Disentangling local heat contributions in interacting magnetic nanoparticles

Recent experiments on magnetic nanoparticle hyperthermia show that the heat dissipated by particles must be considered locally instead of characterizing it as a global quantity. Here we show theoretically that the complex energy transfer between nanoparticles interacting via magnetic dipolar fields can lead to negative local hysteresis loops and does not allow the use of these local hysteresis loops as a temperature measure. Our model shows that interacting nanoparticles release heat not only when the nanoparticle magnetization switches between different energy wells, but also in the intrawell motion, when the effective magnetic field is changed because the magnetization of another particle has switched. The temperature dynamics has a highly nontrivial dependence on the amount of precession, which is controlled by the magnetic damping. Our results constitute a step forward in modeling magnetic nanoparticles for hyperthermia and other heating applications.