Dipolar interactions effect in the relaxation mechanism selection

M. E. DE SOUSA; P. MENDOZA ZÉLIS; F. H. SÁNCHEZ; M. B. FERNÁNDEZ VAN RAAP

Buenos Aires

Workshop; VII Latin American Workshop on Magnetism, Magnetic Materials and their applications; 2013

The influence of dipolar interactions in the relaxation mechanisms is addressed. To this end, specific absorption rates (SAR) were determined in a set of magnetite (Fe3O4 ) nanoparticles, 9 - 12 nm size and 39 -55 nm hydrodynamic sizes, electrostatically stabilized by citric acid coating, well dispersed in aqueous solution and displaying different degree of dipolar interaction. SAR values were obtained calorimetrically at a radio frequency field of 265 kHz and field amplitude of 40.1 kA/m are were analyzed in terms of structural and magnetic colloid properties. It is shown that dipolar interactions among the nanoparticles play a key role in Néel relaxation mechanism and in the dissipation efficiency. Mage netic relaxation becomes slower for interacting colloids and enhances heating efficiency. Largest SAR values arise from the more interacting nanoparticles, a case where Néel relaxation times become larger than Brown relaxation times e leading to Brown dissipation mechanism to prevail. It is shown that SAR data cannot be easily reproduced using the magnetic data and the most currently used theoretical expressions, due to the presence of dipolar interactions. Interaction energy is included in the theoretical expressions through the quantity σ = (Kef V + Eint )/kB T and its value inferred from the blocking temperature shift.