Strontium hexaferrite nanoparticles were synthesized by a microwave-assisted hydrothermal process. The variation of structure, morphology and magnetic properties of the as-produced particles and after annealing temperatures were carefully analysed. Pure M-SrFe12O19 powders were synthesized at T = 200°C using a heating rate of 25 °C.min-1. The particles exhibited a magnetic coercivity of 95 kA.m-1 (µ0Hc = 0.12 T), explained by the shape of the particles that crystallized as very thin platelets with a micrometer size diameter and a very high aspect ratio in which a competition between shape and magnetocrystalline anisotropy takes place. The coercivity was strongly enhanced with Hc = 360 kA.m-1 (µ0 Hc = 0.445 T) by annealing at the optimum temperature of 1000°C. In order to optimize the particle morphology and magnetic properties after annealing, the heating rate of the microwave synthesis was increased. At T = 200°C using a heating rate of 40 °C.min-1 the particle exhibited a size in the range 20-100 nm. The powder crystallized as a mixture of hexaferrite and ferrihydrite. After annealing at 1000 °C, M-SrFe12O19 with a small amount of hematite (<15 %) was obtained. The coercivity was strongly enhanced to reach the value Hc = 465 kA.m-1 (µ0Hc = 0.585 T).

Mixtures of M-type strontium hexaferrite (M-SrFe 12 O 19) 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-SrFe 12 O 19 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.m 2 .kg-1 thanks to a limited amount of hematite.

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