Magnetic properties of Ni films on sapphire investigated using first-order reversal curve (FORC) measurements

Abstract

The first-order reversal curve (FORC) measurement protocol and time efficiency considerations are discussed. The FORC method is then used to analyze and compare the substrate annealing temperature dependence of the magnetic properties and magnetization reversal mechanism of Ni thin films grown on both r and c-plane sapphire substrates. The r-plane of the sapphire substrate has a very ordered step structure on the surface, while the c-plane has larger and more asymmetrical hexagonal steps. The films are also measured at two angles with 90° separation, which for the r-plane samples correspond to a primary and secondary easy magnetization axis of the samples. The FORC measurements revealed that all films show weakly interacting behavior consisting of a local interaction field that is constant at a given particle site, but that varies randomly between grains. This interaction field was observed to, generally, decrease with annealing temperature. Only the r-plane Ni film along the secondary easy axis showed an additional stabilizing mean interaction field after the substrate was annealed. It was additionally revealed that anisotropy causes the domains to reverse by a defect-induced localized magnetization reversal mechanism along the primary easy axis, while the secondary easy axis shows reversal by a domain nucleation/growth mechanism. Lastly, the Hu coordinate of the peaks is observed to increase with increasing annealing temperature in all cases except for the r-plane film along the secondary easy axis. This reflects the increasing coercivity of the films.