Relationship between harmonic spectra and coercive field of immobilized magnetic nanoparticles

We studied the AC hysteresis loop and the harmonic spectra of samples containing immobilized magnetic nanoparticles (MNPs) at different values of the excitation field frequency f and amplitude H₀. First, we measured the dependences of the coercive field 〈H_c〉 on f and H₀. The measured dependences agreed qualitatively with the numerically predicted values. Next, we studied the relationship between 〈H_c〉 and the harmonic spectra, and found strong correlation between 〈H_c〉 and the attenuation rate of these harmonic spectra. We obtained an empirical expression for the harmonic spectra using 〈H_c〉 and a static magnetization curve for the immobilized MNPs. The expression obtained explained the experimental data well. Finally, the harmonic spectra were measured for two MNP samples with different distributions of the magnetic moment m. The MNP sample with the lower m distribution produces richer harmonic spectra for use in magnetic particle imaging.