Magnetic relaxation in nano-particle stacks

Abstract

The remanent magnetisation and magnetic relaxation of Ni-Cu/Cu superlattice nanowires have been investigated. Arrays of superlattice nanowires were prepared by template deposition through polycarbonate nanoporous membranes using a single electrolyte bath. The thicknesses of nickel-rich layers (tNi) and copper layers (tCu) were independently controlled by monitoring the current during deposition. A study of the remanent magnetisation at 5K for tNi = 30Å and a range of values of tCu reveals the existence of inter-layer demagnetising interactions within each array. However the demagnetising interaction strength appears to reach a minimum level, believed to be due to intra-layer interactions caused by island formation within nickel-rich layers. Magnetic relaxation measurements on the same arrays after removal of a saturating (5T) field at various temperatures show M to decrease linearly with ln(t). The data were analyzed using the T ln(t/τ0) scaling technique, revealing the effective energy barrier distribution of the arrays to be constructed of two components, possibly due to non- (or weakly-) interacting particles and strongly-interacting particles respectively. The weakly-interacting component is observed to decrease with decreasing tCu and is believed to be caused by large individual nickel islands (corresponding to inter-layer interactions), while the strongly-interacting component is believed to be due to fragmented nickel islands (corresponding to intra-layer interactions).