Magnetic and thermoelectric properties of Fe$_{\mathrm{3-x}}$Co$_{\mathrm{x}}$O$_{4}$ thin films and CoFe$_{2}$O$_{4}$/Fe$_{3}$O$_{4}$ superlattices

Microcrystalline ferrites are used as a medium for the magnetic recording and storage of information. Magnetite, Fe$_{3}$O$_{4}$, is a ferrimagnet with a cubic inverse spinel structure and exhibits a metal-insulator, Verwey, transition at about 120 K. It is predicted to possess as half-metallic nature, $\sim $ 100{\%} spin polarization, and high T$_{\mathrm{C}}$ (850 K). Cobalt ferrite, Co$_{3}$O$_{4}$, is one of the most important members of the ferrite family, which is characterized by its high H$_{\mathrm{C}}$, moderate magnetization and very high magnetocrystalline anisotropy. Here we report on the magnetic and thermoelectric properties of Fe$_{\mathrm{3-x}}$Co$_{\mathrm{x}}$O$_{4}$ (x $=$ 0 to 1) thin films and CoFe$_{2}$O$_{4}$/Fe$_{3}$O$_{4}$ superlattices grown on MgO (100) by MBE. XRD and RHEED patterns confirmed the inverse spinel structure of the Fe$_{3}$O$_{4}$ films. Magnetic properties of the Fe$_{\mathrm{3-x}}$Co$_{\mathrm{x}}$O$_{4}$ films are markedly sensitive to the Co content. The Verwey transition was disappeared in Co-doped films. A negative MR curve with butterfly shape was observed with low Co content but disappeared for the samples with x $=$ 0.8 and 1. Seebeck coefficients increased with Co concentration; -70 $\mu $V/K for x$=$0 and -220 $\mu $V/K for x$=$1. We will also discuss on the relationship between magnetic and thermoelectric characteristics in CoFe$_{2}$O$_{4}$/Fe$_{3}$O$_{4}$ superlattices with the modulations of 5, 10, and 20 nm.