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Skyrmion and vortex states induced by long-ranged magnetic dipolar interactions in nanoscaled CoPt/Co bilayer and trilayer systems

Spin distributions for exchange-coupled CoPt/Co bilayers and CoPt/Co/CoPt trilayers have been systematically simulated by a three-dimensional (3D) micromagnetic model. It is found that skyrmion structures can occur in all three layers for the trilayer systems when the hard and soft layer thicknesses are about 8 nm and 16 nm respectively. Especially, it is found for the first time that skyrmion states can occur in both the hard and soft phases when both the two layer thicknesses are about 8 nm, i.e., the double skyrmion states will occur. The soft layer thickness ( t s), at which the skyrmion structure occurs in the bilayers, is much smaller than that of trilayers. On the other hand, the corresponding skyrmion numbers obtained in bilayers are only about 8% smaller than those for the trilayers. The exchange constant A and crystalline anisotropy constant K have important influences on the skyrmion numbers and spin distributions, whereas the exchange coupling between the soft and hard layers has negligible effects on them. As A and K change, the spin distributions can transform from vortex into skyrmion as A and K vary.