Microwave-assisted synthesis and magnetic properties of M-SrFe12O19 nanoparticles

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 degrees C using a heating rate of 25 degrees C. min(-1). The particles exhibited a magnetic coercivity of 95 kA.m(-1) (mu(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-1 (mu(0)Hc = 0.445 T) by annealing at the optimum temperature of 1000 degrees 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 degrees C using a heating rate of 40 degrees 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 degrees 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) (mu(0)Hc = 0.585 T). (C) 2017 Elsevier B.V. All rights reserved.