Magnetic hysteresis of Ni nanowires

Magnetization processes in Ni nanowire arrays are investigated. The wires are produced by electrodepositing Ni into porous anodic alumina and exhibit coercivities of the order of 0.05 T (500 Oe) along the wire axis. Transmission-electron microscopy of freed wires shows that the wires are polycrystalline and resemble a chain of nanocrystallites. To model the hysteresis loops, the wires are treated as one-dimensional random-anisotropy magnets where the magnetocrystalline bulk anisotropy is a weak perturbation to the leading anisotropy contribution. The calculation yields an analytical equation for the magnetization as a function of the applied magnetic field. For small and moderate reversed fields, the agreement between theory and experiment is very good, but the applicability of the model breaks down close to the coercive field. This failure is explained by the neglect of higher-order perturbation terms describing, for example, magnetic localization effects.