Reorientational magnetic transition in mesoscopic cobalt dots

We have measured magnetic anisotropy constants and domain structures in arrays of magnetic dots 0.5 μm wide fabricated out of an epitaxial (0001) hcp cobalt film. Occurrence of a concentric stripe domain pattern for which the magnetization is mostly in-plane with small alternately up and down perpendicular components is found for a 25-nm-thick cobalt dot array. Interestingly, this sample produces a spin reorientation from a quasi in-plane to a fully perpendicularly oriented magnetization when the temperature is lowered, due to an increase of the perpendicular anisotropy at low temperature.