Evidence for Roughness Driven 90° Coupling in ${\mathrm{Fe}}_3{O}_4/\mathrm{NiO}/{\mathrm{Fe}}_3{O}_4$ Trilayers

The magnetic interlayer coupling of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ across NiO is studied using ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{NiO}/{\mathrm{Fe}}_3{\mathrm{O}}_4$ trilayers epitaxially grown on (001) MgO substrates. For NiO thicknesses between 0.7 and 5 nm, the magnetic moments of the two ${\mathrm{Fe}}_3{\mathrm{O}}_4$ layers are directed perpendicularly with respect to each other. The 90° coupling strength is determined to be $0.35\pm 0.08\mathrm{mJ}/{\mathrm{m}}^2$ for a 1.4-nm-thick NiO spacer. The 90° coupling can be understood from the effect of an antiferromagnetic spacer in the presence of interface roughness.