Nonlinear Dynamic Hysteresis Driven Self-Heating in Fe_100-xCo_x Alloy Nanoparticles

In this work, stoichiometric variation of the hydrothermally synthesized Fe_100-xCo_x (x = 35, 45, 50, 55, and 65) alloy nanoparticles (NPs) are presented to explain the correlation between composition, magnetic anisotropy, and self-heating performance under alternating magnetic field (AMF). X-ray diffraction was used to confirm the presence of a disordered BCC solid solution phase for all the samples, providing the necessary structural foundation for high saturation magnetization (M_s) in FeCo alloys. VSM analysis was used to establish that the equiatomic composition has the best magnetic characteristics, which include the highest M_s and maximum anisotropy constant, which corresponds to the highest anisotropy energy barrier. Induction heating experiments under a clinically relevant condition with an AMF (14.98 kA/m, 337 kHz) showed that Fe₅₀Co₅₀ reached a maximum specific absorption rate (SAR) of 291 W/g; thus, it was able to surpass conventional ferrites in performance. More importantly, we have found a direct linear relationship between the SAR and the anisotropy energy barrier (ΔE). Such strong dependence cannot be attributed to the Linear Response Theory (LRT), which is only valid at small field amplitudes. Because of the large field and the large dimensionless anisotropy constant ($\frac{\Delta E}{K_{B}T}\approx 1300)$, the process of heat generation is characterized by nonlinear processes, namely, dynamic hysteresis losses. This study reveals that in the high field regime, to maximize the area of the dynamic magnetic hysteresis loop and hence the thermal performance requires a large effective anisotropy barrier of the system. The Fe₅₀Co₅₀ composition is thus proven to be an ideal candidate for magnetic hyperthermia and other field-driven applications.