Structural phase transition and magnetic anisotropy of La-substituted M-type Sr hexaferrite

La-substituted Sr hexaferrite with different La concentrations x were prepared and investigated in order to observe changes in magnetic and structural properties with the substitution. With diffraction methods a change in crystal structure below approximately 100 K from hexagonal with spacegroup P6(3)/mmc to orthorhombic with spacegroup Cmcm was observed for the fully substituted La hexaferrite and attributed to a lattice distortion. A large increase in magnetocrystalline anisotropy of LaFe12O19 at low temperatures was confirmed by measurements and associated with the formation of an orbital momentum of Fe2+ together with the lattice distortion. The localization of the Fe2+ ion at the 2a site was determined from the Mossbauer spectrum, showing also an indication for an orbital momentum in the hyperfine field. In contrast, neither a structural phase transition nor the formation of Fe2+ was observed at x < 1 in x-ray diffraction or NMR spectroscopy.