(201ai) Magnetization Dynamics and Energy Dissipation of Interacting Magnetic Nanoparticles in Dynamic Magnetic Fields

In recent decades, magnetic nanoparticles have attracted a lot of attention because they can be manipulated by applied magnetic fields. As suspensions of magnetic nanoparticles are subjected to alternating magnetic fields (AMFs), the dispersed particles respond to generate heat, which can be employed to actuate release of a drug, or to deposit heat in a cancer tumor, as in magnetic hyperthermia. When a static bias magnetic field gradient is superimposed with a uniform AMF, as that in magnetic particle imaging (MPI) technology, it generates a field free region, in which the response of the magnetization of the particles to the AMF is maximum. As a result, with MPI it is possible to map magnetic nanoparticle tracers in vivo with millimeter to sub-millimeter spatial resolution and excite them for hyperthermia with spatial selectivity. Many prior studies have explored the effect of particle properties and applied field parameters on the so-called specific absorption rate (SAR), which is used to quantify the energy dissipation rate of magnetic nanoparticles. Some of these studies have suggested that magnetic interaction between particles plays an important role in determining SAR of magnetic nanoparticles in AMFs. However, prior computational work studying the role of interactions in energy dissipation fixes particle positions and as such has not considered the potential role of field-induced aggregation on energy dissipation rates.

Here we report a computational study of the magnetization dynamics and energy dissipation rates of spherical single-domain magnetically-blocked nanoparticles in static and alternating magnetic fields, by carrying out Brownian dynamics simulations that account for translation and rotation of the nanoparticles, hydrodynamic drag, thermal fluctuation, magnetic interactions, and a repulsive interaction potential. In the case of a static magnetic field, we studied the magnetic relaxation time of the nanoparticles as the field is suddenly applied and suppressed, for various values of the magnetic interaction strength parameter. Our results suggest that increasing the strength of magnetic interaction leads the equilibrium magnetization of the magnetic suspension under a static field to increase first and then decrease. The results of magnetorelaxometry show that magnetic interactions increase the characteristic relaxation time of the particles to changes in the field. For an applied AMF with and without a static bias field, the particle response is analyzed in terms of the harmonic spectrum of particle magnetization, dynamic hysteresis loops, and SAR as a function of the amplitude and frequency of the AMF, the strength of magnetic interaction, and the magnitude of an applied bias field. Results suggest that for low frequencies of the AMFs, increasing interactions result in an increase in SAR for all excitation field amplitudes studied. For intermediate frequencies of the applied AMF strong magnetic interactions reduce the SAR value slightly at low excitation field amplitudes and enhance the SAR value slightly at high excitation field amplitudes. For the highest frequencies considered, interactions appeared to lower SAR value slightly at low excitation field amplitudes but had no effect at higher excitation field amplitudes. These results provide theoretical insight into the role of particle-particle interactions on the performance of magnetic nanoparticles for application in magnetic hyperthermia and magnetically-triggered drug delivery.