Magnetic Properties of Self-Assembled Nanoparticle AssembliesSara A. Majetich, Dorothy F. Farrell, Yi Ding, Yuhang
Cheng, Madhur Sachan, and Shihai Kan We describe the preparation and properties of highly monodisperse iron, cobalt, and iron platinum nanoparticles, and nanostructures made from them. All of the surfactant-coated nanoparticles are prepared by high temperature solution chemistry methods. We examine several case studies that illustrate the potential of these particles for improving our understanding of nanomagnetism. In the first, the magnetostatic interactions among monodisperse Fe nanoparticles are varied by changing the particle size, spacing, and degree of structural ordering. The magnetic properties of iron nanoparticles are compared for dilute dispersions, self-assembled arrays, and highly ordered nanoparticle crystals. The effects of size and separation can be understood by comparing the ratio of the dipolar and anisotropy energies. The length scale structural order is quantified by small angle x-ray scattering (SAXS), and the length scale of magnetic order is determined from small angle neutron scattering (SANS). These results are correlated with the macroscopic magnetic properties of the assemblies. We find that spin glass-like or mictomagnet behavior dominates when the length scale for structural ordering is less than 300 nm, or about 30 particle diameters. In the second example we describe our efforts to crystallographically align magnetic nanoparticles during self-assembly, both by the control of the particle shape and by the use of magnetic torques on the particle moments. The ability to prepare self-assembled magnetic nanoparticle arrays with crystallographic alignment has been elusive, either because the nanoparticles have cubic crystal structures, and therefore a low magnetocrystalline anisotropy K, or as in the case of FePt, the high anisotropy phase is developed by heating after self-assembly, so the particles cannot move. We have achieved partial alignment using equiaxed hcp Co and CoFe2O4 nanoparticles and a uniform magnetic field during self-assembly. The requirements for alignments are analyzed in terms of the relative Néel and Brownian relaxation rates. The remanence ratio used to determine the magnetic degree of alignment is compared with structural evidence from electron diffraction and x-ray diffraction, as well as dark-field TEM imaging. This research is supported by National Science Foundation, through grants # CTS-0227645 and #ECS-0304453, and by the Petroleum Research Fund of the American Chemical Society through grant #ACS-PRF-37578-AC5. |