Origin of giant magnetoresistance contributions in electrodeposited Ni-Cu/Cu multilayers

The magnetoresistance (MR) and magnetization were studied at room temperature and down to 4.2K for N18Cu19(3 nm)/Cu(d(Cu)) multilayers with 0.5nm less than or equal to d(Cu) less than or equal to 3nm and for a bulk Ni81Cu19 alloy, all produced by electrodeposition. For most of the multilayers, giant magnetoresistance (GMR) was observed with a maximum around 1-1.5 nm Cu-layer thicknesses. The room-temperature MR curves consisted of a rapidly varying low-field component up to about 1 kOe and a slowly varying high-field component persisting beyond 18 kOe. For Cu-layer thicknesses around the maximum GMR, the low-field component dominated whereas for d(Cu) > 1.5 nm, it became comparable to or smaller than the high-field component. For the bulk alloy, anisotropic magneto resistance was observed. According to the room-temperature magnetization curves, both the bulk alloy and the multilayers exhibited ferromagnetic (FM) behaviour with technical saturation around 1-3 kOe. For the multilayers, the low-field MR curves with a small peak splitting were characteristic of uncoupled or weakly coupled magnetic layers. Magnetic measurements from 5 to 300 K indicated that besides an FM contribution, these multilayers also exhibit a superparamagnetic (SPM) component with blocking temperature around 220K. From an analysis of the present results, the low-field MR component could be attributed to the FM domains with their magnetizations exhibiting a random in-plane distribution in the zero-field (demagnetized) state and the high-field MR component to a contribution due to SPM regions. All these considerations are supported by the complete saturation of the MR in a magnetic field of 4 kOe at T = 4.2 K. (C) 2003 Elsevier B.V. All rights reserved.