Magnetic properties of nanoparticles useful for SQUID relaxometry in biomedical applications

► 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.