We have studied the magnetization reversal of ${\mathrm{Co}}_{28}{\mathrm{Pt}}_{72}$ single crystal dots down to $0.2\mu \mathrm{m}$ diameter with perpendicular anisotropy under slow magnetic field sweep rates $({10}^4\mathrm{Oe}∕\mathrm{s})$ as well as with field pulses $(6.7\times {10}^{11}\mathrm{Oe}∕\mathrm{s})$. The magnetic state of the dots in the remanent state is observed by magnetic force microscopy. After application of short field pulses, $1\mu \mathrm{m}$ dots are frequently multidomain, whereas $0.2\mu \mathrm{m}$ dots are always single domain. The nucleation volume is estimated either from aftereffect measurements or by fitting the increase of the dynamic coercive field versus field sweep rate. Its value amounts to a few ${10}^{-19}{\mathrm{cm}}^3$. We show that the wall velocity around nucleation centers is larger than in the rest of the sample.

• Received 12 July 2007

DOI:https://doi.org/10.1103/PhysRevB.77.054416