Magnetic Behavior of Ni/Cu Multilayer Nanowire Arrays Studied by First-Order Reversal Curve Diagrams

Arrays of Ni/Cu multilayer nanowires were fabricated by electrodeposition into alumina templates (pore diameter = 175 nm, interpore distance = 300 nm), with an Ni disc thickness between 20 and 50 nm, and a Cu thickness between 10 and 35 nm. Vector major hysteresis loops were measured with an out-of-plane (along the nanowire axis) and inplane (perpendicular to the nanowire axis) applied field. During the magnetization reversal, we did not observe any net component of transverse magnetization (M_y) , perpendicular to the applied field H_x. Extrapolated first-order reversal curve (FORC) diagrams were measured, out-of-plane and inplane. While the major hysteresis loops of all arrays have similar shapes, their FORC diagrams reveal distinct patterns, suggesting different magnetic behavior. The magnetic anisotropy can be controlled by the ratio of Cu to Ni thickness, the nanowire axis becoming harder as the ratio increases. In the out-of-plane direction, the maximum interaction field is proportional to the Ni thickness. The FORCs show evidence of a large coercive field distribution, which we attribute to the smaller interaction fields in the nanodiscs along the edges of the samples. In the inplane direction, depending upon the easy axis direction, there can be a mixture of nanodiscs reversed by coherent and incoherent rotation, or by incoherent rotation only.