Superexchange and iron valence control by off-stoichiometry in yttrium iron garnet thin films grown by pulsed laser deposition

Controlled off-stoichiometric single phase polycrystalline yttrium iron garnet (YIG) thin films have been grown by pulsed laser deposition, adjusting the oxygen partial pressure PO2 between 5 and 400 mTorr. Atomic stoichiometry by RBS shows an oxygen deficiency for PO2<Pstoich and iron and yttrium deficiency for PO2>Pstoich. Pstoich=30mTorr refers to films showing magnetic and structural properties of the bulk stoichiometric YIG. Curie temperature Tc and saturation magnetization 4πMs decreased for PO2<Pstoich, whereas a new part of the YIG phase diagram is explored for PO2>Pstoich: Increase of Tc (up to +10%) and of 4πMs (up to +20%) and lattice parameter compression. Microscopic interpretation is given in terms of superexchange interaction and creation and site selectivity of iron vacancies.