Field and thermally activated demagnetization processes in ultra-thin films with in-plane anisotropy:: occurrence of non-equivalent reversal modes

We present a micromagnetic analysis of the field and thermally induced magnetization reversal occurring in ultra-thin films having in-plane anisotropy. We used for that purpose a Monte Carlo algorithm which allowed us to work at finite temperature and to simulate the effect on the demagnetization of the thermal fluctuations. Our results show that, for the examined system sizes, demagnetization is ruled by the ratio of the lateral dimension of the system to the magnetostatic correlation length associated with magnetization. When that ratio is small the coherent rotation mechanism is prefered by both the hysteretic and relaxational reversal. The increase of that ratio is accompanied by the occurrence of different incoherent collective reversal mechanisms (which try to minimize the magnetostatic energy). We evidence that those mechanisms can coexist in a given system due to the very similar involved energies. Finally, when the magnetization correlation length becomes small compared to the system size a nucleation-propagation mechanism rules the reversal processes. (C) 2000 Elsevier Science B.V. All rights reserved.