Magnetic property enhancement of cobalt-free M-type strontium hexagonal ferrites by CaCO₃ and SiO₂ addition

An experiment was performed to investigate whether CaCO₃ and SiO₂ addition enhances the magnetic properties of cobalt-free M-type Sr ferrites, in which experimental parameters such as additive composition and the CaCO₃/SiO₂ ratio were changed. The specimens were prepared using conventional ceramic techniques. CaCO₃ addition promoted densification and uniform grain growth, resulting in high remanence; however, with the simultaneous addition of CaCO₃ and SiO₂, coercivity decreased to 3046 Oe in Sr ferrite hard magnets without Co₃O₄. SiO₂ addition suppressed grain growth. The addition of appropriate concentrations of CaCO₃ and SiO₂ as additives after calcination was shown to be very beneficial for tailoring a dense microstructure with relatively small grains. A CaCO₃/SiO₂ ratio of approximately 1.8 was found to be optimum, and the magnetic properties of B_r = 4.3 kG, iH_c = 3.7 kOe, and (BH)_max = 4.5 MGOe were obtained for Sr ferrite hard magnets without Co₃O₄, which are acceptable for motor applications under output power <1 kW.