Tunable magnetic properties of CoPt nanoparticles: Impacts of phase coexistence and thermal annealing

Abstract

Nanostructured materials such as CoPt nanoparticles with tunable hard magnetic properties are desirable for a wide range of technological applications. In CoPt nanoparticle systems, a structurally disordered fcc phase may be transformed into an ordered fct L10 phase via annealing. However, the effects of this thermally induced disorder to order transition on the magnetic properties and phase coexistence still need to be understood. Here, we have studied these effects in Co50Pt50 nanoparticles, which were prepared by chemical reduction combined with ultrasound. By varying the annealing temperatures from 450 °C to 700 °C for 1 h, we observe that the magnetic properties are heavily dependent on the annealing temperature. The nanoparticles annealed at 500 °C exhibit the highest coercivity, while the 600 °C annealed nanoparticles possess the largest saturation magnetization. We demonstrate the variation in coercivity with annealing temperature follows that of the order parameter. This study establishes, for the first time, a correlation between the order parameter and the coercivity for annealed CoPt nanoparticles and paves a pathway for the development of nanostructured magnets with tunable magnetic properties.