Rare-earth substituted strontium ferrite nanopowders SrFe_12-xR_xO₁₉ (R = La, Gd and Er; x = 0.2, 0.5 and 1) were prepared by sol-gel-autocombustion method and subsequent heat treatments. Structural and magnetic properties of SrFe_12-xR_xO₁₉ powders heat treated at 800, 900 and 1000°C, for various times, were characterized with an X-ray diffractometer, a vibrating sample magnetometer and a scanning electron microscope. The results of X-ray diffraction measurements showed the M-type hexagonal structure formation by heat treatments. Magnetic properties, such as specific saturation magnetization, specific remanent magnetization and coercivity, as well as microstructure depend on the heat treatment conditions (temperature and time). The coercivity exhibits a great increase after a critical heat treatment time. When the heat treatment time increases, one obtains an increase in coercivity after a shorter heat treatment time. This jump of the coercivity 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 coercivity. The heat treatment determines a reduced grain growth due to the internal stress generated by R ions. With increasing R content the specific saturation magnetization and specific remanent magnetization decrease due to the dissolution of R ions into the hexaferrite lattice. We believe that by selecting the time and temperature of the heat treatment, microstructure and magnetic properties suitable for magnetic recording media application can be obtained.

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