Full spin polarization of complex ferrimagnetic bismuth iron garnet probed by magneto-optical Faraday spectroscopy

Abstract

We investigate the spin-dependent electronic density of states near and above the Fermi level in bismuth iron garnet (BIG), Bi${}_{3}$Fe${}_{5}$O${}_{12}$, by magnetic circular dichroism and magneto-optical Faraday spectroscopy. BIG is a recently synthesized material, as its preparation requires special nonequilibrium conditions. Its scientific and applicative interest resides in huge specific Faraday rotation of the incident light, useful for magneto-optic applications. We show experimentally the presence of spin gaps in the conduction band as recently predicted theoretically by Oikawa et al. [T. Oikawa, S. Suzuki, and K. Nakao, J. Phys. Soc. Jpn. 74, 401 (2005)]. In the range of photon energies, where full spin polarization is expected, completely asymmetric Faraday hysteresis loops were observed, similar to those observed in half-metals such as (Pr,La)${}_{0.7}$Ca${}_{0.3}$MnO${}_{3}$ and Fe${}_{3}$O${}_{4}$. These results were modeled using even and odd (with respect to magnetization) contributions into hysteresis loops. The odd contribution appears only in the energy ranges where the density of states is fully spin polarized and vanishes at the Curie temperature. These results open a new perspective for the use of bismuth iron garnet in optic spintronics at room temperature and above.