Magnetic particle spectroscopy in two-dimension mixing-frequency magnetic fields

Magnetic particle spectroscopy (MPS) is one of the most versatile tools to characterize the magnetic properties of magnetic nanoparticles (MNPs). The orientation, amplitude and frequency of excitation magnetic fields significantly affect the MPS signal of MNPs. In this study, we investigate the MPS signal of MNPs in two-dimension mixing-frequency magnetic fields with a custom-built MPS system. Figure 1a shows the schematic of the MPS system. In x direction, two solenoid coils are used to generate mixing-frequency magnetic fields, containing a high-frequency fHx and low-frequency fLx magnetic fields. In y direction, a pair of rectangular Helmholtz coils is used to generate a low-frequency fLy magnetic field. To detect the magnetic response of the MNPs, 2D detection coils in both x- and y-direction are designed. With this configuration, the custom-built system allows to comprehensively study the MPS signal of MNPs in 2D mix-frequency magnetic fields with different excitation sequences by tuning their frequencies, amplitudes and phases. Figures 1b and 1c show the simulated MPS signal in x-direction detection coil in 2D mixing-frequency magnetic fields with a low frequency fLy = 100 Hz (Fig. 1b) and fLx = 100 Hz (Fig. 1c), as well as a high frequency fHx = 10 kHz. The magnetization of the MNPs is modulated by the 2D low-frequency magnetic field. Thus, it contains different mixing-frequency components in the MNP spectra. We envisage the study of MPS signal of the MNPs in 2D mixing-frequency magnetic fields is of great significance to magnetic particle imaging and magnetic bio-sensing