The magnetic exchange coupling in La_0.7Sr_0.3MnO₃/NiO and LaMnO₃/NiO bilayers were investigated, where the Curie temperature of the ferromagnetic (FM) layer is lower than the Néel temperature of the antiferromagnetic layer. After small-field cooling, the LSMO/NiO bilayer exhibits an exchange bias with field HEB=60 Oe, whereas the LMO/NiO sample shows weak magnetic interaction (～22 Oe). The unconventional exchange bias in LSMO/ NiO bilayer vanishes as temperature rises above 50 K. The weak magnetic interaction at the LMO/NiO interface is due to a larger Hubbard parameter value and smaller transfer integral value in the Mott insulator LMO compared with that for the FM conductor LSMO. The valence states of Mn and Ni ions across the interfaces for LSMO/NiO and LMO/NiO have been studied using X-ray photoelectron spectroscopy. We speculate that the FM interaction between Ni_2t and Mn_4t gives rise to magnetic regions that pin the ferromagnetic LSMO layer.
Magnetic multilayers consisting of itinerant ferromagnetic SrRuO₃ (SRO) and antiferromagnetic NiO were pulse-laser deposited on (0 0 1) SrTiO₃ substrates. After small-field cooling, enhancements from both the unconventional exchange bias (EB) and the large coercive field were observed in the multilayers. Coercivity values for the in-plane hysteresis loop increased by almost 50 times in comparison with a pure SRO film. The maximum value of the bias moment in the minor loop and the EB field can be achieved in different cooling fields. Moreover, hysteresis indicates a two-step magnetization reversal. We speculate that a pinned layer exists at the SRO/NiO interface producing magnetic regions that pin the ferromagnetic SRO layer.

(a) A low-magnification TEM micrograph of cross-sectional LSMO(10 nm)/NiO(20 nm) bilayer. (b) A High-resolution TEM image of LSMO/NiO bilayer.

Magnetic hysteresis loops of LSMO/NiO and LMO/NiO bilayers at 5 K