Curie temperature, exchange integrals, and magneto-optical properties in off-stoichiometric bismuth iron garnet epitaxial films

Abstract

We have studied the influence of the stoichiometry on the structural, magnetic, and magneto-optical properties of bismuth iron garnet $({\text{Bi}}_{3}{\text{Fe}}_{5}{\text{O}}_{12})$ 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 N\'eel two-sublattice magnetization model giving energies of $\ensuremath{-}48\text{ }\text{K}$ (4.1 meV), $\ensuremath{-}29\text{ }\text{K}$ (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.