Observation of magnetic supercooling of the transition to the vortex state

The magnetic hysteresis of an individual magnetic disc switching in and out of the vortex state has been exhaustively measured using nanomechanical torsional resonator torque magnetometry. Each individual hysteresis loop pinpoints two sharp events, a single vortex creation and an annihilation, with a bias field precision of 0.02 kA m(-1). Statistical analysis of thousands of hysteresis loops reveals a dramatic difference in the sensitivity of the vortex creation and annihilation field distributions to the measurement conditions. The data sets measured at different magnetic field sweep rates demonstrate that the transition from the high-field state to the vortex state is not well modeled as a conventional thermal activation process in which it is assumed that the 'true' nucleation field is lower than any of the observed switching fields. Instead, the results are suggestive of the classic supercooling signature of a first-order phase transition, or more specifically here, its magnetic equivalent. This phenomenological evidence is consistent with a theoretical picture of the vortex nucleation process as a modified Landau first-order phase transition.