source: © 2016 Physical Review Letters
The interaction of magnetic nanoparticles and electromagnetic fields can be determined through electrical signal induction in coils due to magnetization. However, the direct measurement of instant electromagnetic energy absorption by magnetic nanoparticles, as it relates to particle characterization or magnetic hyperthermia studies, has not been possible so far. We introduce the theory of magnetoacoustics, predicting the existence of second harmonic pressure waves from magnetic nanoparticles due to energy absorption from continuously modulated alternating magnetic fields. We then describe the first magnetoacoustic system reported, based on a fiber-interferometer pressure detector, necessary for avoiding electric interference. The magnetoacoustic system confirmed the existence of previously unobserved second harmonic magnetoacoustic responses from solids, magnetic nanoparticles, and nanoparticle-loaded cells, exposed to continuous wave magnetic fields at different frequencies. We discuss how magnetoacoustic signals can be employed as a nanoparticle or magnetic field sensor for biomedical and environmental applications.
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Figure 1
Concept of magnetoacoustic signal induction. (a) Components of the magnetoacoustic setup. Power supply (PS), modulator (M), water chiller (W), driver (D). (b) Magnetoacoustic sensing using a PZT transducer. The sample comprises a steel rod located within the coil. (c) rf-free magnetoacoustic sensing employing a fiber-Bragg-based interferometric ultrasound sensor in a horizontally arranged solenoid (water tank not displayed). The optical sensor comprises optical filters (F), an erbium-doped fiber amplifier (EFDA), a 99/1optical splitter (S), a demodulator (Demod), and the π-shifted FBG sensing unit. (d) Magnetoacoustic sensing of a steel rod specimen using PZT based ultrasound detection. rf interference due to the nonlinearity of the rf amplifier (blue dotted line) and experimental confirmation of the second harmonic magnetoacoustic signal (red line) induced in conducting material at f_MA=2f_rf. Inset shows the quadratic increase of detected magnetoacoustic signal (red crosses) as a function of the linearly rising B field compared to the expected theory (dashed black curve) and a linear relationship (dotted green line).