Recent studies on next-generation permanent magnets have focused on filling in the gap between rare-earth magnets and rare-earth-free magnets, taking into account both the cost-effectiveness and magnetic performance of the magnetic materials. As an improved rare-earth-free magnet candidate, here, Ca-substituted M-type Sr-lean hexaferrite particles within a nano- to micro-scale regime, produced using an ultrasonic spray pyrolysis method, are investigated. Theoretically, the maximum coercivity (Hc) can be achieved in submicron Sr-ferrite crystals (i.e., 0.89 μm). The plate-like resultants showed a significant enhancement in Hc, up to a record high of 7880.4 Oe, with no deterioration in magnetization (M: 71–72 emu/g). This resulted in more favorable magnetic properties than those of the traditional Sr–La–Co ferrites. On the basis of microstructural analysis and fitting results based on the law of approach to saturation method, the Ca-substitution effects on the change in size and anisotropic characteristics of the ferrite particles, including pronounced lateral crystal growth and a strong increase in magnetocrystalline anisotropy, are clearly demonstrated. The cost-effective, submicron, and Ca-substituted Sr-ferrite is an excellent potential magnet and moreover may overcome the limitations of traditional hard magnetic materials.

In this study, we demonstrate a facile strategy for the large-scale preparation of ultrafine magnetic Sm 2 Fe 17 N 3 particles. During the preparation of magnetic nanoparticles (which includes reduction at a high temperature), a blocking agent (e.g., CaO), which enwraps the intermediate phase and discretizes it from the magnetic phase, is often introduced to control the particle size. Contemporary size-control strategies involve immersing solid Sm−Fe intermediates (e.g., Sm 2 O 3 −Fe mixed powder or SmFeO 3 ) in a liquid Ca-based precursor solution followed by a series of chemical reactions. However, a heterogeneous reaction with different phases (e.g., solid−liquid) leads to particulate aggregation, resulting in irregular shapes and broad size distribution of the magnetic particles and deteriorated magnetic performance. In this study, therefore, soluble Ca nitrates were added to Sm− and Fe−nitrate precursor solutions for controlling the size of Sm 2 Fe 17 N 3 submicron particles. Furthermore, the effect of such an addition on the microstructure and phase formation of the Sm−Fe binary system is elucidated. The resulting solutions were used as starting materials for the nitrate salt-based one-pot preparation process. The final products show a significant enhancement in magnetic performance (1.6 times higher energy product), indicating that the method has great potential for practical and inexpensive large-scale commercial applications.