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Oriented single-crystalline thin films of NiO and Fe304, and Fe304/NiO superlattices have been grown on cleaved and polished substrates of MgO(001), using oxygen-plasma-assisted molecular-beam epitaxy. We report the growth mode and structural characterization of the grown films using in situ reflection high-energy electron diffraction (RHEED) and ex situ scanning electron microscopy and x-ray diffraction. The (001) surface of MgO provides an excellent template for the pseudomorphic growth of these thin films and superlattices, for it has a very small lattice mismatch (0.3-0.9%) to the cubic rocksalt structure of Ni0 and to the half unit-cell dimension of the spinel structure of Fe304. Superlattices consisting of alternating layers of NiO and Fe304 have been grown with a repeat wavelength down to 20 0A (approximately one Fe304 unit cell plus two NiO unit cells) thick. These superlattices exhibit strong crystalline ordering and sharp interface formation. RHEED pattern evolution in situ during growth indicates formation of the rocksalt NiO crystalline symmetry and then the spinel Fe304 crystalline symmetry in a periodic sequence as each material is being deposited. Our data indicate single-phase crystal growth in registry with the substrate, with films of overall cubic symmetry. Strain in the grown films exhibits interesting effects that clearly do not follow a simple elastic model. I. INTRODUCTION Growth of high-quality single-crystalline thin films of metal oxides has become increasingly important recently for both technological and fundamental physical reasons. Ceramic oxides such as the ferrite spinels and magnetic garnets exhibit a rich complexity in magnetic coupling and electronic bonding not found in pure metallic and alloy materials. In addition, because of the strong anioncation bonding in oxides, such films are often tribologically and chemically significant more stable than films with free metal or semiconducting surfaces. Oxide ferrites, especially those in layered structures, can also have superior magnetization and microwave resonance properties for use in a variety of planar device structures and other technological applications. Recently, the synthesis of layered oxide materials has received enhanced notoriety because of the discovery of high-temperature superconductivity in the ceramic perovskite oxides. As a result of this combination of factors, strong research interest deals with issues involved in the growth and integration of magnetic and superconducting oxide structures into planar integrated devices. ' Special emphasis in the research community is being focused on thin films and modulated structures of ferrite materials. ' Magnetic spinels, for example, show great promise in planar device applications because of their superiority as high-density magnetic recording media, their resistance to corrosion and wear, and the inherent advantage non-"lossy" insulating ferrites have in microwave resonant circuits. In addition, layered structures of such ferrite materials allow controllable constraints to be placed on stoichiometry, lattice spacing, and strain that couple directly with their magnetic properties. As a result, layered magnetic oxides are ideally suited to a study of a number of fundamental issues dealing with magnetic coupling, ordering and anisotropy in nonitinerant electron magnetic systems. Using the preparation techniques of molecular-beam epitaxy (MBE), Bando and coworkers have recently grown Fe304 films and Fe304lCoO superlattices, and, using reactive sputtering, Ortiz et al. ' '" have grown Fe304 films and Yoshii et al. ' have grown y-Fe203 films. The current investigation extends these studies and demonstrates the ability to produce constrained metal-oxide thin-film stoichiometries, including modulated structures of highcrystalline order and purity with controllable magnetic ordering and properties. In our work, highly oriented single-crystalline thin films of NiO, Fe304 and Fe30~lNiO superlattices have been grown using oxygen-plasma-assisted MBE. We will discuss here the synthesis and characterization techniques used, with special emphasis on the crystal-growth techniques used and the structural characterization of the resultant films using reflection high-energy electron diffraction (RHEED), scanning electron microscopy (SEM), and x-ray diffraction (XRD). We have found the (001)-oriented surface of MgO single crystals to provide an excellent template for the pseudomorphic growth of both NiO and Fe30~, for it has a small (&1%) lattice mismatch to the cubic rocksalt structure of the former and to the half unit-cell dimension of the spinel structure of the latter. We report the preparation of NiO and Fe304 thin films, as well as superlattices consisting of al-ternating layers of Ni0 and Fe304. The Fe304/NiO superlattices have been grown with modulation wavelength A between 20 and 258 A (the lower limit being slightly greater than one Fe304 unit cell plus two NiO unit cells thick), with optimal crystalline order obtained at substrate growth temperature of 240 C. These films exhibit coherent single-crystalline ordering over several hundred repeat wavelengths. Reported elsewhere in the literature are studies we have made on the magnetization and electron transport properties of these thin films and superlattices' ' which show such effects as an unusually large in-plane versus out-of-plane anisotropy () 10 difFerence) in electron transport as well as strong modulation wavelength-dependent magnetic ordering effects. |
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