Thermally induced magnetic switching in thin ferromagnetic annuli

We consider magnetization reversal due to thermal fluctuations in thin, submicron-scale rings. These mesoscopic ferromagnetic particles are of particular interest as potential information storage components in magnetoelectronic devices, because their lack of sharp ends result in a magnetization density that is significantly more stable against reversal than in thin needles and other geometries. Their two-dimensional nature and rotational symmetry allow us to incorporate long-range magnetostatic forces in a fully analytic treatment, which is not possible in most geometries. We uncover a type of 'phase transition' between different activation regimes as magnetic field is varied at fixed ring size. Previous studies of such transitions in classical activation behavior have found that they occur as length is varied, which cannot be realized easily or continuously for most systems. However, the different activation regimes in a single mesoscopic ferromagnet should be experimentally observable by changing the externally applied magnetic field, and by tuning this field the transition region itself can be studied in detail.