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

The room-temperature magnetoresistance (MR) and magnetization characteristics were investigated for electro-deposited 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 (Hp) with Co-content was observed for d(Cu) = I nm up to a Co:Ni ratio of 1:1, beyond which H-p increased faster than H-c, with H-p approximate to 2H(c) when the ratio reached similar to4:1. 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 2H(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.