Micromagnetic model of noncollective magnetization reversal in ultrathin magnetic dots with in-plane uniaxial anisotropy

In most magnetic systems the magnetization reversal is nonuniform, and is initiated in a so-called ``nucleation volume,'' whose dimensions are by far smaller than the total system volume. For simplicity reasons magnetization reversal theories are usually based on the assumption that coherent rotation occurs in this ``nucleation volume.'' In this approach, self-dipolar fields and exchange forces are obviously not well described, because in reality the nucleation volume is coupled with the rest of the system. In the case of ultrathin dots with in-plane uniaxial anisotropy, we could take into account dipolar fields and the exchange stiffness explicitly. The approximations used to derive analytical equations were suggested by experimental results on real dots. The model yields the nonuniform micromagnetic configuration of nucleation volumes. It predicts nucleation and reversal field values, as well as the field dependence of the energy barrier to be overcome to reverse the dot at finite temperature. The (negative) reversal field is found to increase with the dot thickness T and the volume magnetization Mₛ, and to decrease with the material anisotropy K. In the low-thickness limit, the reversal field Hᵣ approaches the Stoner-Wohlfarth reversal field Hₐ with a law close to 1-|Hᵣ/Hₐ|~Mₛ7/2A^-3/4K^-1T3/2, where A is the exchange constant. The relevance of the approximations used is discussed and demonstrated by the good agreement found for all predictions between experiment and/or numerical calculations on the one hand and the model on the other hand.