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 $100\mathrm{K}$ from hexagonal with spacegroup $P{6}_3∕mmc$ 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 ${\mathrm{LaFe}}_{12}{\mathrm{O}}_{19}$ at low temperatures was confirmed by measurements and associated with the formation of an orbital momentum of ${\mathrm{Fe}}^{2+}$ together with the lattice distortion. The localization of the ${\mathrm{Fe}}^{2+}$ ion at the $2a$ site was determined from the Mössbauer spectrum, showing also an indication for an orbital momentum in the hyperfine field. In contrast, neither a structural phase transition nor the formation of ${\mathrm{Fe}}^{2+}$ was observed at $x<1$ in x-ray diffraction or NMR spectroscopy.

• Received 23 December 2005

DOI:https://doi.org/10.1103/PhysRevB.73.144408