electrical resistivity, ρ, on the angle θ between current density J and magnetization M
With an electrically opaque cobalt/overlayer interface, the AMR ratio is shown to vary in inverse proportion to the cobalt film thickness; an indication that in-plane AMR is a consequence of anisotropic scattering with both volume and interfacial contributions
Enhanced AMR due to structurally symmetric adjacent layers with strong SOI has been suggested by Liu et al.[3]. Overall, such results suggest that the in-plane AMR in a single ultra-thin FM metal film may have a previously unconsidered interfacial contribution, as a result of anisotropic interface scattering and/or interfacial spin-orbit interactions
Summary
Anisotropic magnetoresistance (AMR) was first observed by William Thomson[1] and, despite concerted study over almost 150 years[2], detailed understanding of the physical phenomenology underpinning AMR continues to emerge.[3,4,5] In ferromagnetic (FM) metals AMR is considered to be a consequence of the spin-orbit interaction (SOI) within the bulk and is manifest as a dependence of the electrical resistivity, ρ, on the angle θ between current density J and magnetization M. AMR in Rashba systems has been studied theoretically, and it has been shown that the Rashba SOI produces in-plane AMR in the FM layer with the same sign, i.e. ρ∥ > ρ⊥, and symmetry, i.e., cos[2] θ dependence, as ‘conventional’ AMR.[18] Unconventional AMR in an adjacent layer with strong SOI may arise due to Rashba SOI, but has a different angular symmetry to conventional AMR.[19] Enhanced AMR due to structurally symmetric adjacent layers with strong SOI has been suggested by Liu et al.[3] Overall, such results suggest that the in-plane AMR in a single ultra-thin FM metal film may have a previously unconsidered interfacial contribution, as a result of anisotropic interface scattering and/or interfacial spin-orbit interactions. Iridium has strong SOI (larger atomic number), which should enhance any Rashba contribution to the AMR These structures allow the isolation of the in-plane AMR contribution due to scattering at an electrically opaque interface — using the Co/Ir interface as an illustrative model system
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