High-Performance Hexaferrite Ceramic Magnets Made from Nanoplatelets of Ferrihydrite by High-Temperature Calcination for Permanent Magnet Applications

Highly aligned ceramic hexaferrite magnets with high-energy products ( BH ) max and a density exceeding 90% of theoretical density have been fabricated. The precursors were an antiferromagnetic powder, a six-line ferrihydrite mixed with SrCO 3, and a grain growth inhibitor SiO 2 . Conventional cold compaction of the precursor powders was employed prior to calcination at temperatures of 1050, 1150, and 1250 °C. The influence of calcination temperature and magnetic properties has been systematically studied in the produced ceramic magnets. Conventional cold compaction is a favorable route for industrial production when compared with other compaction techniques like spark plasma sintering, hot compaction, or electroforging. A high ( BH ) max of 25.2 kJ/m 3 was obtained for the best magnet along with an appreciable coercivity, H c, of 187 kA m –1, a high squareness ratio, M r / M s, of 0.84, and a saturation magnetization, M s, of 73 A m 2 /kg. Texture and crystallite size analysis were extracted from 2D synchrotron transmission powder diffraction measurements. We have demonstrated that high-performance bulk magnets for permanent magnet applications can be produced from nonmagnetic interacting crystallites mixed with a grain growth inhibitor without applying a magnetic field for alignment.