Add to ORKGThe stoichiometry of a series of 300 Angstrom thick Fe3-deltaO4 layers grown by means of molecular beam epitaxy on MgO(100) has been investigated both by magneto-optical Ken spectroscopy and by Mossbauer spectroscopy, The layers consisted of a 200 Angstrom thick Fe3-deltaO4 layer grown with the Fe-57 isotope, sandwiched between two 50 Angstrom thick layers grown with natural Fe, It was found that delta varied between 0 and 0.33. From the fits of the polar Ken spectra of the Fe3-deltaO4 layers measured between 1.4 and 4.0 eV we find, within experimental accuracy, a stoichiometry that is identical to that determined by Mossbauer spectroscopy. This confirms that the magneto-optical Ken spectra can also be used to determine the stoichiometry of...