Giant magnetoresistance and magnetic properties of electrodeposited Ni–Co–Cu/Cu multilayers

Abstract

The room-temperature magnetoresistance (MR) and magnetization characteristics were investigated for electrodeposited Ni–Co–Cu(3 nm)/Cu( d Cu) multilayers with d Cu=1 and 2 nm as a function of the ratio of Co to Ni in the magnetic layer. The maximum giant magnetoresistance (GMR) was obtained when the Co- and Ni-contents of the magnetic layer were approximately equal for d Cu=1 nm, whereas a significantly smaller GMR with no systematic dependence on Co-content was observed for d Cu=2 nm. Concurrent increase of the coercive field ( H c) and the MR peak position ( H p) with Co-content was observed for d Cu=1 nm up to a Co:Ni ratio of 1:1, beyond which H p increased faster than H c, with H p≈2 H c when the ratio reached ∼4:1. For films containing approximately equal quantities of Co and Ni, the MR vs. H curves could be successfully fitted by a Langevin function. This was interpreted by ascribing the magnetization contribution for magnetic fields above about 2 H c to superparamagnetic (SPM) regions which form due to the electrochemical deposition conditions between the non-magnetic Cu layer and the ferromagnetic (FM) Ni–Co–Cu layer. The formation of such intermixed interfaces is a general phenomenon in electrodeposited multilayers, leading to a strongly reduced antiferromagnetic coupling of the magnetizations of the neighbouring FM layers. In such cases, the observed GMR curves exhibit a typical concave shape and arise due to the slowly saturating SPM behaviour at the intermixed interfaces.