Applying nonharmonic driving-field waveforms to nanoparticles for magnetic hyperthermia has beneficial effects on their ability to generate heating power. The response of superparamagnetic nanoparticles described as double-well systems with randomly distributed easy axes is studied by means of a rate-equation approach in the typical range of frequencies adopted in therapeutic applications. Magnetic hysteresis loops obtained at various frequencies and vertex fields show that sawtooth and square field waveforms substantially enhance the specific loss power (SLP) with respect to a harmonic excitation of the same frequency and amplitude. This improvement is related to the presence of fast, quasiadiabatic transformations where the occupancy numbers in the two potential wells are virtually frozen, resulting in largely off-equilibrium magnetization states and in hysteresis loops with a greater area. The square driving-field waveform is particularly promising for magnetic hyperthermia, the SLP being not only most effectively enhanced but also independent of particle size over a large interval of diameters. The special features of the hysteresis loops generated by the square driving-field waveform are studied in detail; a peculiar effect of loop instability is described, and a condition for having stable hysteresis loops is defined. The considered driving-field waveforms can be made to comply with the physiological restrictions imposed by magnetic hypertermia treatments in vivo.

• Received 9 May 2019
• Revised 4 September 2019

DOI:https://doi.org/10.1103/PhysRevApplied.12.034041