Pulsed laser deposited epitaxial oxide thin films for microwave and spintronics applications
Oxides are an important class of materials exhibiting properties useful in the modern technological applications. A wide range of oxides are inherently stable, both chemically and thermodynamically. Some members of the oxide family display ferroelectric, ferromagnetic and multiferroic characteristics etc., which are important for next generation non–volatile memory and microwave applications. For example, ferrites are particularly interesting due to their high magnetization, high Neel/Curie temperatures and insulating nature which aids in fabricating more efficient spintronic devices. In this work we address two crucial problems existing in the path of successful application of ferroelectric material-based spintronic memory and ferrite-based microwave devices. First, ferroelectric tunnel junctions that have a metal top electrode (also used as interconnects) are unstable due to uncompensated depolarizing field. This makes the memory devices to lose the stored information over time. We show that this problem can be addressed by introducing a ferroelectric–dielectric barrier layer in the tunnel junction. Second, a specific structural defect referred to as antiphase boundary is present in all spinel ferrite thin films reported in the literature. This type of defect significantly degrades the structural and magnetic properties of the spinel ferrite thin films as compared to their bulk counterparts and hinders the utilization of ferrite thin films for potential applications. We show that these defects can be eliminated in nickel ferrite thin films by using isostructural substrates that have a small lattice mismatch (<0.8%) and thereby obtain magnetic and microwave properties comparable to the bulk single crystal. We demonstrate that this principle can also be applied to other members of ferrite family (magnetite, cobalt ferrite etc.) to obtain improved structural and magnetic properties. In magnetite thin films grown on isostructural and lattice-matched substrates, we do not find any antiphase boundaries and hence the magnetic properties are much improved along with sharp metal to insulator transition at temperature close to that of bulk.