M-SrFe12O19 and ferrihydrite-like ultrathin nanoplatelets as building blocks for permanent magnets: HAADF-STEM study and magnetic properties

Abstract

Mixtures of M-type strontium hexaferrite (M-SrFe12O19) and ferrihydrite-like particles were prepared by a microwave-assisted hydrothermal process at 200 °C with heating rates in the range 40–50 °C min−1. The particles exhibited a platelet shape with a diameter comprised between 20 and 200 nm and a thickness between 2 and 5 nm. HAADF-STEM observations and EDS analysis were carried out for a better understanding of nucleation and growth process. EDS showed that most of the particles contained Sr and HAADF-STEM revealed that very thin particles with a hexaferrite core extending over less than a unit cell and with surface disorder crystallized along with well crystallized hexaferrite and defect free ferrihydrite particles. The symmetric multilayer structures (SRS) of the ultrathin particles suggested that the nucleation step of the hexaferrite particles involved clusters containing Sr atoms. In comparison with the M-SrFe12O19 micrometer sized platelets prepared with heating rate of 25 °C min−1, the mixtures of ultrathin hexaferrite- and ferrihydrite-like particles combined after annealing a higher coercivity reaching 465 kA m−1 thanks to the smaller initial particle size and a high magnetization reaching 65 A m2 kg−1 thanks to a limited amount of hematite.