Magnetic Anisotropy of doped Hexaferrites

Hexaferrites are among the oldest and technically most important permanent magnets. In this project we intend to combine the local magnetic probe of nuclear magnetic resonance (NMR) and state of the art density functional theory bandstructure calculations to improve our understanding of the microscopic mechansim responsible for the magnetic anisotropy of the hexaferrites. In this way we hope to provide a sound scientific basis for the search for further improvements of the hard magnetic ferrites. Hexagonal ferrites cover even after several decades of applications more than 50% of the permanent magnet market because the lower price compensates for the superior magnetic performance of the rare earth based permanent magnets developed in the last years. Therefore, any improvement of the properties of the classical barium-hexaferrite is of special relevance for a large market. In view of the technical importance of the ferrites many attempts have been made to improve their magnetic properties by doping or substitutions. In this way the material can be fine tuned to achieve special coercivity or grain sizes required e.g. in recording media, but until recently no significant improvement of the key magnetic properties, saturation magnetization and the anisotropy field, was achieved. Instead, doping on the Ba and Fe-sublattices was in general found to reduce both parameters. However, some recent results from materials research based simply on trial and error showed significant improvement of the hard magnetic properties of hexaferrites. The finding triggered considerable research interest but the underlying mechanism of the effect remained unclear. Magnetism in the hexaferrites is carried by iron-ions on five positions in a complex magnetic structure. Knowledge of the local properties of iron on these different positions is crucial for a better understanding of the macroscopic properties. The experimental technique of nuclear magnetic resonance used here is the most precise method available to determine the five possible magnetizations of iron in the complex magnetic structure. At the same time the density functional theory is the most accurate theoretical tool available to calculate the local electronic and magnetic properties. Both techniques together provide a powerful tool for the investigation of the magnetic anisotropy of hexaferrites.

The magnetic anisotropy is a key material property of permanent magnets. Despite the fact that it determines the possible applications of a magnetic material in a huge market ranging from sensors and actuators in industry to refrigerator door seals, speakers; or hard disks in consumer electronics our understanding of the microscopic of the magnetic anisotropy is based an relatively crude assumptions an the electronic properties of the solid state and, therefore, rather poor. This holds even for the Hexaferrites which dominate the permanent magnet market since nearly live decades. The aim of the project was to arrive at a state of the art description of the magnetic anisotropy in these fairly cornplex rnaterials. This was achieved using modern computational methods to calculate the electronic band structure and identiy among the possible solutions of the cornplex equations the actual one by direct comparison with microscopic magnetic experimental data. Two different sources of magnetic anisotropy are known: The dipolar contribution, which is due to all magnetic moments in the solid, and angular magnetic moments, which are a local property of special sites in a structure. We found that a significant part of the magnetic anisotropy of the undoped Hexaferrites is, in fact, of classical dipolar origin. The major part is due to a small angular magnetic momentum at the Fe-atoms in the structure. The energy resolution of the current computational methods is, however; too poor to reliably assign this contribution to one er more of the live possible Fe sites. In contrast, our calculations as well as the experiments of Hexaferrites doped with Lanthanum clearly proved that the very large additional magnetic anisotropy in these new rnaterials can be attributed to one special Fe site in the crystal structure carrying a large angular magnetic momentum which is fixed in a direction along one crytalografic axis. The investigation of the temperature dependence of the effect also showed that, unfortunately, strucual instabilities make it very unlikely that this advanced feature of the La-doped Hexaferrites can be used in technical applications at room temperature.