EQ-08: Static and Dynamic Magnetic Properties of Iron Oxide Nanoparticles in Colloid and Embedded in Polymer Nanocomposites

Magnetic nanoparticles (MNPs) have been the subject of intense study for many decades, due, in part, to their many applications such as water/gas purification to data storage to drug delivery. While it is well-known that the physical properties of MNPs, such as composition, size and size distribution, crystallinity, shape, etc. determine the magnetic properties, it is not as widely discussed what impact the MNP environment has on the measured properties. For example, MNPs can physically move in a liquid under the influence of an external magnetic field, forming structures like rings and chains [1]. When dispersed in a solid matrix, or nanocomposites, their physical movement can be significantly restricted, if not eliminated, leaving only magnetic (Neel) rotations. Here, we compared colloidal dispersions of iron oxide MNPs in water and toluene with nanocomposites of the iron oxide MNPs made by dispersion of the MNPs in polymer followed by either UV polymerization (water-based) or rapid precipitation (toluene-based). Measurement of magnetization as a function of applied field show two distinct behaviors: the nanocomposite virgin curve is centered in the first quadrant major loop before merging with the falling major loop, in stark contrast with the MNPs in liquid which track the rising side of the major loop (Fig. 1). Furthermore, in the nanocomposites, there is a peak in the imaginary AC susceptibility component based on the magnetic properties of the individual MNPs. In liquid, the peak in the imaginary component shifts out of range based on the magnetic properties of a chain. (Fig. 2) The changing structures possible in liquid (which are restricted in nanocomposites) also modify the interactions, resulting in a dynamic magnetic anisotropy [2]. This has profound implications for magnetic applications, especially as many of these use AC magnetic fields where the magnetic anisotropy is crucial to their detailed response.References: 1. Abel et.al. ACS Applied Materials and Interfaces, 2023 2. Dennis, et al, Nanotechnology 2018.