Mg shallow doping effects on the ac magnetic self-heating characteristics of γ-Fe₂O₃ superparamagnetic nanoparticles for highly efficient hyperthermia

The effects of Mg doping on the magnetic and AC self-heating temperature rising characteristics of γ-Fe₂O₃ superparamagnetic nanoparticles (SPNPs) were investigated for hyperthermia applications in biomedicine. The doping concentration of nonmagnetic Mg²⁺ cation was systematically controlled from 0 to 0.15 at. % in Mg_x-γFe₂O₃ SPNPs during chemically and thermally modified one-pot thermal decomposition synthesis under bubbling O₂/Ar gas mixture. It was empirically observed that the saturation magnetization (M_s) and the out-of-phase magnetic susceptibility (⁠$χm″$⁠) of Mg_x-γFe₂O₃ SPNPs were increased by increasing the Mg²⁺ cation doping concentration from 0.05 to 0.13 at. %. Correspondingly, the AC magnetically induced self-heating temperature (T_ac,max) in solid state and the intrinsic loss power in water were increased up to 184 °C and 14.2 nH m²kg⁻¹ (Mg_x-γFe₂O₃, x = 0.13), respectively, at the biologically and physiologically safe range of AC magnetic field (H_appl × f_appl = 1.2 × 10⁹ A m⁻¹s⁻¹). All the chemically and physically analyzed results confirmed that the dramatically improved AC magnetic induction heating characteristics and the magnetic properties of Mg_x-γFe₂O₃ SPNPs (x = 0.13) are primarily due to the significantly enhanced magnetic susceptibility (particularly, $χm″$⁠) and the improved AC/DC magnetic softness (lower AC/DC magnetic anisotropy) resulting from the systematically controlled nonmagnetic Mg²⁺ cation concentrations and distributions (occupation ratio) in the Fe vacancy sites of γ-Fe₂O₃ (approximately 12% vacancy), instead of typically well-known Fe₃O₄ (no vacancy) SPNPs. The cell viability and biocompatibility with U87 MG cell lines demonstrated that Mg_x-γFe₂O₃ SPNPs (x = 0.13) has promising bio-feasibility for hyperthermia agent applications.