Figure 1: Skyrmions in bilayer system.
(a) The skyrmions on each layer have the same skyrmion number Nsk,1 = Nsk,2 due to the ferromagnetic interlayer coupling Jinter > 0, and those helicity are the same as each other. (b) The helicity of the skyrmion on layer 2 is opposite to that in the case (A1). (c) Compared to the case (A1), the magnetic texture on layer 2 is totally reversed topologically and hence Nsk,1 = −Nsk,2, due to the antiferromagnetic interlayer coupling Jinter < 0. In this case, we do not apply the external magnetic field and instead the magnetic anisotropy K is introduced to stabilize skyrmions. (d) The helicity of the skyrmion on layer 2 is opposite to that in the case (B1).
