Long-range magnetic order inFe3O4/NiO superlattices

Using neutron diffraction, x-ray scattering, and bulk-magnetization methods, we have characterized the magnetic structure for ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$/NiO superlattices grown by molecular-beam epitaxy. The antiferromagnetic NiO order extends through several superlattice bilayers, even though the intervening ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ layers are ferrimagnetic. The structural and magnetic coherence of the ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ is limited by interfacial stacking faults between adjacent layers resulting from symmetry differences between the NiO rocksalt and ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ 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 ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ and NiO interlayers. The NiO appears to order at temperatures larger than ${\mathit{T}}_{\mathit{N}}$ for bulk (520 K) due to coupling to the ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ layers (${\mathit{T}}_{\mathit{C}}$=858 K). The dependence of this enhancement on the relative NiO composition is qualitatively consistent with the predictions of mean-field theory.