Emission Mössbauer spectroscopy study of theFe3O4(100)surface andFe3O4/MgO(100)andFe3O4/CoO(100)interfaces

Abstract

${}^{57}\mathrm{Co}$ probe atoms were deposited on the surface of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(100)$ films grown on MgO(100) substrates. In situ emission M\ossbauer spectroscopy and reflection high-energy electron-diffraction measurements were performed on the surface after the deposition and after an annealing at $250\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ After the deposition, the probe atoms are in ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ charge states in a disordered atomic arrangement at the surface. They do not participate in the electron-exchange process that takes place in the bulk part of the film. After annealing at $250\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ the probe atoms partly diffuse below the surface. All of the probe atoms now participate in the electron-exchange process. We observed enhanced fluctuations of the spins at and close to the surface at room temperature. Deposition of a MgO layer on top of the magnetite film significantly reduces the spin fluctuations. At 130 K the probe atoms at the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}/\mathrm{MgO}$ interface are in a ${\mathrm{Fe}}^{3+}$ charge state. Deposition of CoO on a magnetite film results in an interface that consists of a cobalt substituted magnetite layer $({\mathrm{Co}}_{x}{\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{O}}_{4}).$ We estimate the cobalt content of this interface layer to be $0.5\ifmmode\pm\else\textpm\fi{}0.1.$