  By performing density functional calculations, we find a giant
magnetocrystalline anisotropy (MCA) constant in abundant element cerium (Ce)
substituted M-type hexaferrite, in the energetically favorable strontium site,
assisted by a quantum confined electron transfer from Ce to specific iron (2a)
site. Remarkably, the calculated electronic structure shows that the electron
transfer leads to the formation of Ce$^{3+}$ and Fe$^{2+}$ at the $2a$ site
producing an occupied Ce($4f^1$) state below the Fermi level that adds a
significant contribution to MCA and magnetic moment. A half Ce-substitution
forms a metallic state, while a full substitution retains the semiconducting
state of the strontium-hexaferrite (host). In the latter, the band gap is
reduced due to the formation of charge transferred states in the gap region of
the host. The optical absorption coefficient shows an enhanced anisotropy
between light polarization in parallel and perpendicular directions. Calculated
formation energies, including the analysis of probable competing phases, and
elastic constants confirm that both compositions are chemically and
mechanically stable. With successful synthesis, the Ce-hexaferrite can be a new
high-performing critical-element-free permanent magnet material adapted for use
in devices such as automotive traction drive motors.
