Effect of Sintered Grain Growth on Chemical Ordering in Binary FePt/Cu Nanoparticle Arrays

Recent studies have shown a strong correlation between grain growth and chemical ordering in chemically synthesized FePt nanoparticles. In order to study this effect, we have prepared a series of samples in which 3.5 nm FePt nanoparticles are dispersed in a matrix of Cu nanoparticles. The samples were annealed at 600 °C and at 800 °C. Grain size was determined by XRD Scherrer analysis and time-dependent remanent coercivity measurements were made to determine the intrinsic remanent coercivity, H_cr0. For samples annealed at 600 °C, H_cr0 increases strongly with grain size up to ∼5 nm and increases weakly with additional grain growth. By contrast, after annealing at 800 °C, H_cr0 appears nearly independent of grain size. The results suggest that isolated 3.5 nm FePt nanoparticles can be weakly ordered when annealed at 600 °C and sintering is necessary for significant chemical ordering.