This project has produced a number of significant results centered on the synthesis of magnetic nanoparticles and has led to a better understanding of the physics underlying magnetodielectric coupling in these systems. The research has progressed along two separate, but interrelated, arcs: 1) investigations focused on tuning the properties of magnetic nanoparticles during synthesis, and 2) investigations focused on probing the magnetodielectric coupling in nanoparticles. The most significant results arising from these research directions and their significance for the broader study of magnetic nanomaterials are summarized in the following.
Tuning the magnetic properties of nanoparticles
Because the magnetic properties of nanoparticles depend sensitively on morphology, structure, and composition, the behaviour of these systems can be controlled by modifying the synthesis conditions. This allows considerable flexibility in tuning the properties of nanoparticles for specific applications.
1. Tuning the magnetocrystalline anisotropy in Fe3O4 nanoparticles by Co doping. The magnetic properties of nanoparticles are determined mainly by their size (V) and by their magnetocrystalline anisotropy (K), which sets the blocking temperature TB=KV/kB. Because the magnetocrystalline anisotropy of Fe3O4 is much smaller than that of CoFe2O4 it is possible to dope Co into Fe3O4 nanoparticles during synthesis to increase the anisotropy, which modifies the magnetic properties. We established that up to 10 at% Co could be incorporated into Fe3O4 nanoparticles to modify their magnetic characteristics without significantly affecting their size.
2. Tuning the magnetic properties in g-Fe2O3 nanoparticles by controlling their size. We have also demonstrated that the saturation magnetization and blocking temperature of magnetic nanoparticles can be modified by controlling the size of the nanoparticles. Larger nanoparticles exhibit a larger magnetization per unit mass and a higher blocking temperature than smaller nanparticles. We find that the increase is slower than predicted by theory, which we attribute to the presence of non-magnetic surface layers in the nanoparticles.
3. Tuning the magnetohydronamic response of magnetic nanoparticles. Magnetic nanoparticles forming stable suspensions in solution are referred to as “ferrofluids”; these ferrofluids often show dramatic effects in external magnetic fields, including nanoparticle chain formation. We have investigated the hydrodynamic response of nanoparticles having different morphologies to external magnetic fields by measuring the thermal dissipation and optical scattering in ferrofluids. Our measurements demonstrated that the nanoparticle rotation and diffusion depend on both the magnetic characteristics of the ferrofluid and on the surfactant coating of the nanoparticles.
Magnetodielectric response of nanoparticles
The central theme of this project was to study the coupling between the dielectric response and magnetic properties of nanoparticles. We have completed two separate studies into these effects.
1. Magnetodielectric coupling in ferrofluids. We have investigated the temperature and magnetic field dependence of the dielectric constant in insulating, mineral-oil based ferrofluids containing Fe3O4 nanoparticles using a custom designed hermetically sealed cell capable of operating at low temperatures (2 K). We find that the dielectric loss exhibits a distinct maximum associated with superparamagnetic blocking, but that the dielectric response does not seem to depend strongly on the distribution of nanoparticles in the ferrofluid (whether chains or random distribution).
2. Magnetodielectric coupling in Mn3O4 nanoparticles. Our earlier studies demonstrated that bulk Mn3O4 shows clear dielectric anomalies associated with long wavelength magnetic order. We find no signatures of these dielectric anomalies in Mn3O4 nanoparticles, which we attribute to the fact that the large magnetic structures are suppressed by finite size effects. This has important implications for using bulk magnetodielectrics as the basis for nanoscale devices.
Impact
The research conducted for this project has led to a number of significant results concerning mechanisms for controlling the magnetic properties of nanoparticles, magnetohydrodynamics in ferrofluids, and magnetodielectric coupling in nanoscale systems. Additionally, this project has played an integral role in two M.S. degrees awarded in the Department and provided support for one African-American undergraduate student. The scientific accomplishments are outlined in the following publications.
