Abstract:
Chemical synthesis routes for two Mn-based high magnetocrystalline anisotropy energy density (K_u) nanoparticles (MnBi nanoparticles and MnAl nanoparticles) and other nanoparticles including BiClO, Bi, Mn, and Al nanoparticles were developed. The MnBi and MnAl nanoparticles have great potential for ultrahigh density magnetic recording applications as well as high energy permanent magnets applications. Bi, Mn, and Al nanoparticles were studied as building blocks of the binary nanoparticle and also because of their own distinctive properties. Bi nanoparticles were synthesized by the reduction of BiCl_3 by 1,2-hexadecanediol. In the presence of moisture, BiClO nanorods were formed. With trioctylphosphine surfactant, faceted Bi nanoparticles with large particle size variation were formed. With oleic acid surfactant, spherical Bi nanoparticles 4.7 nm ± 0.3 nm were synthesized. The reduction with n-butyllithium as a reducing agent resulted in complete reduction of BiCl_3 but large particle size variation. Mn nanoparticles were synthesized with two precursors, MnCl_2 and Mn_2 (CO)_10. The as-synthesized Mn nanoparticles were covered with a layer of low crystallinity MnO due to surface oxidation. Larger particles showed the metallic α-Mn core and low crystallinity MnO shell, while small particles only showed low crystallinity MnO. It was not clear if the smaller particles had an amorphous metal core. Coating the Mn nanoparticles with Au can form a Mn/Au core/shell structure and provided oxidation resistance to the Mn. Al nanoparticles was produced by decomposition of triisobutylaluminum. Binding of the perfluoroundecanoic acid surfactant to the nanoparticle surface was confirmed by FT-IR. MnBi nanoparticles were synthesized in sequential reduction/decomposition method and seed mediated growth method. MnBi nanoparticles with pure Bi nanoparticles were formed in both methods. Magnetic measurements detected ferromagnetic or superparamagnetic phases in the products, indicating the formation of MnBi with small crystallite size. MnAl nanoparticles were synthesized by co-reduction of MnCl_2 and AlCl_3 with strong reducing agent n-butyllithium. The as-synthesized nanoparticles were roughly monodisperse with a size of 4.6 nm ± 0.6 nm. The composition of the co-reduction method products was also in the range of the ferromagnetic τ-phase MnAl, with an average composition of Mn_56 Al_44. However, τ-phase MnAl was not found in the sample both before and after annealing by XRD and magnetic measurement. Evaporation of Al and oxidation of Mn affected the stoichiometry of the nanoparticles and inhibited the formation of τ-phase MnAl.
