Investigation of structural and magnetic properties of thermal plasma-synthesized Fe_1-xNi_x alloy nanoparticles

Abstract

Structural and magnetic properties of thermal plasma synthesized Fe_1-xNi_x (x = 0.25, 0.33, 0.50, 0.67 and 0.75) alloy nanoparticles have been investigated using x-ray diffraction, neutron diffraction, transmission electron microscopy, scanning electron microscopy, dc magnetization, and Mössbauer spectroscopy techniques. High temperature gas phase nucleation and growth environment inside thermal plasma reactor facilitate stabilization of the disordered γ-FeNi (fcc) phase throughout (except for x = 0.25) the composition range under investigation. For x = 0.25 composition, a small (∼8%) amount of the disordered bcc phase along with prominent fcc (∼92%) phase is present, as inferred from the neutron diffraction data analysis. The spherical shaped nanoparticles possess high crystallinity. The average crystallite size (in 30–40 nm range) as well as particle size distribution show insignificant change as a function of composition. The observed value of the saturation magnetic moment for these nanoparticles are very close to that for their bulk counterparts, indicate highly crystalline nature of the thermal plasma synthesized nanoparticles. Room temperature Mössbauer spectroscopic data reveals that the alloy nanoparticles contain two different sites for Fe corresponding to high moment/low moment states. The neutron diffraction data indicates ferromagnetic ordering for all the compositions of the series. The average magnetic moments/f.u., derived from neutron diffraction and dc magnetization, are found to match with each other as well as with the values reported in the literature for bulk Fe_1-xNi_x alloys. The highest ordered magnetic moment was found to be 1.4 μ_B per f.u. for Fe_0.50Ni_0.50 composition. Overall, thermal plasma based synthesis is found to be an excellent route to produce high-quality nanoparticles of the binary metallic alloys.