The power of torque magnetometry: defect induced switching in hexaferrite nano-structures.

The presence of magnetic heterogeneities in ferromagnetic systems can reduce their performances in applications such as data storage media and permanent magnets. The detection and the full characterization of these heterogeneities is difficult especially when they are present in a very low concentration within ferromagnetic systems. Here, torque magnetometry is applied to investigate the magnetism of heterogeneities in two-dimensional hexaferrite structures. The study was conducted on a two-dimensional BaFe₁₂O₁₉ structure with decoupled nano-platelets grown on oxidized silicon buffered with ZnO. Measured torque curves reveal anomalous effects manifested by the presence of reversible and hysteretic kinks at large and low magnetic fields respectively. These kinks represent the contribution of magnetic heterogeneities to the global anisotropy of the film in addition to the two-fold symmetry of the major perpendicular anisotropy component. The heterogeneities consist of two types of nano-metric crystallites with the same magnitude of anisotropy as the major magnetic phase, but tilted about -74° and 74° from the normal to the film plane. These results are supported by various types of material characterization (SEM, XPS, XRD, MFM and VSM) and are well reproduced with a simple theoretical model that replicates the symmetry, switching and the easy axes alignment of the heterogeneities magnetization.