Thermally activated vortex nucleation as a function of thickness in submicron patterned permalloy thin films

Arrays of submicron patterned magnetic thin films often show tails in their hysteresis loops, which are usually attributed to either the coercivity distribution in the array or vortex formation/domain wall trapping in individual bits. In arrays of submicron-sized elliptical patterned bits, the tails can appear at less than the critical thickness for vortex formation, suggesting that vortices are unlikely. Thermal fluctuations, however, allow small magnetic systems to sample magnetic states that are normally inaccessible at zero temperature, which can result in different switching processes than would be expected based on zero-temperature simulations. In order to study the effect of thermal fluctuations on switching processes, arrays of submicron patterned magnetic films of different sizes and over a range of thickness were measured using the magnetooptic Kerr effect (MOKE) and compared with Langevin micromagnetic simulations. The comparison indicates that the presence of hysteresis loop tails in arrays of patterned bits is a result of vortex formation in individual bits.