The Influence of Heat-Treatment on Structural and Magnetic Properties of Rare-Earth Substituted Y-type Sr-Hexaferrites

Rare-earth substituted strontium ferrite nanopowders SrGa₂Fe_12−xR_xO₂₂ (R = La, Gd and Er; x = 0.2, 0.5 and 1) were prepared by chemical co-precipitation method and subsequent heat treatments. The results of X-ray diffraction measurements showed the Y-type hexagonal structure. Magnetic properties, such as specific saturation magnetization Ms, specific remanent magnetization Mr and coercivity Hc, as well as microstructure depend on the heat-treatment conditions (temperature and time). The coercivity Hc exhibits a great increase after a critical heattreatment time. This jump of Hc 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 Hc. With increasing R content both the Ms and Mr decrease due to the dissolution of R ions into the hexaferrite lattice.