Formation of Nonequilibrium Magnetic Nanoparticle Structures in a Large Alternating Magnetic Field and Their Influence on Magnetic Hyperthermia Treatment

We simultaneously simulated thermally activated magnetization reversal and magnetic torque-driven rotation of mono- and polydisperse magnetite nanoparticles exposed to a large alternating magnetic field. We found that if the nanoparticles are rotatable, their easy axes are stationarily oriented to the directions parallel, perpendicular, or oblique to the magnetic field, depending on the relative conditions between the size of the nanoparticles and the amplitude and frequency of the magnetic field. The formations of such nonequilibrium oriented structures lead to significant variations of dynamic hysteresis loops. This knowledge would help reconcile good efficacy with a low side-effect profile in targeted magnetic thermotherapy using magnetite nanoparticles.