Effective Néel relaxation time constant and intrinsic dipolar magnetism in a multicore magnetic nanoparticle system

The coexisting Néel and Brownian relaxation behaviors of magnetic nanoclusters in a viscous medium lead to a highly nonlinear field-dependent magnetization response, which can benefit magnetic particle imaging and hyperthermia. To empirically correlate the moment and particle dynamics with the core and cluster sizes, we performed spectroscopic susceptometry to assess frequency-dependent complex magnetic susceptibility of water-dispersed magnetic nanoclusters at very low field amplitude. The superparamagnetic core particles of nanoclusters should undergo fast moment dynamics. However, for the nanoclusters experiencing the field-driven Brownian relaxation, their constituent core particles appear to collectively behave as a large effective core with a long Néel relaxation time constant. We later numerically interpolated the phase-delay spectra of the immobilized nanoclusters to estimate the Néel relaxation time constant attributed to the intrinsic dipolar interparticle magnetism. From additional static magnetometry, the overlapping bimodal magnetic moment distribution predicts the secondary core sizes larger than the actual sizes from the electron microscopy images. The different estimates of the effective Néel relaxation time constant obtained from the (nearly field-free) frequency-dependent and (static) field-dependent magnetization responses further indicate the activation energies limiting the relaxation behavior of magnetic nanoclusters. This finding highlights the number of effective cores affecting the intracluster interaction energy.