Surface reconstructions on Mn3O4(001) films

Abstract

The structural complexity of surfaces of spinel-like oxides together with the existence of competing phases of different or mixed valencies have limited surface structure determinations so far. Only recently, the concept of subsurface cation vacancies (SCVs) could be demonstrated for an ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ single crystal surface. Here, we present the appearance of SCVs for ultrathin and thin films of ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$(001) depending on film thickness. The structures of films grown by molecular beam epitaxy have been investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and near-edge x-ray absorption fine structure spectroscopy (NEXAFS). For coverage up to 4 (Mn ions)/(Ag atom) of ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ and an annealing temperature of 640 K, a $p(2\ifmmode\times\else\texttimes\fi{}1)$ reconstruction has been observed in STM and LEED. Using NEXAFS, the identity of the manganese oxide has been narrowed down to the spinel ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ and its related vacancy compound $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$. Thus the $p(2\ifmmode\times\else\texttimes\fi{}1)$ reconstruction corresponds to an ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$(001) surface with $\mathrm{Mn}{\mathrm{O}}_{2}$ termination. For a lower annealing temperature of 450 K or thicker films [5 and 10 (Mn ions)/(Ag atom) ], additional $p(4\ifmmode\times\else\texttimes\fi{}2)$ and $c(4\ifmmode\times\else\texttimes\fi{}4)$ reconstructions have been observed. These extra reconstructions stem from lateral shifts of top-sublayer ions induced by SCVs.