Low temperature epitaxy and magnetic property of ferromagnetic alloy nanoparticles with atomic order

用透射电镜和电子衍射研究了L1_0型FePt和FePd纳米粒子的低温外延生长。对于在空气中裂解的NaCl单晶衬底上连续沉积Pt和Fe制备的FePt纳米颗粒，发现衬底温度高于623 K和沉积后在高于873 K的温度下退火是获得具有大矫顽力的良好隔离的L1_0-FePt纳米颗粒所必需的。在FePt纳米颗粒中加入少量Ag原子百分比，导致有序化温度降低100 K。此外，Cu添加到FePd纳米颗粒导致的垂直各向异性的增强，以及在约823 K的降低的退火温度下的L1_0结构的形成。采用射频磁控溅射法制备的FePtCu纳米粒子在衬底温度低至613 K时，不经任何后退火处理，形成了取向良好、隔离良好、分散致密的L1_0纳米粒子。目前观察到的L1_0有序化所需的退火温度降低为260 K。我们还制备了Fe/FePd纳米复合粒子，并观察到由于Fe和FePd之间的交换耦合，饱和磁化强度的增加。

Low temperature epitaxial growth of L1_0-type FePt and FePd nanoparticles have been studied by transmission electron microscopy and electron diffraction. For FePt nanoparticles fabricated by successive deposition of Pt and Fe onto NaCl single crystal substrate cleaved in air, it was found that substrate temperatures higher than 623 K and post-deposition annealing at the temperatures higher than 873 K are necessary for obtaining well-isolated L1_0-FePt nanoparticles with large coercivity. Addition of a few atomic percent of Ag into FePt nanoparticles resulted in a decrease of the ordering temperature by 100 K. Also Cu addition into FePd nanoparticles lead to an enhancement of perpendicular anisotropy as well as a formation of the L1_0 structure at the reduced annealing temperature around 823 K. FePtCu nanoparticles fabricated by RF magnetron sputtering formed a well-oriented, well-isolated and densely dispersed L1_0 nanoparticles under the substrate temperature as low as 613 K without any post-deposition annealing. The presently observed decrease of the annealing temperature necessary for the L1_0 ordering was 260 K. We also fabricated Fe/FePd nanocomposite particles and observed an increase of saturation magnetization due to the exchange coupling between Fe and FePd.