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

Highlyaligned ceramic hexaferrite magnets with high-energy products(BH)(max) and a density exceeding 90% oftheoretical density have been fabricated. The precursors were an antiferromagneticpowder, a six-line ferrihydrite mixed with SrCO3, and agrain growth inhibitor SiO2. Conventional cold compactionof the precursor powders was employed prior to calcination at temperaturesof 1050, 1150, and 1250 degrees C. The influence of calcination temperatureand magnetic properties has been systematically studied in the producedceramic magnets. Conventional cold compaction is a favorable routefor industrial production when compared with other compaction techniqueslike spark plasma sintering, hot compaction, or electroforging. Ahigh (BH)(max) of 25.2 kJ/m(3) wasobtained for the best magnet along with an appreciable coercivity, H (c), of 187 kA m(-1), a high squarenessratio, M (r)/M (s), of 0.84, and a saturation magnetization, M (s), of 73 A m(2)/kg. Texture and crystallite size analysiswere extracted from 2D synchrotron transmission powder diffractionmeasurements. We have demonstrated that high-performance bulk magnetsfor permanent magnet applications can be produced from nonmagneticinteracting crystallites mixed with a grain growth inhibitor withoutapplying a magnetic field for alignment.