Exchange-bias effect at La0.75Sr0.25MnO3/LaNiO3 interfaces

We show that ferromagnetic/paramagnetic La0.75Sr0.25MnO3/LaNiO3 multilayers present an unexpected magnetic exchange-bias effect (EBE), observed in field-cooled magnetization loops. The exchange-bias field and the enhancement of the coercivity vanish around 50 K. We demonstrate that the oxidation state of the Ni and Mn cations changes from Mn3+-Ni3+ to Mn4+-Ni2+ in the layers close to the interface probed by x-ray absorption spectroscopy measurements. The variation of the valence states is accompanied by a change in the magnetic behavior of the cations at the La0.75Sr0.25MnO3/LaNiO3 interface, possibly giving rise to the formation of magnetic or magnetically frustrated regions that may pin the ferromagnetic La0.75Sr0.25MnO3 layers and explain the EBE.