  Highly-ordered and spatially-coherent cobalt slanted columnar thin films were
deposited by glancing angle deposition onto silicon substrates, and
subsequently oxidized by annealing at 475 $^{\circ}$C. Scanning electron
microscopy, Raman scattering, generalized ellipsometry, and density functional
theory investigations reveal shape-invariant transformation of the slanted
nanocolumns from metallic to transparent metal-oxide core-shell structures with
properties characteristic of spinel cobalt oxide. We find passivation of
Co-SCTFs yielding CoAl$_2$O$_3$ core-shell structures produced by conformal
deposition of a few nanometers of alumina using atomic layer deposition fully
prevents cobalt oxidation in ambient and from annealing up to 475 $^{\circ}$C.
