鉄白金ナノ粒子の極微構造観察と規則相低温合成

抄録

Two-dimensionally dispersed L1₀-FePt nanoparticles as small as 12nm with orientation was fabricated by electron-beam evaporation and post-deposition annealing at 873K. After a prolonged annealing at 873K for 86.4ks, the increase of coercivity was almost saturated and coercivity reached 5.5kOe at 300K, which is very small compared to the values reported on FePt nanoparticles with larger particle sizes. Long-range order parameter of the fabricated nanoparticles was determined by electron diffraction at 200kV taking the multiple scattering into consideration. Obtained order parameter remained 0.55 for the nanoparticles with an axial ratio c/a of 0.961 ± 0.003. The low degree of order is thought to be responsible for the small coercivity in spite of the long-time annealing as high a temperature as 873 K. In order to enhance the L1₀ phase formation under the lower annealing temperature, effect of Cu addition on the atomic ordering of FePt nanoparticles were examined. As a result, a low temperature synthesis of isolated and oriented L1₀-FePtCu nanoparticles with high-density dispersion has been achieved by conventional rf-magnetron sputtering technique with a single crystal NaCl substrate. Hard magnetic L1₀-type ordered phase with a coercivity of about 1.4 kOe was directly formed at low substrate temperature as low as 613 K without any post-deposition annealing. Hard magnetic properties were sensitive to Pt concentration. It is considered that additive Cu replaces the Fe-site in the L1₀-FePtCu phase.