Control of Composition and Size in Discrete Co_<i>x</i>Fe_2–<i>x</i>P Nanoparticles: Consequences for Magnetic Properties

Abstract

In this work, a solution-phase method was developed for the synthesis of Co x Fe 2– x P nanoparticles over all x (0 ≤ x ≤ 2). The nanoparticles vary in size, ranging from 17 to 20 nm in diameter with standard deviations ≤ 14%. The synthesis involves preparation of Co x Fe 1– x alloy nanoparticles and high temperature conversion into crystalline ternary phosphide nanocrystals. The target composition can be controlled by the initial metal precursor ratio, and the size of Co x Fe 2– x P (from 12 to 22 nm) can be tuned by varying the oleylamine/metal ratio. Mössbauer data show that Fe has a strong preference for the square pyramidal site over the tetrahedral site. Magnetic measurements on Co x Fe 2– x P nanoparticles showed a strong compositional dependence of the Curie temperature ( T C ); CoFeP and Co 0.7 Fe 0.3 P have T C ’s > 340 K and are superparamagnetic at room temperature.