Atomic ordering of 10-nm-sized L10-FePt, FePd and CoPt nanoparticles have been studied by transmission electron microscopy and electron diffraction. Proceeding of the atomic ordering was confirmed by the increase of coercivity as well as electron diffraction. Among these three kinds of L10 nanoparticles, the atomic ordering reaction proceeded at the lowest temperature as low as 773 K for FePd nanoparticles. Coercivities of FePt and FePd nanoparticles largely increased with particle diameter and reached 10 kOe for 20-nm-sized FePt particles at room temperature. The large coercivity is attributed to the rotation magnetization of single-domain particles. For CoPt nanoparticles the observed coercivity was very small even after annealing at 1023 K for 1h. The slow rate of the L10 ordering can be attributed to the lowest T c/Tm value in CoPt nanoparticles. L10 ordering was also confirmed even in very small FePt particles as small as 2-3 nm in diameter. By contrast, FePd nanoparticles with sizes less than 5 nm showed disordered fcc even after annealing at 873 K. The coercivity of 4-nm sized L10-FePt nanoparticles were quite low and the observed largest coercivity was 490 Oe at 10 K after annealing at 873 K for 12 h.

Keywords: L10 structure, FePt, FePd, CoPt, Nanoparticle, Large coercivity, TEM, Electron diffraction, Atomic ordering