Magnetic properties of ultrafine M-type hexaferrite nanoparticles with tunable sizes prepared by glass-ceramic method

Ultrafine M-type hexaferrite Ba 1- x Sr x Fe₁₂O₁₉ nanoparticles ( x = 0–0.8) were synthesized via a glass-ceramic route where the grain size was controlled by controlling the BaO/SrO ratio in precursor glasses. Sr 2+ substitution induces lattice contraction and reduces the average particle size from 78 nm ( x = 0) to 27 nm ( x = 0.8) through promoting the non-uniform nucleation of the strontium-rich borate phase. Meanwhile, the Sr substitution results in an increase in coercivity from 3.4 to 4.3 kOe and a slight rise in saturation magnetisation from 67.0 to 68.5 emu/g. To decrease the coercivity of the Ba 1- x Sr x Fe₁₂O₁₉ nanoparticles, subsequent Co Ti co-substitution ( y = 0–0.10) was further investigated on the x = 0.8 composition, which reduces coercivity from 4.3 kOe to 2.7 and 2.4 kOe, while M s decreases marginally to 62.1 emu/g at y = 0.10. Temperature-dependent magnetic measurements reveal distinct spin-glass-like freezing, evidenced by a peak in zero-field-cooled curves and irreversibility between zero-field-cooled and field-cooled magnetization, with peak temperature increasing with Co Ti content. This work establishes a quantitative framework linking compositional design, microstructural evolution, and magnetic performance, demonstrating the glass-ceramic route as a versatile platform for engineering hexaferrite nanoparticles with tunable size and coercivity for high density magnetic recording applications.