Magnetic nanoparticles are of increasing interest due to their unique physical properties such as superparamagnetism, the exchange-bias effect, and particle-size-dependent static and dynamic properties [1-3]. These novel characteristics make magnetic nanoparticles very attractive for bio-applications including magnetic hyperthermia and MRI contrast agents. As an example, soft magnetic nanoparticles in single-domain states exhibit collective Larmor precession of individual spins. In cases where the frequency of time-varying magnetic fields equals the Larmor precession frequency, individual magnetic moments efficiently absorb energies that are transferred from externally applied AC magnetic fields, after which those energies dissipate into other forms due to their intrinsic damping of given materials. Such energy dissipations of magnetic nanoparticles are of crucial importance in hyperthermia bio-applications for high specific loss power (SLP). Larmor precession motions of individual spins in magnetic particles excited by relatively high-frequency (several hundred MHz) AC magnetic fields can give rise to a higher efficiency of energy dissipation than those by Brownian rotation of nanoparticles and/or by Néel relaxation of nanoparticles’ magnetizations.
In the present study, we explored robust non-linear magnetization dynamics and the associated high-efficiency energy-dissipation effect using soft single-domain-state magnetic nanospheres 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. 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.
The research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1A2A1A10056286).
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