Synthesis, Structural Evolution, and Magneto-mechanical Effect of SrFe₁₂O₁₉ Nanoparticles for Biomedical Applications

Strontium hexaferrite (SrFe₁₂O₁₉) is a chemically robust hard-magnetic material with high magnetic anisotropy, making it a promising candidate for non-thermal magneto-mechanical cancer therapy. In this work, SrFe₁₂O₁₉ nanoparticles were synthesized via a citrate precursor route followed by rapid annealing (5 and 30 min at 950°C), high-energy planetary ball milling in an oleic-acid/octadecene medium (up to 48 h), and PEG functionalization to enable aqueous dispersibility. Processing-dependent changes in phase composition, crystallite size, lattice parameters, magnetic properties, and colloidal behavior were correlated with biological performance. X-ray diffraction confirmed predominantly single-phase SrFe₁₂O₁₉ after annealing, while prolonged milling eliminated residual hematite, reduced the crystallite size from ~ 23–25 nm to ~ 8–12 nm, and induced lattice expansion and partial amorphization, resulting in decreased coercivity, remanence, and saturation magnetization. PEGylation produced stable aqueous dispersions with hydrodynamic diameters of ~ 165–175 nm and low polydispersity, although the magnetic response was further reduced due to the non-magnetic polymer shell and size-selective fractionation. In vitro studies on 4T1 murine adenocarcinoma cells showed low intrinsic cytotoxicity up to 50 µg/mL. A pronounced magneto-mechanical effect was observed under a low-frequency alternating magnetic field (100 mT, 22 Hz) for the nanoparticles annealed for 30 min and milled for 48 h, reducing cell viability from ~ 55% to ~ 25% at 200 µg/mL (IC₅₀ ≈ 150 µg/mL). These results identify SrFe₁₂O₁₉ as a cobalt-free platform for field-induced cancer therapy.