Ultra-Sensitive Embedded AC Magnetic Tracers Through Nano-Scale Ordering

In magnetic nanoparticle (MNP) systems, inter-particle interactions as affected by the spatial MNP arrangement will strongly govern the macroscopic magnetic properties. Tuning these interactions can have significant impact on the nonlinear magnetic response of tracers for magnetic particle imaging (MPI) and thermometry. In colloidal systems under MPI field and frequency conditions, it has been shown that dynamic formation of chains at a critical AC drive field amplitude can greatly enhance the sensitivity and resolution for imaging [1,2]. This is likely related to the formation of a colloidal anisotropy [3]. Here we fabricate highly ordered aligned bundles of chains composed of 19 nm to 20 nm iron oxide nanoparticles coated with polystyrene on various surfaces (3D printed objects, sapphire crucibles, Si/SiO₂, plastics). The arrangement of the nano-objects within the polystyrene has been studied by scanning electron microscopy revealing a close-packed structure. Magnetic particle spectroscopy (MPS) measurements show a similar sensitivity as they do in colloidal conditions once chains form, with both exhibiting a “square” AC hysteresis. The “square” hysteresis in solids is preserved down to DC field conditions and is correlated with micromagnetic simulations of single chains and close-packed structures.

[1] Z. W. Tay et al. Small Methods., Vol. 5, p. 2100796 (2021)

[2] K. L. Fung et al. Nano Lett., Vol. 23, p. 1717 (2023)

[3] C. D. Dennis et al. Nanotechnology., Vol. 29, p. 215705 (2018)