Fluence dependence of ion implantation-induced exchange bias in face centered cubic Co thin films

The fluence dependence of exchange bias induced by oxygen ion implantation has been studied in highly textured face centered cubic Co films. These films exhibit a strong magnetocrystalline anisotropy prior to implantation. Upon implantation, the crystalline order is strongly reduced, even for the lowest implantation fluence, as shown by an isotropic magnetic behavior. Detailed analysis of the structural changes shows that the crystallite size remains basically unaltered upon implantation, suggesting that Co xO y is formed at the Co grain boundaries. A large suppression of the magnetocrystalline anisotropy is observed after implantation. This anisotropy has no influence on the unidirectional anisotropy associated to the exchange bias effect. Our study identifies a narrow implantation fluence window in which exchange bias by oxygen ion implantation is established. With increasing oxygen fluence, an increase in the magnitude of the exchange bias effect for higher fluences and, finally, a saturation of the exchange bias effect is observed in the studied fluence window. Moreover, the particular shape of the measured hysteresis loop is ascribed to a distribution of switching fields, which results from the implantation depth profile of oxygen throughout the Co film.