Exchange anisotropy in Co/NiO bilayers: time-dependent effects

The time dependence of the exchange anisotropy wasstudied in Co/NiO bilayers. In order to only observe therelaxation phenomena inside the antiferromagnetic (AF) layer andto eliminate the dynamic behaviour inside the ferromagnetic (F)layer, we have developed an experimental method where a small a.c.magnetic field is applied perpendicular to the main anisotropyaxis. All data are obtained by magneto-optical (m.o.) experiments.We observe a logarithmic time dependence of Hud, the exchangeunidirectional anisotropy. We prove that the key parameter for therate of relaxation is the anisotropy of the AF layer which dependsstrongly of the preparation method. We use the random field modelas proposed by Malozemoff and suppose a breakdown of the AFinterface into regular domains of size close to the crystallitesize (10 nm width). If we further develop a Fulcomer and Charaprelaxation model, we can propose from the distribution ofrelaxation times an analysis in terms of a spread of AF anisotropyenergies. High magnetic pulsed field experiments (55 T) completethe experimental study and the results are analysed assuming thatthe Zeeman energy balances the anisotropy energy of the AF domainsand switches them into the opposite direction.