Magnetic Properties of Nanoparticle Systems of Fe, Co, Ni, and FePt, Created by Ion Implantation

Magnetization measurements have been performed on nanoparticle arrays of Fe, Co, Ni, and FePt in single-crystal substrates. Materials are formed by ion implantation into a layer followed by thermal annealing to give electrically isolated but crystallographically textured arrays. Studies show competition between effective single-particle anisotropy (due to magnetocrystalline, shape, surface, and stress effects) and macroscopic anisotropy due to dipolar interactions.

Results of these measurements are compared to expectations found in the well-known Stoner-Wohlfarth model of non-interacting, uniaxial particles. While this model is extensively used in magnetic materials research, the missing effects from dipolar interactions between particles give very different results than those measured here. To gauge the importance of interactions with respect to anisotropy, the magnetic length scales of Holz and Scherer are used.

Finite size effects associated with shrinking lengths to the nanometer scale have also been measured. Some examples of these effects are an enhanced moment in Ni nanoparticles due to a transition from ferromagnetic to paramagnetic behavior and an enhanced critical exponent β in FePt.

Micromagnetic simulations have been used to model these arrays. They confirm the effects of stabilized magnetization from dipolar interactions found in experiment. Also, they predict a differing behavior between more typical 2D arrays and the thicker arrays formed in this project.