Strongly enhanced exchange bias of top-pinned Co/IrMn films with Py spacers

This work investigates the effects of inserting an ultrathin ferromagnetic Py or Co spacer layer (SL) at the interface of ferromagnet/antiferromagnet films, namely Co/SL(Py)/IrMn and Py/SL(Co)/IrMn. The exchange bias field, HEB, of the two series of films changes with the spacer thickness in very different manners. Whereas HEB of the Py/SL(Co)/IrMn series decreases monotonously with the Co layer thickness, a fourfold initial bias enhancement is obtained for Py spacers with thicknesses up to 1 nm. Moreover, the Co/SL(Py)/IrMn series shows a maximum HEB more than 50% higher than that of the Py/IrMn film which, in turn, is almost three times higher than the HEB of the control Co/IrMn film. Simulations of experimental magnetization curves through a polycrystalline model for exchange bias showed that these variations, which could yield improvement in the performance of modern spintronic devices, are predominantly governed by modifications of the interfacial exchange interactions. Several mechanisms that could be responsible for the observed behavior are discussed.