Different magneto-optical response of magnetic sublattices as a function of temperature in ferrimagnetic bismuth iron garnet films

In this paper we investigate the magneto-optical (MO) and magnetic properties of bismuth iron garnet $\mathrm{B}{\mathrm{i}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ thin films over a wide range of photon energies (1.6–3.5 eV) and temperatures (5–740 K). Depending on the photon energy range, the Faraday rotation (${\mathrm{\Theta }}_F$) and ellipticity (${\varepsilon }_F$) vary nonmonotonously with temperature. This behavior cannot be explained by a magnetization variation that can only decrease with increasing temperature. ${\mathrm{\Theta }}_F$ and ${\varepsilon }_F$ spectra have therefore been analyzed using a model based on two optical transitions of a diamagnetic nature, representing the tetrahedral and octahedral iron sites. Thus, the contribution of each magnetic sublattice has been extracted from the global macroscopic MO response and investigated as a function of temperature. The magnetic properties of octahedral and tetrahedral sublattices depend differently on temperature, suggesting a different anisotropy due to oxygen coordination. We have demonstrated that this relatively simple macroscopic measurement with a subsequent analysis can grant access to the information on the properties at a microscopic level. These results can advance the fundamental understanding of MO properties in multisublattice magnetic materials.

Figure 1

Temperature dependence of the (a) ${\mathrm{\Theta }}_F$ and (b) ${\varepsilon }_F$ spectra of the BIG thin film. The dashed line in (a) and the arrow in (b) are guides to the eye.

Figure 2

(a)–(d) Experiential measurement (symbols) and theoretical simulation (solid lines) of (a), (c) ${\mathrm{\Theta }}_F$ and (c), (d) ${\varepsilon }_F$ spectra for two selected temperatures of $T=5$ and 660 K. The dashed and the dashed-dotted lines represent the contribution from the tetrahedral and octahedral diamagnetic electric dipole transitions. (e), (f) Comparison of experimental (symbols) and theoretical simulation (solid lines) of ${\mathrm{\Theta }}_F$ and ${\varepsilon }_F$ spectra obtained at various temperatures.

Figure 3

Temperature dependence of the energy transition ${\omega }_{0i}$ characterizing the tetrahedral (circle) and octahedral (square) diamagnetic electric dipole transitions. The dashed lines are guides to the eye.

Figure 4

Temperature dependence of the half linewidth ${\mathrm{\Gamma }}_i$ and the $N_i{f}_i$ parameters characterizing the tetrahedral (circle) and octahedral (square) diamagnetic electric dipole transitions. For better visualization, the scales on the left and right axis are different. The dashed lines are guides to the eye.

Figure 5

Temperature dependence of the (a), (c) ${\mathrm{\Theta }}_F$ and (b), (d) ${\varepsilon }_F$ measured at photon energies of 2.25 and 2.76 eV (open circles). The solid circles and squares represent the temperature dependence of the contribution of the tetrahedral and octahedral sublattices, respectively. The stars represent the sum of the contributions of the two sublattices. The solid lines are guides to the eye.