Magnetic particle spectroscopy of magnetite-polyethylene nanocomposite films: A novel sample for MPI tracer design

Several harmonic spectra are plotted in Figure 2. An important result is the comparison of free particles embedded in agar and films (Figure 2c). The particles embedded in the films are “frozen” in place, while those in agar, unless bound within the matrix, should still have some freedom to rotate. Particles in both should have identical Neel relaxation times, as that is dependent only on the structure of the iron core and not local environment, provided magnetic interactions can be excluded. A calculation of the Neel relaxation time yields values in the range of 0.12-171 microseconds (for crystal anisotropies in the range of 11-21kJ/m³). The Brownian relaxation time for the films is assumed to be infinite, such that relaxation is only possible through the Neel mechanism. However, the Brownian relaxation times for particles in agar are long enough that relaxation is also assumed to be dominated by the Neel mechanism. While Brownian relaxation may play a role in explaining the signal difference, it is likely due to magnetic interactions between neighboring particles that steepen the magnetization curve. Figure 2c demonstrates that a low interparticle distance generally results in a higher signal. In samples with high local concentrations (such as the films presented here, or samples that exhibit significant aggregation), the average distance between two particles is small enough that dipole-dipole interactions must be considered. Such interactions are one possible explanation for the performance of Feridex IV and Resovist.