Unraveling the roles of thermal annealing and off-time duration in magnetic properties of pulsed electrodeposited NiCu nanowire arrays

Abstract

Magnetic alloy nanowires (ANWs) have long been studied owing to both their fundamental aspects and possible applications in magnetic storage media and magnetoresistance devices. Here, we report on the roles of thermal annealing and duration of off-time between pulses (toff) in crystalline characteristics and magnetic properties of arrays of pulsed electrodeposited NiCu ANWs (35 nm in diameter and a length of 1.2 lm), embedded in porous anodic alumina template. Increasing toff enabled us to increase the Cu content thereby fabricating NiCu ANWs with different crystallinity and alloy compositions. Although major hysteresis curve measurements showed no considerable change in magnetic properties before and after annealing, the first-order reversal curve (FORC) analysis provided new insights into the roles of thermal annealing and toff. In other words, FORC diagrams indicated the presence of low and high coercive field regions in annealed Ni-rich ANWs, coinciding with the increase in toff in as-deposited ANWs. The former has a small coercivity with strong demagnetizing magnetostatic interactions from neighboring NWs and may correspond to a soft magnetic phase. The latter has a greater coercivity with weak interactions, corresponding to a hard magnetic phase. On the other hand, for as-deposited and annealed Cu-rich NiCu ANWs, a mixed phase of the soft and hard segments could be found. Furthermore, a transition from the interacting Ni-rich to non-interacting Cu-rich ANWs took place with a magnetic field applied parallel to the NW axis. Thus, these arrays of ANWs with tunable magnetic phases and interactions may have potential applications in the nanoscale devices.