Dipolar interactions effect in the relaxation mechanism selection

DE SOUSA,M.ELISA; MENDOZA ZÉLIZ, PEDRO; F. H. SÁNCHEZ; FERNANDEZ VAN RAAP, MARCELA

Congreso; X-Latin American Workshops on magnetismo, magnetic materials and their applications; 2013

Magnetite (Fe3O4) nanoparticles are proper materials for Magnetic Fluid Hyperthermia applications whenever these conjugate stability at physiological (neutral pH) medium and high specific dissipation power (SAR). Here, magnetite nanoparticles (MNPs) 9−12 nm in size, electrostatically stabilized by citric acid (CA) coating, with hydrodynamic sizes in the range 17−30 nm, and well dispersed in aqueous solution were prepared using a chemical route. SAR values were obtained calorimetrically under a radio frequency excitation 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. Largest SAR values arise from the more interacting nanoparticles, a case where Neel relaxation times become larger than Brown relaxation times, 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= (KeffV + Eint)/kBT and its value inferred from the blocking temperature shift.