A First Principle Study on Magneto-Optical Effects and Magnetism in Ferromagnetic Semiconductors Y₃Fe₅O₁₂ and Bi₃Fe₅O₁₂
Wei-Kuo Li1*, Guang-Yu Guo1,2
1Department of Physics and Center for Theoretical Physics, National Taiwan University, Taipei 10617, Taiwan
2Physics Division, National Center for Theoretical Sciences, Hsinchu 30013, Taiwan
* Presenter:Wei-Kuo Li, email:weikuo38@gmail.com
The magneto-optical (MO) effects not only are a powerful probe of magnetism and electronic structure of magnetic solids but also have valuable applications in high-density data-storage technology. Yttrium iron garnet (Y₃Fe₅O₁₂) (YIG) and bismuth iron garnet (Bi₃Fe₅O₁₂) (BIG) are two widely used magnetic semiconductors with significant magneto-optical effects. In particular, YIG has been routinely used as a spin current injector. In this work [1], we present a thorough theoretical investigation on magnetism, electronic, optical and MO properties of YIG and BIG, based on the density functional theory with the generalized gradient approximation plus onsite Coulomb repulsion. We find that YIG exhibits significant MO Kerr and Faraday effects in UV frequency range that are comparable to ferromagnetic iron. Strikingly, BIG shows gigantic MO effects in visible frequency region that are several times larger than YIG. We find that these distinctly different MO properties of YIG and BIG result from the fact that the magnitude of the calculated MO conductivity (σxy) of BIG is one order of magnitude larger than that of YIG. Interestingly, the calculated band structures reveal that both valence and conduction bands across the semiconducting band gap in BIG are purely spin-down states, i.e., BIG is a single spin semiconductor. They also show that in YIG, Y sd orbitals mix mainly with the high lying conduction bands, leaving Fe d orbital dominated lower conduction bands almost unaffected by the SOC on the Y atom. In contrast, Bi p orbitals in BIG hybridize significantly with Fe d orbitals in the lower conduction bands, leading to large SOC-induced band splitting in the bands. Consequently, the MO transitions between the upper valence bands and lower conduction bands are greatly enhanced when Y is replaced by heavier Bi. This finding suggests a guideline in search for materials with desired MO effects. Our calculated Kerr and Faraday rotation angles of YIG agree well with the available experimental values. Our calculated Faraday rotation angles for BIG are in nearly perfect agreement with the measured ones. Thus, we hope that our predicted giant MO Kerr effect in BIG will stimulate further MOKE experiments on high quality BIG crystals. Our interesting findings show that the iron garnets not only offer an useful platform for exploring the interplay of microwave, spin current, magnetism, and optics degrees of freedom, but also have promising applications in high density MO data-storage and low-power consumption spintronic nanodevices.

[1] W. -K. Li and G. -Y. Guo, A First Principle Study on Magneto-Optical Effects and Magnetism in Ferromagnetic Semiconductors Y₃Fe₅O₁₂ and Bi₃Fe₅O₁₂, arXiv:2005.14133.

Keywords: Magneto-Optical Effect, Density Functional Calculations, Magnetic Semiconductors, Electronic Structure