Study of the magnetic and magneto-optical properties of [110]-oriented Fe3O4/NiO superlattices

Abstract

The magnetic behavior of a set of [110]-oriented single crystalline Fe3O4/NiO superlattices was studied by SQUID magnetometry and by magneto-optical Kerr-effect (MOKE) measurements. Samples for this study were grown on MgO substrates by means of plasma-assisted molecular beam epitaxy with the NiO layer thickness kept constant, while the Fe3O4 layer thickness and the number of bilayers were varied in such way as to preserve the total Fe3O4 thickness throughout the entire set. The structural ordering was studied by x-ray diffraction and reflection high energy electron diffraction. The growth mode was single crystalline, but with rough interfaces. The hysteresis profiles vary with the Fe3O4/NiO thickness ratio; with thinner Fe3O4 layers the degree of influence of the high-anisotropy antiferromagnet (NiO) spin system over that of the ferrimagnet (Fe3O4) increased. The optical constants of the substrate and the component layers at 632.8 nm wavelength were obtained in two steps by unconstrained and constrained optimization algorithms based on the Fresnel equations and ex situ ellipsometric measurements. These optical constants were found to differ by a small but distinct amount from the available standard values. The magneto-optical constant (also at 632.8 nm) of the Fe3O4 layer was determined from longitudinal and polar Kerr effect measurements using one of the existing matrix methods and was found to be a direction-dependent function of the Fe3O4 layer thickness.