Scanning magnetoresistive microscopy study of quasi-static magnetic switching in mesoscopic square dots: observation of field-driven transition between flux-closure states

We used newly developed scanning magnetoresistance microscopy (SMRM) to observe time-lapsed magnetic domain images during magnetic switching in square Permalloy dots. Unlike magnetic force microscopy, the SMRM images measure the absolute local magnetic field from any microstructure. SMRM works in an external magnetic field, allowing the measurement of evolution of quasi-static domain structures. We observed a complete magnetic reversal in 5 μm Permalloy dots from a positively saturated state to a negatively saturated state via a mixture of flux-closure states. We also observed field-driven transitions between the various flux closure states at low fields. There appears to be a distribution of switching field and mode, even for geometrically identical dots. This work demonstrates the novel capability of the SMRM technique and the rich switching physics of magnetic square dots.