Structural and magnetic properties of stoichiometric epitaxialCoO∕Feexchange-bias bilayers

Abstract

We present a detailed study of the magnetic and structural properties of the CoO/Fe bilayers by using a combination of x-ray diffraction (crystalline structure), Rutherford backscattering (chemical composition), and SQUID magnetometry (magnetic characterization) measurements. We prepared stoichiometric and single crystalline CoO thin films by a post-deposition annealing process in ultrahigh-vacuum conditions from sputter deposited hyperstoichiometric, polycrystalline CoO films. A simultaneous increase of the CoO crystalline quality and chemical phase purity has been achieved. Subsequently, the annealed CoO layers served as a template for the growth of epitaxial Fe films. The epitaxial relation between CoO and Fe was found to be of the Nishiyama-Wassermann type, where the Fe[001] direction is parallel aligned to the CoO[1-10] direction. The magnetic properties of CoO/Fe bilayers are strongly influenced by the stoichiometry of the antiferromagnetic CoO layer. Especially the blocking temperature is sensitive to the oxygen concentration within the CoO layer. While the blocking temperature of stoichiometric, epitaxial CoO/Fe exchange-bias bilayers almost equals the Neel temperature of bulk CoO, it is strongly reduced for superstoichiometric, polycrystalline CoO.