Investigation of Microstructure and Magnetic Properties of CH₄ Heat Treated Sr-Hexaferrite Powders during Re-Calcination Process

The microstructure and magnetic properties of methane (CH4) heat-treated Sr-hexaferrite powders during the re-calcination process were investigated and compared with the magnetic properties of conventionally synthesized Sr-hexaferrite powder. Gradual changes in the magnetic behavior of the produced powder in each re-calcination stage were investigated using magnetization curves obtained from the vibration sample magnetometry (VSM) technique. First, the initial Sr-hexaferrite powder was prepared by the conventional route. Then the powder was heat treated in a dynamic CH4 atmosphere in previously optimized conditions (temperature: 950 degrees C, gas flow rate:15 cc min(-1) and time: 30 min), and finally, re-calcined in various temperatures from 200 to 1200 degrees C. By investigating the hysteresis loops, we found the transition temperature of soft to hard magnetic behavior to be 700 degrees C. The maximum ratio M-r/M-s was obtained at temperatures of 800-1100 degrees C. At 1100 degrees C, and despite the Sr-hexaferrite single phase, the magnetic behavior showed a multiphase behavior that was demonstrated by a kink in the hysteresis loop. Uniform magnetic behavior was observed only at 900 degrees C and 1000 degrees C. Although the ratio M-r/M-s was almost the same at these temperatures, the values of M-r and M-s at 1000 degrees C were almost double of 900 degrees C. At 1000 degrees C, the second quadrant of hysteresis curve had the maximum area. Therefore, 1000 degrees C was the optimum temperature for re-calcination after CH4 gas heat treatment in the optimized conditions. Due to the presence of a small amount of hematite soft phase at 1000 degrees C, the most probable reason for the exclusive properties of the optimized product may be the exchange coupling phenomenon between the hard Sr-hexaferrite phase and the impurity of the soft hematite phase.