Increase in the Curie temperature and magnetic anisotropy in FePd/Pt-iron oxide core-shell nanoparticles

Partially oxidized fcc FePd and FePt nanoparticles with mean diameters of 5 and 3 nm, respectively, were synthesized by a reverse micelle polyol process. In situ measurements of magnetic and structural properties during annealing showed a large increase in the magnetocrystalline anisotropy and in the Curie temperature of the nanoparticles due to (i) a phase transition from A 1 to L 1 0 and (ii) a simultaneous phase separation between a metallic core and an iron oxide shell. These occurred at 675 K in the FePd nanoparticles and at above 850 K for the FePt. The Curie temperature of the nanoparticles was found to be about 850 K, an increase of more than 100 K from the bulk L 1 0 phase. The ferromagnetic resonance results are discussed and compared with a phenomenological model that makes it possible to estimate the magnetocrystalline anisotropy as 1.6 × 10 5 and 1.5 × 10 6 J m − 3 in FePd and FePt, respectively. Exchange coupling between the core and the shell explains both the high magnetocrystalline anisotropy of the core and the high Curie temperature of the shell.