AbstractIn the present work the Er‐substituted Sr hexaferrite, SrFe12–x Erx O19, prepared by sol–gel self combustion and subsequent heat treatments, is investigated. The combusted powders were annealed at 800, 900 and 1000 °C for 5, 10, 20, 40, 80, 160 and 320 min. The influence of annealing temperature and time on the structure and magnetic properties (specific saturation magnetization M s, specific remanent magnetization M r and coercivity H c) is studied. The structural characterization was made using X‐ray diffraction and scanning electron microscopy. The magnetic measurements were carried out with a vibrating sample magnetometer. During the heat treatments the soft magnetic powder obtained by sol–gel self combustion is changed to a hard magnetic powder. The coercivity increases to a value much greater than its value before annealing. The remarkable increase of H c is explained by crystallization of the hard magnetic hexaferrite phase and corresponds to a transition from superparamagnetic to stable single‐domain particles, as a result of the grain growth during the heat treatment. The H c values of the samples heat treated for longer times (over 80 min) are fairly satisfactory. This means that the particle sizes were well under control to prevent abnormal growth of the particles. M s and M r depend on both annealing time and temperature. M s and M r begin to increase when H c sharply increases and tend to a constant value with increasing annealing time. Also, M s and M r decrease with increasing Er content. This dependence may be ascribed to the lowering of the total magnetic moment when Er3+ ions substitute magnetic Fe3+ ions in the M‐hexagonal structure or to the formation of more non‐magnetic phase like hematite, α‐Fe2O3. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)