Lanthanum M-type hexaferrite: a magnet, a semiconductor, and a quantum information material

Iron-based hexaferrites are critical-element-free permanent magnet components of magnetic devices. Of particular interest is electron-doped M-type hexaferrite i.e., LaFe₁₂O₁₉ (LaM) in which extra electrons introduced by lanthanum substitution of barium/strontium play a key role in uplifting the magnetocrystalline anisotropy. We investigate the electronic structure of lanthanum hexaferrite using a density functional theory with localized charge density, which reproduces semiconducting behavior and identifies the origin of the very large magnetocrystalline anisotropy. Localized charge transfer from lanthanum to the iron at the crystal's 2a site produces a narrow 3d_z² valence band strongly locking the magnetization along the c axis. The calculated uniaxial magnetic anisotropy energies from fully self-consistent calculations are nearly double the single-shot values, and agree well with available experiments. The chemical similarity of lanthanum to other rare earths suggests that LaM can host other rare earths possessing nontrivial 4f electronic states for, e.g., microwave-optical quantum transduction.