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We investigated the effect of core size distribution on the performance of a magnetic nanoparticle thermometer (MNPT) in circumstances when Néel relaxation dominates the dynamic behavior of particles. Numerical simulations revealed the effects of excitation field strength and core size distribution on the temperature dependence of the amplitude and phase of harmonics. In MNPT, the field dependences of sensitivity deviated significantly from those calculated when the core size distribution was neglected. These simulation results were compared with those from experiments for which reasonable agreement was obtained. These findings must be carefully considered when designing an optimal MNPT system.

We performed magnetic fractionation and characterization of Resovist ® magnetic nanoparticles (MNPs) for use in magnetic particle imaging (MPI). We showed that the core size of the fractionated MNPs can be controlled by adjusting the separation field. We also showed that particles with core size of approximately 27 nm are optimal for MPI applications. To obtain MNPs of an appropriate core size and a narrow size distribution, we performed combined positive and negative fractionation. In the obtained fractionated MNPs, the third harmonic magnetization, which is used as the MPI signal, was 2.75 times as large as that of the original sample.

It is well known that the core size of practical magnetic nanoparticle (MNP) samples is widely distributed. This study investigated an empirical expression for harmonics of AC magnetization of MNPs for magnetic nanoparticle thermometer (MNPT) and used two main core diameters to approximate the effect of the core size distribution. When the frequency of AC magnetic field was 200 Hz, the harmonics calculated from the Langevin function with two-core diameters (two-core-diameters model) agreed well with the experimental data. When the frequency was 3 kHz, where the effect of the Brownian relaxation could not be ignored, the harmonics could also be empirically expressed by the two-core-diameters model by adequately choosing the parameters of the model. The simple two-core-diameters model is useful when MNPs sample with core size distribution is used for MNPT.

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