Influence of static magnetic field strength on the temperature resolution of a magnetic nanoparticle thermometer

This paper investigates the influence of dc magnetic field strength on the resolution of a magnetic nanoparticle (MNP) thermometer, which employs the fundamental f₀ and 2f₀ harmonics of the MNP magnetization induced by ac and superimposed dc magnetic fields. In ac and parallel dc magnetic fields, the strength of dc magnetic field modulates the harmonics of the MNP magnetization, which affects their temperature sensitivities and measurement signal-to-noise ratios (SNRs). A temperature-adjustable fluxgate-based magnetic particle spectrometer was used to measure the spectra of the MNP magnetization at different temperatures. To determine the temperature, the amplitudes of the measured f₀ and 2f₀ harmonics were modeled based on the static Langevin function. AC susceptibility measurements on a MNP sample demonstrate the applicability of the static Langevin function for the description of the MNP magnetization spectra at a low frequency ac magnetic field without taking into account the MNP dynamics. Our simulations and experiments show that with increasing dc magnetic field from 0.2 mT to 2.0 mT, both the amplitude of the 2f₀ harmonic and the temperature sensitivity of the amplitude ratio of the 2f₀ to f₀ harmonics increase by a factor of about 10 in an ac magnetic field with a frequency of 70 Hz and an amplitude of 1 mT. Concomitantly, the SNR of the 2f₀ harmonic significantly increases by about 20 dB. Consequently, the temperature resolution of the MNP thermometer is improved from 1.97 K to 0.26 K.