Synthesis of Magnetic Intermetallic FeAl Nanoparticles from a Non-Magnetic Bulk Alloy

摘要

We report the synthesis of intermetallic FeAl nanoparticles using the laser vaporization controlled condensation technique. The nanoparticles have an average particle diameter between 6 and 9 nm. Measurements of the d spacing from X-ray and electron diffraction studies confirm that the FeAl nanoparticles have the same crystal structure (B2) as the bulk FeAl. High-resolution TEM images reveal that the nanoparticles consist of a crystalline core encased within a thin amorphous layer formed upon the exposure of the particles to air. We also report the synthesis of FeAl oxide and carbide nanoparticles. The oxide nanoparticles have the hercynite FeAl_2O_4 composition and the carbide nanoparticles have the AlFe_3C_(0.5) composition. While the crystalline bulk FeAl (20-30% weight Al) materials are nonmagnetic, the analogues nanoparticles are found to be ferromagnetic even well above room temperature. Coercivities of the order of 50 Oe almost independent of temperature are observed from room temperature down to 40 K, below which the coercivities values increase up to 700 Oe at 5 K. The nanoparticles show a definitive exchange anisotropy effect probably arising from the intersurface anisotropy at the adsorbed oxygen layers on these particles. The observed displacement of the hysteresis loops along the field and the magnetization axes are reversible with the direction of the applied field. The FeAl nanoparticles may lead to a novel form of advanced materials that combine high electrical resistivity, oxidation resistance, ductility and unique magnetic properties.