Unusual magnetic behaviour of hcp-Co on bcc-V, Cr, and Mo

CoX (X = V, Cr, Mo) bilayers were prepared on MgO(110) by MBE. Their structure was determined, by in situ RHEED and ex situ X-ray, as hcp-Co on bcc-X(211). The Co thickness was 200 Å and X thickness varied between 6 and 100 Å. AFM images revealed surface topography characterized by oblong grains. Magnetization reversal was examined ex situ, at room temperature, by angular MOKE measurements which showed strong in-plane uniaxial anisotropy. The angular dependence of coercivity agrees well with a modified Kondoroski model, suggesting that magnetization reversal is due to domain wall motion and the squareness of the loops shows that wall pinning is virtually absent. The reversal process is consistent with an in-plane Co columnar structure and the existence of very square minor loops suggests that these columns can be switched individually.