  We demonstrate that the transition from the high-field state to the vortex
state in a nanomagnetic disk shows the magnetic equivalent of supercooling.
This is evidence that this magnetic transition can be described in terms of a
modified Landau first-order phase transition. To accomplish this we have
measured the bulk magnetization of single magnetic disks using nanomechanical
torsional resonator torque magnetometry. This allows observation of single
vortex creation events without averaging over an array of disks or over
multiple runs.
