Propagation of antiferromagnetic order across paramagnetic layers in CoO/NiO superlattices

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Using neutron-diffraction techniques, we have probed the magnetic structure of nickel-oxide/cobalt-oxide superlattices grown by reactive sputtering. Collinear antiferromagnetic order propagates through several bilayers despite the short-range nature of the spin interaction in the components. The magnetic order for 36 Å period superlattices develops simultaneously in the NiO and CoO layers at a Néel temperature between the values for bulk CoO and NiO. When the superlattice periodicity is increased to 72 Å, the Ni and Co spins appear to order separately at temperatures shifted from their bulk TN. The magnetic coherence length decreases as the temperature is increased, but remains greater than the width of a single bilayer above TN for the CoO interlayers. Mean-field calculations demonstrate that exchange coupling at the interfaces is responsible for the anomalous magnetic behavior near the phase transition.