The role of Co²⁺ cation addition in enhancing the AC heat induction power of (Co_xMn_1-x)Fe₂O₄ superparamagnetic nanoparticles

The physical role of magnetically semi-hard Co²⁺ cation addition in enhancing the AC heat induction temperature (T _AC) or specific loss power (SLP) of solid (Co _x Mn_1−x)Fe₂O₄ superparamagnetic iron oxide nanoparticles (SPIONPs) was systematically investigated at the biologically safe and physiologically tolerable range of H _AC (H _AC,safe = 1.12 × 10⁹ A m⁻¹ s⁻¹, f _appl = 100 kHz, H _appl = 140 Oe (11.2 A m⁻¹)) to demonstrate which physical parameter would be the most critical and dominant in enhancing the T _AC (SLP) of SPIONPs. According to the experimentally and theoretically analyzed results, it was clearly demonstrated that the enhancement of magnetic anisotropy (K _u)-dependent AC magnetic softness including the Néel relaxation time constant τ _N (≈τ _eff, effective relaxation time constant), and its dependent out-of-phase magnetic susceptibility χ ″ primarily caused by the Co²⁺ cation addition is the most dominant parameter to enhance the T _AC (SLP). This clarified result strongly suggests that the development of new design and synthesis methods enabling to significantly enhance the K _u by improving the crystalline anisotropy, shape anisotropy, stress (magnetoelastic) anisotropy, thermally-induced anisotropy, and exchange anisotropy is the most critical to enhance the T _AC (SLP) of SPIONPs at the H _AC,safe (particularly at the lower f _appl < 120 kHz) for clinically safe magnetic nanoparticle hyperthermia.