Magnetic behavior of novel alloyed L10-phase Co1-xFexPt nanoparticles

Highlights: • Novel alloyed L1₀-phase Co_1-xFe_xPt nanoparticles synthesized by polyol process. • Varying Co:Fe ratios correlated to the lattice constants but not particle sizes. • A random distribution of Co and Fe atoms in the layered structure was found. • Magnetic hysteresis depended on composition, enabling magnetic properties tuning. • Co_1-xFe_xPt NPs were magnetically stable against temperature with high T_c. In this work, alloying of CoPt and FePt nanoparticles (NPs), i.e. the Co_1-xFe_xPt NPs (x = 0, 0.25, 0.5, 0.75, 1), were synthesized by the polyol process. These as-synthesized NPs show the A1 phase with a particle size less than 5 nm. After annealing at 700 °C, the A1 (cubic) phase was transformed to L1₀ (tetragonal) phase in all samples. The lattice parameters varied as a function of the composition. The particle size grew larger after annealing and the size distribution was wide ranging from 100 nm. The size and distribution was however independent of the Co(Fe) concentration. X-ray absorption spectroscopy indicated that there was a random distribution of Co and Fe atoms in the layered structure. Magnetic measurements of the annealed NPs showed that the magnetic hysteresis loop depends on the composition. The coercivity (H_c) was very high for the CoPt and FePt NPs, whereas the M_s value was maximized for the Co_0.5Fe_0.5Pt NPs. The variation of H_c was attributed to the change in lattice parameters which could alter the exchange interaction, and thus the magnetocrystalline anisotropy. On the other hand, higher polarization and increased magnetic moments of Fe atoms were believed to be the reason for the enhanced M_s in the Co(Fe)Pt NPs. In addition, all NPs were magnetically stable against temperature variation with changes in M_s of less than 10%. The Curie temperature was expected to be as high as 800–900 K. Given these properties, these new forms of magnetic nanoparticles may find use in advanced magnetic recording technology.