FePt nanoparticles crystallizing in the face-centered tetragonal (fct) L 10 structure, where the Fe and Pt atoms are accumulated in different monoatomic layers stacked alternately along the c axis, may be potentially aoolied to ultra-high density magnetic recording beyond 1 Tbit/in2 because these possess a very high magnetic uniaxial anisotropy (Ku, ca.6×106 J/m3).S.Y.prepared 6 nm-sized particles of this ordered alloy by using a "SiO2-nanoreactor" method1-3 as follows.