Spin dynamics of magnetic nanoparticles and its application for magnetic hyperthermia

Abstract

Localized magnetic particle hyperthermia heating treatment using magnetic nanoparticles continue to be an active area of cancer research. In magnetic hyperthermia, magnetic nanoparticles are subjected in alternating magnetic field, thus forced to release energy-dissipation as a heat (via various mechanisms: Néel-Brown relaxation, hysteresis loss) in their surrounding while neighboring cancer cells undergo severe thermal shock and potential destruction. One of the critical issues of the mechanisms for describing the conventional hyperthermia treatment is relatively low heating power, which is not enough to kill cancer cells. To improve the heating performance of magnetic nanoparticles, there have been numerous researches using various analysis methods. However, since the mechanism of heat generation from magnetic nanoparticles is not newly proposed, the reported researches have limitations on the degree of improvement in heating power. Here, we have proposed a new mechanism for the energy-dissipation of magnetic nanoparticles based on resonance of collective spin dynamics that can maximize the heating power. First, we explored robust non-linear magnetization dynamics and the associated high-efficiency energy-dissipation effect in single-domain soft magnetic nanospheres, as excited by oscillating magnetic fields of different frequencies and amplitudes under given static magnetic fields. We conducted micromagnetic simulations to explore the novel magnetization dynamics of soft magnetic particles and additional analytical derivations of the energy-dissipation rate for the steady-state regime by varying the frequency and strength of rotating magnetic fields for different Gilbert damping constants and static magnetic field strengths. All of the simulation results and analytical calculations agree well quantitatively. The dynamic origin of such a high-efficiency energy-dissipation mechanism is completely different from those of the typical ones used in bio-applications. Furthermore, we have extended the object of the energy-dissipation study by the magnetic resonance phenomenon from the single-domain state to the magnetic-vortex state. Using both micromagnetic simulations and semi-analytical analysis, we addressed the similarities and differences between the single-domain state and magnetic-vortex state in terms of the temporal evolutions of the spin dynamics and energy-dissipation calculated for all variables given to the system. The energy-dissipation generated by resonant excitation of magnetic vortex was smaller than the energy-dissipation of single-domain state, and it is directly related to , where is the average magnetization component over the sphere volume in the vortex-core orientation. Finally, we identify the existence of spin-dynamics driven energy-dissipation in magnetic nanoparticles from ferromagnetic resonance experiments using vector network analyzer and compare the results with analytical calculation and micromagnetic simulation. From the experiments, we observed that the energy-dissipation released by the resonance of spins inside the nanoparticles well corresponded to the calculated results. This work provides further insights into the fundamentals of magnetization dynamics in magnetic particles and the associated energy dissipation effect, and suggests a highly efficient means of magnetic-hyperthermia-applicable energy dissipation.