High-field magnetoresistance, giant magnetoresistance, and superparamagnetism in Co/Cu multilayers

Abstract

High-field magnetoresistance (HMR) in fields up to μ0H=16 T was measured on dc magnetron sputtered Co/Cu multilayers (MLs) with tCu=0.6–1.1 nm and tCo=0.3–4 nm, in the temperature range T=4.2–300 K. With decreasing single layer thickness the HMR at 300 K exceeds 10% and it is far from saturation up to μ0H=16 T. Independent of the HMR in the low field region μ0H⩽3 T a strongly Cu layer dependent GMR=0–60% is observed. Decreasing the temperature below 150 K down to 4.2 K the HMR changes into a GMR type field dependence with pronounced saturation. The superparamagnetic-like HMR dependence on H and T for high temperatures does not scale with H/T. Kerr microscopy on MLs, showing both HMR and GMR, reveals ferromagnetically and antiferromagnetically coupled domains, whereas in MLs showing HMR but not GMR only large ferromagnetic domains are visible. The results, including magnetization and roughness measurements, suggest the existence of very small Co clusters (almost in the atomic scale) with canted magnetic moments, causing the HMR to be independent of the antiferromagnetic and ferromagnetic coupling of the Co layers, respectively.