Figure 11

Short-time Fourier transform (STFT) procedure for obtaining the harmonic PSF at excitation and bias field frequencies \(f_0 = 31.75\) kHz and \(f_{bias} = 10\) Hz, respectively. (a) The applied field (sum of excitation and bias) as a function of time. The inset shows the \(f_0\) excitation field. The red line (i) denotes the time when the bias field is 0 mT, and the blue line (ii) the time when the bias field is at its maximum of 40 mT. (b) The magnetization response at position (i) at the local maximum (thick red line) with the integration time window (thin red lines) and (ii) at the the local minimum (thick blue line) with the integration time window (thin blue lines) used for calculating the STFT with Eq. 2. Note that the x-axis is now converted to the magnetic field of \(f_{bias}\). The inset shows the zoomed-in magnetization response at position (i). (c) Spectrogram of the MNP response signal using a triangular shaped window function that results in a 50 % overlap in the integration domain. The red and blue lines correspond to the time positions (i) and (ii), respectively. (d) Fourier spectra for the two different time positions (i) and (ii).