Effect of Various Composition Ratios on the Exchange Spring Effect in SrFe₁₂O₁₉/CoFe₂O₄ Nanocomposite Magnets

This study aims to examine the effect of various composition ratios on the exchange spring effect of strontium ferrite (SrFe₁₂O₁₉)/cobalt ferrite (CoFe₂O₄) nanocomposite magnets. Nanoparticles of hexagonal hard phase SrFe₁₂O₁₉ and spinel soft phase CoFe₂O₄ were each prepared through mechanical alloying and high-power ultrasonic irradiation processes. Furthermore, the constituent of the hard-soft particles was weighed based on its ratios and the mixture was homogenized under a sonicator-type Qsonica at a frequency of 20 kHz for an hour. The bulk composite samples were obtained in cylindrical pellets form through compaction of mixture powders at 5000 kg/cm², followed by sintering at 1200 °C. The exchange spring effect enhanced the magnetic properties, surpassing the normal limit for non-interacting magnetic particles due to enhanced grain interactions. The results showed that the degree of property enhancement depended on the various composition ratios and microstructure of the composite magnets. The magnetization of saturation (M_s) of the samples increased compared to a single SrFe₁₂O₁₉. Furthermore, the highest coercivity (H_c) and product energy maximum (BH)_max values were observed in the sample which had mass ratios of SHF:COF of 85:15 with coded S80C20. The occurrence of exchange spring effects was observed in material characterized by a microstructure consisting of a hard-magnetic phase with elevated magnetocrystalline anisotropy and saturation magnetization values, along with a soft magnetic phase exhibiting high saturation magnetization value.