Characterization of the structural and magnetic ordering of Fe3O4/NiO superlattices grown by oxygen-plasma-assisted molecular-beam epitaxy

Abstract

Oriented single-crystalline thin films of NiO and Fe3O4 and Fe3O4/NiO superlattices have been grown on cleaved and polished substrates of MgO(001) using oxygen-plasma-assisted molecular-beam epitaxy (MBE). We report the growth mode and structural characterization of these films using in situ RHEED and ex situ scanning electron microscopy and x-ray diffraction, and their magnetic characterization using SQUID magnetometry. Also reported are preliminary results of magnetotransport measurements. MgO has a very small lattice mismatch to the cubic rocksalt structure of NiO and to the half-unit-cell dimension of the spinel structure of Fe3O4. Pseudomorphic growth of superlattices consisting of alternating layers of NiO and Fe3O4 with repeat wavelength down to 17 Å and of single thick layers of either of these materials are readily obtained. The grown films exhibit cubic single-crystalline symmetry in registry with the substrate, with sharp interfaces and strongly layer-thickness-dependent strain. RHEED pattern evolution in situ during growth indicates development of the rocksalt-like NiO and spinel Fe3O4 crystalline symmetries as each of the alternating layers of the superlattices are deposited. Magnetic hysteresis curves for superlattices with repeat wavelength less than 75 Å exhibit a crossover from the conventional ferrimagnetic response expected of Fe3O4 found in longer wavelength superlattices and bulklike thick films to a more linear magnetic response that correlates strongly to the layer thickness dependence of lattice distortions and strain. Large magnetoresistance (6% at 55 kOe) is also observed for these short-wavelength superlattices, with the observed change in resistance nearly linear with field and with small hysteresis.