We have studied the influence of the stoichiometry on the structural, magnetic, and magneto-optical properties of bismuth iron garnet $\left({\text{Bi}}_3{\text{Fe}}_5{\text{O}}_{12}\right)$ thin films grown by pulsed laser deposition. Films with different stoichiometries have been obtained by varying the Bi/Fe ratio of the target and the oxygen pressure during deposition. Stoichiometry variations influence the Curie temperature $T_C$ by tuning the (Fe)-O-[Fe] geometry: $T_C$ increases when the lattice parameter decreases, contrary to what happens in the case of stoichiometric rare-earth iron garnets. The thermal variation of the magnetization, the Faraday rotation, and the Faraday ellipticity have been analyzed in the frame of the Néel two-sublattice magnetization model giving energies of $-48\text{K}$ (4.1 meV), $-29\text{K}$ (2.5 meV), and 84 K (7.3 meV) for the three magnetic exchange integrals $j_{aa}$, $j_{dd}$, and $j_{ad}$, respectively. Magneto-optical spectroscopy linked to compositional analysis by Rutherford backscattering spectroscopy shows that Bi and/or Fe deficiencies also affect the spectral variation (between 1.77 and 3.1 eV). Our results suggest that bismuth deficiency has an effect on the magneto-optical response of the tetrahedral Fe sublattice, whereas small iron deficiencies affect predominantly the magneto-optical response of the octahedral sublattice.

We demonstrate how controlled changes in oxygen stoichiometry tune the magnetic properties and the superexchange interaction, thus providing a simple and efficient method to elaborate magnetic oxides with strongly modified magnetic and semiconductor properties. Structural and magnetic properties of the model system of yttrium iron garnet (YIG) thin films were tuned by controlling oxygen stoichiometry. The YIG phase diagram has been explored by creating cation and oxygen vacancies during film growth by pulsed laser deposition. Especially, unexplored in thermodynamic equilibrium growth conditions, excess oxygen stoichiometry in a YIG unit cell was investigated: a significant increase of Curie temperature $(+10\%)$ and magnetization $(+20\%)$ were observed. These changes are understood in the framework of a microscopic model implying the formation of ${\mathrm{Fe}}^{4+}$ and ${\mathrm{Fe}}^{2+}$ in the films with oxygen excess and vacancies, respectively.