• SrFe12O19 based ceramics with ultra-low dielectric loss in the millimetre-wave band

      Yu, Chuying; Zeng, Yang; Yang, Bin; Wylde, Richard; Donnan, Robert; Wu, Jiyue; Xu, Jie; Gao, Feng; Abrahams, Isaac; Reece, Mike J.; Yan, Haixue; Queen Mary University of London; Hunan University; National University of Defence Technology; University of Chester; Thomas Keating Ltd; Northwestern Polytechnical University (AIP Publishing, 2018-04-02)
      Non-reciprocal devices such as isolators and circulators, based mainly on ferromagnetic materials, require extremely low dielectric loss in order for strict power-link budgets to be met for millimetre (mm)-wave and terahertz (THz) systems. The dielectric loss of commercial SrFe12O19 hexaferrite was significantly reduced to below 0.002 in the 75 - 170 GHz band by thermal annealing. While the overall concentration of Fe2+ and oxygen vacancy defects is relatively low in the solid, their concentration at the surface is significantly higher, allowing for a surface sensitive technique such as XPS to monitor the Fe3+/Fe2+ redox reaction. Oxidation of Fe2+ and a decrease in oxygen vacancies is found at the surface on annealing, which is reflected in the bulk sample by a small change in unit cell volume. The significant decrease in dielectric loss property can be attributed to the decreased concentration of charged defects such as Fe2+ and oxygen vacancies through annealing process, which demonstrated that thermal annealing could be effective in improving the dielectric performance of ferromagnetic materials for various applications.