The impact of temperature on thermal fluctuations in magnetic nanoparticle systems

Abstract

We investigate the effect of temperature on the thermal magnetic noise signal of magnetic nanoparticle (MNP) systems as models for non-interacting macrospins. An analytical expression for the amplitude of the fluctuations in a magnetic field is derived for the Brownian and the Néel fluctuation mechanisms and compared with numerical results at different temperatures. To experimentally validate our findings, magnetic noise spectra of two commercially available polydisperse MNP systems (Ferucarbotran and Perimag) were measured at different, biomedically relevant temperatures. A distinctive effect of temperature on the power spectral noise densities is measurable already for 5 K temperature differences and, within the bandwidth of the experiment, higher noise amplitudes are found for lower temperatures. However, a crossing of the spectra at higher frequencies is revealed in simulations so that the total fluctuation amplitude is conserved. These findings contribute to a profound understanding of temperature influences on MNP fluctuation and relaxation mechanisms.