Synthesis and Size Dependence of Strongly Correlated Ferrite Nano-Objects

Recent results have shown that high quality ferrite particles functionalized with oleic acid can produce significant gains in magnetic particle imaging (MPI) signal and imaging resolution when stimulated with an AC field with an amplitude greater than 10 mT.¹ The proposed mechanism is the formation of linear chains due to strongly correlated inter-particle interactions and is characterized by a square AC hysteresis loop, enhanced MPI resolution, and a large change in MPI signal at a threshold field. Here we have synthesized ferrite particles from approximately 15 nm to 25 nm by a one-step colloidal method. The particles were physically characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and dynamic light scattering. The strongly correlated nature of the particles was investigated by drive field dependent magnetic particle spectroscopy (MPS) measurements at 5 kHz frequency. Particles with a mean size of about 19 nm and larger show distinct features of a strongly correlated system: a threshold field where the MPI signal sharply increases and nearly square AC hysteresis loops at fields above the threshold regime. The 15 nm particles do not clearly show this behavior. The results suggest that a minimum critical size is required for the strongly correlated behavior to occur in ferrite particles.

1. Tay et al. Small Methods, 5, 2100796 (2021)