Magnetic anisotropy in CoNi nanowire arrays: Analytical calculations and experiments

Ordered arrays of Co${}_x$Ni${}_{1-x}$ nanowires ($0<x<1$) were fabricated by a template-assisted method using electrodeposition into anodic aluminum oxide membranes. Tuning of the Co-alloy composition by changing the Ni content enables control of the effective anisotropy axis, which is determined by the balance between the hcp and fcc magnetocrystalline and shape anisotropies. We report on the nanowires’ structural and magnetic properties (e.g., hysteresis curves and their parameters as well as first-order reversal curve analysis), paying particular attention to their angular dependence. It is confirmed that the crystal phase of nanowires with length 2.5 $\mu$m and diameter 35 nm shifts from hcp to fcc as the Ni content increases. That results in a significant modification of the magnetization process and, accordingly, of the magnetic properties of the array. Analytical calculations of the angular dependence of the coercivity allow us to confirm that the magnetization reversal is mostly ascribed to the propagation of a transverse domain wall. Fitting of the experiment to these calculations indicates the presence of a transverse crystalline anisotropy (ascribed to the hcp phase) in Co wires, while this changes to an axial anisotropy (fcc phase) as the Ni content increases.

Figure 1

Cross-sectional SEM image of Co${}_{50}$Ni${}_{50}$ NWs embedded in the AAO template. Dendrite structures shown at the bottom of the array are filled with Au (only the connection part of the dendrites to the main cylindrical NW is shown).

Figure 2

XRD $\theta -2\theta$ diffraction patterns of (a) Co, (b) Co${}_{80}$Ni${}_{20}$, and (c) Co${}_{50}$Ni${}_{50}$ NWs.

Figure 3

Reduced magnetization ($M/M_s$) hysteresis loops of Co, Co${}_{80}$Ni${}_{20}$, and Co${}_{50}$Ni${}_{50}$ NWs for (a) parallel and (b) perpendicular applied magnetic field.

Figure 4

Experimental and calculated (according to the different modes) angular dependence of parallel coercivity for (a) Co, (b) Co${}_{80}$Ni${}_{20}$, (c) Co${}_{50}$Ni${}_{50}$, and (d) Ni NWs.

Figure 5

Fitted magnetocrystalline anisotropy constant $K_{mc}$ as a function of Ni concentration.

Figure 6

FORC diagrams for (a),(b) Co and (c),(d) Co${}_{50}$Ni${}_{50}$ NWs, obtained with the applied magnetic field parallel (a),(c) and perpendicular (b),(d) to the NW axis. The color scale goes from blue (representing the minimum) toward red (maximum values). The gray-scale strip in the diagrams denotes the reversibility indicator which ranges from white to black, corresponding to fully reversible and irreversible behavior, respectively.