Comparison of the anisotropy energy obtained from temperature dependent AC and DC magnetometry in iron oxide nanoparticles (IONPs) with controlled dipolar interactions

Core-shell structures of two core sizes and three different interparticle distances for each core, provided by individual silica shells, were employed as test bench to compare energetic parameters that can be obtained from DC and AC magnetometry analysis. Samples were structurally characterized through X-Ray powder diffraction (XRD), confirming the spinel structure of the cores with different crystallite sizes and different proportions of amorphous silica. Transmission electron microscopy (TEM) revealed the spherical core–shell structures of the samples with core sizes of 8.5 and 12.6 nm and shells thickness that ranges from 2 to 20 nm. Zero field cooling and field cooling (ZFC-FC) routines, saturation magnetization as a function of temperature and AC susceptibility measurements were analyzed to calculate activation energy values and thermal parameters related to anisotropic contributions, attributed to dipolar and surface effects. Both contributions manifest by shifting and broadening the activation energy distribution and causing a slowing down of the spin-relaxation rate on approaching the critical temperature, a complementary approach is employed to analyze them.