Field dependent transition to the non-linear regime in magnetic hyperthermia experiments : comparison between maghemite, copper, zinc, nickel and cobalt ferrite nanoparticles of similar sizes

Abstract

Further advances inmagnetic hyperthermiamight be limited by biological constraints, such as using sufficiently low frequencies and low field amplitudes to inhibit harmfuleddy currents inside the patient’s body. These incite the need to optimize the heatingefficiency of the nano particles, referred to as the specific absorption rate (SAR).Among the several properties currently under research, one of particular importance is the transition from the linear to the non-linear regime that takes place as the field amplitude is increased, an aspect where the magnetic anisotropy is expected to play a fundamental role. In this paper we investigate the heating properties of cobalt ferrite and maghemite nano particles under the influence of a 500 kHz sinusoidal magneticfield with varying amplitude, up to 134 Oe. The particles were characterized byTEM, XRD, FMR and VSM, from which most relevant morphological, structural and magnetic properties were inferred. Both materials have similar size distributions and saturation magnetization, but strikingly different magnetic anisotropies. Frommagnetic hyperthermia experiments we found that, while at low fields maghemiteis the best nanomaterial for hyperthermia applications, above a critical field, close to the transition from the linear to the non-linear regime, cobalt ferrite becomesmore efficient. The results were also analyzed with respect to the energy conversionefficiency and compared with dynamic hysteresis simulations. Additional analysiswith nickel, zinc and copper-ferrite nanoparticles of similar sizes confirmed the importance of the magnetic anisotropy and the damping factor. Further, the analysis of the characterization parameters suggested core-shell nanostructures, probably due to a surface passivation process during the nanoparticle synthesis. Finally, we discussed the effect of particle-particle interactions and its consequences, in particular regarding discrepancies between estimated parameters and expected theoretical predictions.