Studies of Highly Monodisperse Magnetic Nanoparticles
Sara Majetich
Department of Physics
Carnegie Mellon University
Pittsburgh, PA 15213
We have synthesized highly monodisperse, iron and iron alloy nanoparticles and
investigated their structural and magnetic properties. These particles were produced from
decomposition of metal carbonyl precursors or reductions of metal salts at high
temperatures. The particles are coated with surfactants to maintain stable dispersions.
The effects of magnetic interactions in dilute dispersions of the monodisperse
nanoparticles were studied through zero field cooled and field cooled magnetization
measurements, and by magnetic relaxation. Light scattering techniques were used to
determine typical agglomerate sizes. The magnetic relaxation results are interpreted in
terms of statistical contributions from magnetically coupled aggregates of various sizes.
The surfactant-coated monodisperse nanoparticles self-assemble into arrays when
the solvent is evaporated. The dependence of the array structures on the particle size, the
surfactant, and the solvent are described. The results are interpreted in terms of the
underlying van der Waals, steric, and magnetic forces. A mechanism for the formation of
the different array structures is postulated, explaining the region of the evaporating
droplet where the array is nucleated and the role of the surface in stabilizing a non-closest
packed structure. The magnetic properties of the arrays are examined both theoretically
and experimentally. The arrays were characterized magnetically by field cooled and zero
field cooled magnetization measurements, hysteresis loops, magnetic relaxation, and
neutron scattering. Evidence for dipolar ferromagnetism is presented.
Biographical Info.:
Undergrad, Chemistry – Princeton
Masters, Chemistry – Columbia
PhD, Physics – University of Georgia
Postdoc – Cornell
Faculty, Physics – Carnegie Mellon, since 1990, now full professor