FePt and CoPt nanoparticles prepared by micellar method: Effects of A1 -> L1 0 transition on oxidation resistance and magnetic properties

Abstract

FePt and CoPt alloy nanoparticles with diameters of 2-12 nm and interparticle distances of 20-140 nm are prepared with reverse micelles. X-ray magnetic circular dichroism (XMCD) measurements on 5.8 nm FePt nanoparticles after hydrogen plasma reduction at 300°C reveals the magnetic moment per Fe atom and magnetic anisotropy energy matching chemically disordered FePt in A1 phase. Annealing at 650°C transform portion of FePt particles to chemically ordered L1 0 phase. The presence of nanoparticles in L1 0 phase is identified by high-resolution transmission electron microscopy (HRTEM), where it is also observed that large fraction of the particles contain defects such as twin boundaries. By increasing the annealing temperature or prolonging annealing time, ratio of transformed particles increases. The average magnetic anisotropy energy of the transformed particles is below 30% of the value of bulk FePt in L1 0 phase. Annealing at above 750°C , however, decreases the average magnetic anisotropy in the sample. Similar A1 to L1 0 transition is observed in FePt nanoparticles with different diameters as well as in CoPt nanoparticles. The spin moment of Fe in FePt nanoparticles decreases with smaller particle diameter, while the orbital moment stays almost constant. Magnetic moments at room temperature are significantly reduced compared to those at low temperature, suggesting the Curie temperatures in FePt and CoPt nanoparticles are significantly lower than in the bulk. The annealing also induces Pt segregation towards the surface in FePt nanoparticles, which is identified by the decreased apparent Fe content measured by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The segregation of Pt enhance the resistance against oxidation in nanoparticle larger than 5 nm by a factor of 100-1000, but has no influence on smaller particles. Such difference is explained by the thickness of Pt segregation, which is estimated by a core-shell modeling.