Impact of nanoparticle dimensions on size-induced magnetic transitions in Fe₃O₄–PBTTT-C₁₄ hybrid films

Polymer magnetic films (PMFs) were fabricated by incorporating Fe₃O₄ nanoparticles (9–45 nm) into PBTTT-C₁₄ semiconductor films to systematically investigate size-induced magnetic transitions. Magnetic force microscopy (MFM) and superconducting quantum interference device (SQUID) measurements revealed a critical size-dependent transition: nanoparticles with diameters of 9.81–15.90 nm exhibited superparamagnetic behavior with negligible coercivity, while those larger than 25.89 nm displayed distinct ferromagnetic behavior, with coercivity increasing with Fe₃O₄ nanoparticle size up to a maximum of 127.8 Oe. The 32.94 nm particle sample achieved the highest saturation magnetization of 238.67 emu/cm³, establishing an optimal size for enhanced magnetic performance. X-ray photoelectron spectroscopy (XPS) confirmed strong chemical interactions between Fe and the polymer backbone through prominent Fe 2p_3/2 and 2p_1/2 peaks at 711 eV and 725 eV. Time-resolved photoluminescence (TRPL) revealed size-dependent exciton decay and size-induced magnetic transition of Fe₃O₄ nanoparticles from 9.81 nm superparamagnetism to 44.84 nm ferromagnetism with corresponding spin coupling and magnetic modulation. Raman spectroscopy demonstrated that 32.94 nm particles produced the narrowest full width at half maximum (FWHM) at 1489 cm⁻¹, reflecting optimal polymer chain ordering. This study elucidates the size-dependent mechanisms governing magnetic behavior in polymer-Fe₃O₄ composites, providing fundamental insights for developing advanced spintronic devices.