Interlayer coupling betweenFe3O4layers separated by an insulating nonmagnetic MgO layer

The magnetic interlayer coupling between two magnetic layers separated by an insulator was investigated on ${\mathrm{Co}}_{0.2}$ ${\mathrm{Fe}}_{2.8}$ ${\mathrm{O}}_4$/${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$/MgO/${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ samples grown by means of molecular beam epitaxy on (001) ${\mathrm{MgAl}}_2$ ${\mathrm{O}}_4$ substrates. The samples were designed to observe interlayer coupling of either sign. Hysteresis loop measurements show that the ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ layers are ferromagnetically coupled in the thickness range 0–45 nm MgO. Below a MgO spacer thickness of 1.3 nm, the coupling strength increases drastically with decreasing MgO thickness and is ascribed to the existence of ferromagnetic bridges through the MgO spacer. The small ferromagnetic coupling above 1.3 nm seems to arise from a magnetostatic coupling due to correlated interface irregularities.