Interface exchange coupling in a CoPt/NiO bilayer

The effects originating from the proximity between the ferromagnetic and the antiferromagnetic phase of a CoPt/NiO bilayer, grown at 670 K by Pulsed Laser Deposition, have been investigated from the point of view of the chemical properties (through Hard X-ray Photoelectron Spectroscopy, HAXPES) and of the magnetic behavior (by measuring hysteresis loops in the temperature range 5-300 K both after cooling in zero external field and after cooling from T = 380 K in a field of 1 T). At T = 5 K, the coercivity, measured after zero-field-cooling, is ~ 168 mT, to be compared to that of a reference CoPt layer of ~ 87 mT. Such magnetic hardening of the ferromagnetic CoPt phase is ascribed to the magnetic exchange interaction at the interface with the antiferromagnetic NiO phase, which is also responsible for the horizontal shift of the loop, observed only after field-cooling (exchange bias effect). Actually, the latter effect persists up to room temperature (exchange fields ?0Hex ~ 60 mT and ~ 8 mT were observed at T = 5 and 300 K, respectively). Hence, it can be deduced that the CoPt and NiO phases are efficiently coupled by the exchange interaction, despite the chemical inhomogeneity observed at the interface region. In fact, the HAXPES analysis reveals that a chemical reduction of the NiO phase takes place in the interface region, resulting in the formation of metallic Ni. On the other hand, this inhomogeneity of the interface is proposed to be at the origin of the peculiar shape of the field-cooled loop at T = 5 K, featuring a double reversal of the magnetization.