Size dependence of the magnetic relaxation and specific power absorption in iron oxide nanoparticles

In this study, magnetic and power absorption properties of a series of iron oxide nanoparticles with average sizes 〈d〉 ranging from 3 to 23 nm were reported. The nanoparticles were prepared by thermal decomposition of Iron(III) acetylacetonate in organic media. From the careful characterization of the magnetic and physicochemical properties of these samples, the specific power absorption (SPA) values experimentally found were numerically reproduced, as well as their dependence with particle size, using a simple model of Brownian and Néel relaxation at room temperature. SPA experiments in ac magnetic fields (H0 = 13 kA/m and f = 250 kHz) indicated that the magnetic and rheological properties played a crucial role determining the heating efficiency at different conditions. A maximum SPA value of 344 W/g was obtained for a sample containing nanoparticles with 〈d〉 = 12 nm and dispersion σ = 0.25. The observed SPA dependence with particle diameter and their magnetic parameters indicated that, for the size range and experimental conditions of f and H studied in this study, both Néel and Brown relaxation mechanisms are important to the heat generation observed.