Abstract
We present a general study of the frequency and magnetic field dependence of the specific heat power produced during field-driven hysteresis cycles in magnetic nanoparticles with relevance to hyperthermia applications in biomedicine. Employing a kinetic Monte-Carlo method with natural time scales allows us to go beyond the assumptions of small driving field amplitudes and negligible inter-particle interactions, which are fundamental to the applicability of the standard approach based on linear response theory. The method captures the superparamagnetic and fully hysteretic regimes and the transition between them. Our results reveal unexpected dipolar interaction-induced enhancement or suppression of the specific heat power, dependent on the intrinsic statistical properties of particles, which cannot be accounted for by the standard theory. Although the actual heating power is difficult to predict because of the effects of interactions, optimum heating is in the transition region between the superparamagnetic and fully hysteretic regimes.
Highlights
The effective local field acting on particle i is given by the sum of the external applied field, H, and the interaction field, Hidip
We have developed a kinetic Monte-Carlo model of the underlying heating mechanisms associated with the hyperthermia phenomenon used in cancer therapy
The so-called superparamagnetic regime is characterised by a dynamic hysteresis which transforms gradually to more conventional hysteresis arising from thermally activated transitions between metastable states
Summary
We present a general study of the frequency and magnetic field dependence of the specific heat power produced during field-driven hysteresis cycles in magnetic nanoparticles with relevance to hyperthermia applications in biomedicine. The kMC model naturally includes the time scales of intrinsic thermal fluctuations without any further need for calibration, as opposed to Time-Quantified Metropolis Monte-Carlo methods[27], and as we will show, genuinely reproduces the superparamagnetic regime, the ‘metastable’ and ‘dynamic’ hysteresis mechanisms[28], and their frequency dependent crossover. This allows for systematic validation of the standard theories, such as RT, and establishing their range of validity in practical applications where interactions cannot be ignored. Our approach allows us to quantify the importance of metastability in the heat production in MNP assemblies and thereby gauge its significance with respect to the superparamagnetic heating anticipated according to RT and Eq (1)
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