Magnetic behavior of superparamagnetic Fe nanocrystals confined inside submicron-sized spherical silica particles

Abstract

We have studied the magnetic behavior of superparamagnetic Fe nanocrystals (4--7 nm in diameter) dispersed in submicron-sized spherical silica particles $(\ensuremath{\sim}150\mathrm{nm}$ in diameter). Spherical composites that could be useful for biomedical applications were prepared by an aerosol-assisted method. M\"ossbauer studies have allowed us to determine that the magnetic response of the composites must be the result of a competition between intraparticle anisotropy and interparticle dipolar interactions. Evidence of an interacting superparamagnetic (ISP) regime that is characterized by a ${H/M}_{s}$ scaling law of the reduced magnetization isotherms instead the $H/T$ scaling law of the ideal superparamagnetic regime has been found in the composites. The ISP regime, as recently reported in similar nanostructured systems, appears as an intermediate regime, separating the high-temperature, conventional superparamagnetic regime from the low-temperature, blocked-particle regime. We have also found that the normalized values of ${M}_{s}$ at room temperature are function of the Fe metallic particle size. Finally, we have found that the magnetic anisotropy constant of superparamagnetic Fe nanoparticles depend on the nature of their coating shell.