Hard magnetic materials a b s t r a c t 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-SrFe 12 O 19 powders were synthesized at T = 200 °C using a heating rate of 25 °C.min À1. The particles exhibited a magnetic coercivity of 95 kA.m À1 (m 0 Hc = 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 À1 (m 0 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 À1 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-SrFe 12 O 19 with a small amount of hematite (<15%) was obtained. The coercivity was strongly enhanced to reach the value Hc = 465 kA.m À1 (m 0 Hc = 0.585 T).