Characterization of Magnetic Core-Shell Nanoparticle Suspensions Using AC Susceptibility for Frequencies up to 1 MHz

Abstract

The complex susceptibility was measured on various magnetic core‐shell nanoparticle suspensions in the frequency range between 100 Hz and 1 MHz and compared to theoretical models. It is shown that the spectrum of the imaginary part of the ac susceptibility χ’’ measured on aqueous suspensions of multicore magnetite nanoparticles with wide distributions of core and hydrodynamic sizes contains contributions from Brownian and Neel relaxation in agreement with theory. Furthermore, it is demonstrated that the magnetic field dependence of the Brownian relaxation time can be utilized to determine the core size distribution from measurements on magnetic nanoparticle suspensions whereas the mean hydrodynamic diameter can be obtained from the Brownian peak in χ’’ in zero field. For more concentrated nanoparticle suspensions where magnetic dipole‐dipole interactions are expected, a second maximum in χ’’ below the Brownian peak with a resonant‐like appearance was observed.