The influence of heat-treatment on microstructure and magnetic properties of rare-earth substituted SrFe12O19

Rare-earth substituted strontium ferrite nanopowders SrFe12-xRxO19 (R=La, Gd and Er; x=0.2, 0.5 and 1) were prepared by sol-gel-autocombustion method and subsequent heat treatments. The results of X-ray diffraction measurements showed the M-type hexagonal structure. Magnetic properties, such as specific saturation magnetization M-s, specific remanent magnetization sigma(r) and coercivity H-c, as well as microstructure depend on the heat-treatment conditions (temperature and time). The coercivity H-c exhibits a great increase after a critical heat-treatment time. This jump of H-c was explained by a transition from the superparamagnetic state to normal state of the single domain nanoparticles. The occurrence of an agglomerated structure composed of magnetically interacting ultrafine crystallites also contributes to the increase of H-c. With increasing R content both the M-s and M-r decrease due to the dissolution of R ions into the hexaferrite lattice. (C) 2007 Published by Elsevier B.V.