Relaxation processes of superparamagnetic iron oxide nanoparticles in liquid and incorporated in poly(methyl methacrylate)

Commercially available superparamagnetic iron oxide nanoparticles (SPION) of 12 nm are investigated with respect to the contribution of both Neel and Brown relaxation to the magnetic heating of these particles. For this purpose experiments have been performed to heat the particles, suspended in a liquid as well as incorporated in a poly(methyl methacrylate) (p(MMA)) matrix, using an alternating (ac) magnetic field of 745 kHz at three different field strengths up to 2850 A m(-1). It is shown that the specific absorption rates (SAR) of the particles in the ferrofluld are identical within experimental error to the SAR of the particles incorporated in p(MMA) at all measured field strengths. As Brown relaxation is not possible if the SPION are incorporated in the polymer matrix, it can be concluded that Neel relaxation is the only relaxation process that contributes to heating the ferrofluld at 745 kHz as well. This is confirmed by calculation of the relaxation times for Neel and Brown relaxation. Furthermore, calculations suggest that Neel relaxation is also the predominant relaxation process for small superparamagnetic particles at lower frequencies.