Observation of two kinds of ESR spectra in FePt nanoparticles

The size effects on magnetic properties of FePt nanoparticles were investigated through ESR experiment in X-band at room temperature. For particle size (D) of ≥ 2.6 nm, the absorption spectra were the Gaussian type, whereas for D = 2.0 nm, it was reproduced with the summation of the Gaussian and Lorentzian types. It has been well known that a face-centered-tetragonal (fct) structure becomes unstable for D of below 2-4 nm. Two kinds of magnetic components for FePt nanoparticles with D = 2.0 nm, observed in the ESR spectrum at room temperature, are considered to be attributed to the coexistence of the particles with fct structure and those with face-centered cubic structures.