Temperature and set field dependence of exchange bias training effects in Co/NiO/[Co/Pt] heterostructures with orthogonal easy axes

Training effects in a new class of exchange biased ferromagnet/antiferromagnet/ferromagnet trilayers (Co/NiO/[Co/Pt]3) with mutually orthogonal easy axes have been measured and successfully modeled. Previous experiments have demonstrated an enhanced blocking temperature as well as the ability to isothermally field tune the magnitude of the room temperature in-plane exchange bias. These effects have been attributed to the presence of the [Co/Pt] multilayer with perpendicular magnetic anisotropy, which variably pins the backside NiO domains. Here we show that the tuning of the exchange bias and the blocking temperature enhancement are highly dependent on both the temperature and the in-plane remanence of the normally out-of-plane [Co/Pt] multilayer, achieved using modest in-plane set fields. Training effects and their dependence on temperature and in-plane remanence are modeled using a thermodynamic approach. The in-plane remanence of the [Co/Pt] acts only to set the equilibrium exchange bias value and sets the scale for the blocking temperature; it has no effect on the training. We conclude that training effects occur only at the Co/NiO interface and that the relaxation towards equilibrium is confined to this interface. The field enhanced blocking temperature and isothermal tuning of exchange bias in these magnetic heterostructures with mutually orthogonal easy axes could play a role in the enhancement of exchange bias effects in future spin-valve devices. A thorough knowledge of the training effects is essential to account for the fundamental relaxation mechanisms that occur with repeated field cycling.