An investigation of magnetic reversal in submicron-scale Co dots using first order reversal curve diagrams

First order reversal curve (FORC) diagrams are a powerful method of investi gating the physical mechanisms giving rise to hysteresis in magnetic system s. We have acquired FORC diagrams for an array of submicron-scale Co dots f abricated by interference lithography. These dots reverse magnetization thr ough the nucleation and annihilation of a single-vortex state. Using FORC d iagrams, we are able to obtain precise values for the nucleation and annihi lation fields involved in magnetic reversal. Our results indicate, however, that there are actually two distinct paths for vortex annihilation: When a complete magnetic reversal takes place, a vortex enters on one side of a d ot and exits out the opposite side. But if the magnetization is returned to its original orientation before a complete reversal has occurred, then the vortex will exit on the same side from which it has entered. We are unable to obtain a precise field value for this later path of annihilation; howev er, it is shown that, statistically, the vortex annihilates with greater ea se when it exits out the same side from which it has entered. (C) 1999 Amer ican Institute of Physics. [S0021-8979(99)08409-1].