Growth of epitaxial bcc Co(001) electrodes for magnetoresistive devices

Abstract

The applicability of the strain-induced bcc phase of Co in magnetoresistive devices was studied. Ultrathin bcc Co(001) films and the influence of the additional layers needed for magnetoresistive devices were examined by means of ${}^{59}\mathrm{Co}$ nuclear magnetic resonance (NMR). NMR is shown to be a discriminating technique for determining the presence of structurally and magnetically pure bcc Co. The maximum stability for uncovered and Fe-covered layers grown on Fe(001)/GaAs(001) and Fe(001)/Ge(001) seed layers is found to be about 2 nm. Growth of an ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ top layer preserves the bcc phase, in contrast to a Cu film which causes a transformation of the bcc structure to the fcc or the hcp phase. The bcc-preserving effects of ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ imply the possibility to fabricate magnetic tunnel junctions with bcc Co(001) bottom electrodes. Although bcc Co is a force-induced structure, thin layers are shown to be stable over a few years when ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ has been grown on top. Junction structures using bcc Co(001) bottom electrodes were grown and characterized.