We studied the effect of viscosity on the harmonic signals of magnetic nanoparticles (MNPs) for thermometry application. Although thermometry is based on the temperature dependence of MNPs harmonic signals, they are also affected by the viscosity of the surrounding media. Therefore, we first perform numerical calculation to qualitatively show the effect of viscosity on the first (M1), third (M3) and fifth (M5) harmonic signals when an excitation field with medium amplitude (3 mT) and high frequency (10 kHz) is applied. We then present the experimental results for Resovist MNPs samples when viscosity is changed from 0.89 to 5.2 mPa·s. We obtained qualitative agreement between numerical calculation and experimental results on the viscosity dependences. Next, we study the correlation between the changes in harmonic signals caused by viscosity variation. The M1 and M3 had negative correlation in their change, while M3 and M5 had positive correlation. Due to the positive correlation, the ratio M5/M3 became almost independent of viscosity when appropriate amplitude and frequency were chosen for the excitation field. We also proposed a method to accurately estimate both viscosity and temperature of the MNPs sample from the measured harmonic signals even when the viscosity is not known in advance, which will be useful for precise thermometry.
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