Long-range magnetic order in Fe3O4/NiO superlattices

Using neutron diffraction, x-ray scattering, and bulk-magnetization methods, we have characterized the magnetic structure for Fe3O4/NiO superlattices grown by molecular-beam epitaxy. The antiferromagnetic NiO order extends through several superlattice bilayers, even though the intervening Fe3O4 layers are ferrimagnetic. The structural and magnetic coherence of the Fe3O4 is limited by interfacial stacking faults between adjacent layers resulting from symmetry differences between the NiO rocksalt and Fe3O4 spinel unit cell. The diffraction data manifest this interfacial disorder via a broadening of selected reflections. Using a structure-factor model based upon a Hendricks-Teller description of the random-stacking sequence, we have separated the magnetic order parameters of the Fe3O4 and NiO interlayers. The NiO appears to order at temperatures larger than TN for bulk (520 K) due to coupling to the Fe3O4 layers (TC=858 K). The dependence of this enhancement on the relative NiO composition is qualitatively consistent with the predictions of mean-field theory.