Organization and controlled coupling between soft and hard magnetic nanocrystals using mesoporous materials

Abstract

Mesoporous materials provide a unique host for controlling interactions between nanoscale guests. Here, we use polymer-templated mesoporous silicas to control magnetic dipole-dipole coupling between soft (superparamagnetic) face-centered-cubic and hard (ferromagnetic) face-centered-tetragonal FePt nanocrystals. We find that mixed soft-hard coupled FePt chains show enhanced magnetic coercivity, compared to single-component chains, while randomly associated nanocrystals show no change. A semi-quantitative analysis of temperature dependent magnetization data indicates that the free-energy barrier to spin flipping has both significant enthalpic and entropic components. Linear channels, thus, appear to be an effective way to organize magnetic nanocrystals with constructive dipolar coupling and tunable magnetic properties.