Spin structure in ferrimagnetic/antiferromagnetic Fe3O4/NiO superlattices (abstract)

Abstract

Neutron diffraction studies indicate that synthetic single crystalline [001] and [110] Fe3O4/NiO superlattices, prepared by oxygen-plasma-assisted molecular-beam epitaxy, exhibit long range antiferromagnetic ordering through the NiO layers despite the contrasting symmetries of the NiO rocksalt and Fe3O4 spinel unit cells, and that this ordering extends through multiple superlattice repeats. The structural and magnetic coherence in the Fe3O4 layers is confined to individual bilayers (<10 nm) due to interfacial stacking faults and this reduction gives rise to broadening of the selected neutron reflections. The coherence lengths for the antiferromagnetic and ferrimagnetic layers also differ in-plane, suggestive of magnetic frustration through the interfaces. Strong magnetic coupling at the Fe3O4/NiO interfaces, which leads to exchange biasing and to an enhancement of the NiO Neél temperature above the bulk value of 520 K (and approaching the TCurie=858 K for Fe3O4), has been observed. In contrast to results observed for a 1 μm NiO film, polarized neutron diffraction measurements of the [Fe3O4(6.8 nm)|NiO(3.4 nm)] superlattice found no preferred direction for NiO spins within a single 111 domain in a small field, but did show a tendency to reorient under application of a 1.5 T magnetic field. This seems consistent with the picture of an exchanged coupled but frustrated interface, since we have observed that the magnetic spins follow the field direction.