Magnetic properties of nanoparticles useful for SQUID relaxometry in biomedical applications

We use dynamic susceptometry measurements to extract semiempirical temperature-dependent, 255-400 K, magnetic parameters that determine the behavior of single-core nanoparticles useful for SQUID relaxometry in biomedical applications. Volume susceptibility measurements were made in 5 K degree steps at nine frequencies in the 0.1-1000 Hz range, with a 0.2 mT amplitude probe field. The saturation magnetization (Ms) and anisotropy energy density (K) derived from the fitting of theoretical susceptibility to the measurements both increase with decreasing temperature; good agreement between the parameter values derived separately from the real and imaginary components is obtained. Characterization of the Neel relaxation time indicates that the conventional prefactor, 0.1 ns, is an upper limit, strongly correlated with the anisotropy energy density. This prefactor decreases substantially for lower temperatures as K increases. We find, using the values of the parameters determined from the real part of the susceptibility measurements at 300 K, that SQUID relaxometry measurements of relaxation and excitation curves on the same sample are well described. - Research Highlights: → Dynamic susceptibility of magnetite nps gives T-dependent K and τ0 values. → Anisotropy K and prefactor τ0 are highly correlated. → Fits show τ0 upper limit is around 0.1 ns at 255-400 K. → Optimal diameter magnetite np for SQUID relaxometry is 25-26.5 nm. → Parameters from susceptibility also model relaxometry results.