Polarized neutron reflectivity on CoO / Co exchange biased multilayers

The exchange bias (EB) phenomenon is associated with the interfacial coupling between ferromagnetic and antiferromagnetic layers which results in an unidirectional anisotropy. The macroscopic effects related to exchange biased systems are the shift of the hysteresis loop towards positive or negative directions and an increase of the coercivity field as upon cooling the system in an applied magnetic field. However, another macroscopic effect, namely, time relaxation of the exchange bias field has gained little attention so far. The system we used for measuring the three macroscopic effects and, especially, the time relaxation one is a CoO/Co multilayer. The sample was prepared by rf-sputter on an a-plane sapphire substrate. The growth parameters were optimized as to obtain a low interface roughness, in expense of the crystallinity. The measurements were carried out at the ADAM reflectometer (ILL) by Polarized Neutron Spectrometry. The samples have been characterized by x-ray reflectivity and MOKE measurements. Neutron hysteresis loops were measured at 310 K (TN of CoO is 291 K) and 240 K by scanning the magnetic field and detecting the 4 reflectivities (R++,R+-, R-+, R--) at the position of the first multilayer peak. From such curves we see that the reversal of magnetization in the sample occurs not by in-plane rotation but rather through domain walls movement. In-plane magnetic moment rotation would have induced an increase of the spin-flip reflectivities (R+- and R-+) at the crossing-point. There was no difference of behaviour of the magnetization reversal process from room temperature to low temperature and from positive to negative part of the hysteresis loop. We noticed, as well, that the exchange bias field was not stable in time. It decreases from about 180 Oe towards 0 Oe. The half-life time obtained by fitting the time dependence of the exchange bias field (TDEBF) curve with an 'exponential decay' type of function was 580 s. (authors)