Varying steps in hysteresis loops of Co square nano-frames

Magnetic nanostructures are investigated nowadays for diverse applications as well as in basic research. They often show unexpected magnetization reversal processes which are not only of high interest for understanding magnetization reversal processes on the nano-scale, but also due to their possible use in magneto-electronic devices, spintronics, neuromorphic computing and other areas. Square nano-frames prepared from iron, e.g., belong to the magnetic nano-particles exhibiting additional stable states at remanence which can be used for quaternary storage devices. The reason for this study is that nano-particles prepared from permalloy or nickel with their very low magneto-crystalline anisotropy do usually not show steps in the hysteresis loop and corresponding stable intermediate state, while the magneto-crystalline anisotropy of cobalt often dominates over the shape-anisotropy, resulting in a large number of steps which are not always stable. Here we report on experimental investigations by magneto-optic Kerr effect (MOKE) and micromagnetic simulations of square Co nano-frames, exhibiting steps along the slopes of the hysteresis loops. The positions and numbers of these steps varied during measurements without intentionally changing the experimental setup. Micromagnetic simulations were carried out to explain the experimental findings which could be attributed to different Co crystallite orientations, resulting in a modified magneto-crystalline anisotropy and thus different hysteresis loops for smallest variations of the laser beam position on the samples during MOKE measurements.