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.