We evaluated the complex harmonic signals from magnetic nanoparticles (MNPs) for magnetic particle imaging (MPI). We measured the complex harmonic signals from Resovist MNPs, one of the candidates for an MPI tracer, when only an ac excitation field and both the ac and the dc excitation fields were applied. To evaluate the complex harmonic magnetization spectra, we performed a numerical simulation accounting for the distributions of magnetic moments, anisotropy energy barriers, and hydrodynamic sizes of MNPs in the sample. The simulation results agreed well with experimentally obtained amplitude of the harmonic signals. The simulation results of the phase lag, which was caused by the finite relaxation time, agreed with the experiments for low harmonic numbers. The phase lag deviation between them became significant for higher harmonic numbers. For quantitative evaluation of the phase lag for higher harmonic numbers, a more accurate estimation of the Néel relaxation time is required.
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