Magnetic textures in hemispherical thin film caps with in-plane exchange bias

Abstract

Hemispherical caps of in-plane exchange biased IrMn/CoFe layer systems have been fabricated on top of regularly arranged spherical silica particles by magnetron sputtering, creating magnetic Janus particles. In this thin film layer system cap, the magnetic shape anisotropy of the topographically non-flat hemispheres competes with the unidirectional anisotropy induced by the exchange bias. The magnetic properties of this non-trivial system have been investigated by longitudinal magneto-optical Kerr effect magnetometry, where a characterization method has been developed considering both the curved layer system and the signal contributions of flat parts of the sputtered thin film system. Both remagnetization curves, from Kerr magnetometry and the magnetic force microscopy images, reveal an onion state in the magnetic caps of the ensemble. Additional micromagnetic simulations show a stabilization of the onion state due to the introduced unidirectional anisotropy also in individual hollow hemispheres as compared to the vortex state exhibited by purely ferromagnetic caps.