The effect of size, shape and oxidation on the magnetic properties of nanoparticles

Abstract

Magnetic nanoparticles are widespread in the natural environment and have potentially important biomedical and technological applications. The effect of the shape, size and level of oxidation on the magnetic properties of nanoparticles is either poorly understood, or subject to a number of conflicting results. Such effects are difficult to study experimentally and lend themselves to numerical calculations. In this thesis a number of numerical techniques were used to study magnetic nanoparticles.

A numerical mean-field model was developed to help understand the effect of particle size and shape on the Curie temperature. The Curie temperature was found to scale with particle size in agreement with the finite size scaling equation, and values of the scaling exponent were in agreement with theoretical values. The effect of shape on Curie temperature was investigated for the first time. At very small particle sizes, the Curie temperature was discovered to have a strong dependence on the particle shape below a threshold size. The threshold size was found to be controlled by the crystal structure of the magnetic material.

The Wien2k density functional theory package was used to estimate nearest neighbour exchange energies in magnetite and maghemite. Estimates of exchange energies were obtained for magnetite, but irresolvable difficulties arose in maghemite due to the presence of vacancies in the maghemite structure.

Magnetite-maghemite core-shell nanoparticles were studied using a Monte Carlo model to investigate the effect of oxidation on the magnetic properties of these systems. The Curie temperature of core-shell particles was found to increase non-linearly with increasing oxidation, likely due to the dominance of the surface in reducing the number of nearest neighbours in the maghemite shell at low levels of oxidation. Simulations of hysteresis were performed and the coercivity of a core-shell nanoparticle was found to decrease with increasing oxidation, with possible implications for magnetic hyperthermia treatment.