Perspectives of Magnetic Particle Spectroscopy for Magnetic Nanoparticle Characterization

Magnetic Particle Spectroscopy (MPS) has been used to estimate the performance of magnetic nanoparticles for Magnetic Particle Imaging. It was demonstrated that one can reconstruct the particles core size distribution from the measured MPS spectrum. However, using MPS as an analytical instrument for the characterization of magnetic nanoparticle samples includes systematic errors. First, previous reports on MPS-based core size estimation only take the Néel process of the relaxation into account. However, other methods, e.g. ac susceptometry, show that for excitation frequencies in the lower kilohertz regime the Brownian magnetization process also plays an important role. We developed an extended MPS setup that enables parameter measurements of the harmonics, depending on the amplitude and frequency of the excitation signal and the amplitude of a static offset field. Based on a dynamic magnetization model, we utilize a multi-variate fitting routine to describe the sample’s core size and hydrodynamic size distribution. Second, existing implementations assume mono-modal log-normal distributions of core sizes, which cannot be guaranteed to match the actual size distribution of the sample. For that reason, we also use a regularized singular value decomposition (SVD) based method for reconstructing arbitrary core size distributions from the harmonic spectrum.