Magnetic and viscous dynamics of spheroidal nanoparticles

The shape of magnetic nanoparticles (MNPs) greatly affects their dynamics and their potential as heating agents, e.g., for magnetic hyperthermia treatment of cancer. To investigate shape effects theoretically, we present a computationally efficient model of a uniformly magnetized, spheroid-shaped MNP, suspended in liquid at finite temperature, where the magnetic dynamics are coupled to mechanical rotation. Using exact solutions for the shape-dependent input parameters we vary the aspect ratio from a flat disk, through a sphere to a thin needle shape. By computing the energy absorption rate under an alternating magnetic field, and decomposing the dissipated energy into a frictional and magnetic loss channel, we determine the main heating mechanism in different parameter regimes. We find a critical aspect ratio for maximizing frictional losses, which yields the largest total absorption for a broad range of intermediate applied fields. Depending on the applied field strength, this may increase the absorption rate by over 200% relative to both spherical and highly elongated particles. For stronger fields (larger than about half the anisotropy field given maximal shape anisotropy), magnetic losses dominate for all shapes and extreme elongation is preferable.