Magnetoresistance Anisotropy of Polycrystalline Cobalt Films: Geometrical-Size- and Domain-Effects

TL;DR

This study investigates the anisotropic magnetoresistance (AMR) in polycrystalline cobalt films, revealing a geometrical-size-effect (GSE) linked to spin-orbit interactions and domain structures, with implications for understanding magnetic scattering.

Contribution

It demonstrates the existence of a geometrical-size-effect in AMR of Co films and links it to spin-orbit interaction effects and domain structures, expanding understanding of MR anisotropy.

Findings

Polar AMR can be twice as large as transverse AMR at saturation.

The GSE ratio remains unchanged with temperature despite overall AMR reduction.

Domain structures significantly influence MR behavior below magnetic saturation.

Abstract

The magnetoresistance (MR) of 10 nm to 200 nm thin polycrystalline Co-films, deposited on glass and insulating Si(100), is studied in fields up to 120 kOe, aligned along the three principal directions with respect to the current: longitudinal, transverse (in-plane), and polar (out-of-plane). At technical saturation, the anisotropic MR (AMR) in polar fields turns out to be up to twice as large as in transverse fields, which resembles the yet unexplained geometrical size-effect (GSE), previously reported for Ni- and Permalloy films. Upon increasing temperature, the polar and transverse AMR's are reduced by phonon-mediated sd-scattering, but their ratio, i.e. the GSE remains unchanged. Basing on Potters's theory [Phys.Rev.B 10, 4626(1974)], we associate the GSE with an anisotropic effect of the spin-orbit interaction on the sd-scattering of the minority spins due to a film texture. Below magnetic saturation, the magnitudes and signs of all three MR's depend significantly on the domain structures depicted by magnetic force microscopy. Based on hysteresis loops and taking into account the GSE within an effective medium approach, the three MR's are explained by the different magnetization processes in the domain states. These reveal the importance of in-plane uniaxial anisotropy and out-of-plane texture for the thinnest and thickest films, respectively.