Activation energy and mechanisms for skyrmion collapse in synthetic antiferromagnets

The mechanisms of the collapse of skyrmion structures in synthetic antiferromagnets and the activation energy of such processes are studied within the transition state theory based on the analysis of the multidimensional energy surface of the system and the construction of minimum energy paths between the corresponding states. Synthetic antiferromagnets consist of two thin ferromagnetic films separated by a nonmagnetic metal spacer, the conduction electrons of which provide antiferromagnetic interlayer exchange interaction. A discrete Heisenberg-type model is used, which includes symmetric and antisymmetric exchange in each layer, interaction with the applied magnetic field, and the aforementioned interlayer exchange interaction. The experimentally observed magnetic structures are reproduced. It is shown that the most probable mechanism for the collapse of skyrmion pairs involves an asymmetric state with a skyrmion in one layer. The activation energy for such a process is calculated. It is 16% lower than the numerical estimates based on the micromagnetic ansatz, but is a factor of 1.4 higher than that corresponding to the annihilation of a skyrmion of the same size in one layer.