M-type hexaferrites play an important role at the permanent magnet market since several decades because of their excellent performance cost ratio. Even slight improvements in magnetic properties would be of great importance for industrial application, which triggered many attempts to tune the magnetic properties by substituting elements at various lattice sites. While most of them led either to a decrease of magnetization or to a reduction of the magnetocrystalline anisotropy, the La-Co substitution causes an increased magnetocrystalline anisotropy. Recently this increase was explained by the influence of the Co ion. But already in the 1970's an increased anisotropy was found also for the pure La hexaferrite. To clarify the role of the rare earth element for enhancing magnetocrystalline anisotropy, detailed investigation of rare earth substituted hexaferrites. Emphasis was put on the La substituted Sr hexaferrite including the pure La hexaferrite.
The investigations contained the preparation of the material, the characterization with spectroscopic methods (X-ray, Mössbauer, NMR), examinations of microstructure with SEM and optical microscope as well as magnetization and anisotropy measurements. Electron structure calculations based on the density functional theory were performed in order to give a theoretical explanation of the magnetic properties of this compound. A strongly increased anisotropy was observed for the La hexaferrite at low temperatures, while La substituted Sr hexaferrite with concentrations smaller than La/Sr=3/1 exhibited a decrease with decreasing temperature. It was possible to fit all experiments and calculations to a picture, giving a qualitative explanation of the enhanced anisotropy.