Magnetism in amorphous and crystalline FeCo and FeCo/SiO2 nanoparticles

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Newman, Kelly
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Amorphous FeCo nanoparticles were synthesized using a scalable metal halide reduction. Post-synthesis treatments were used to create larger, more crystalline nanoparticles. Both the amorphous and crystalline particles were coated in silica (SiO2) to produce an interfacial phase (Fe-orthosilicate) with substantial contribution to the nanoparticle magnetism. X-ray diffractometry, x-ray absorption spectroscopy, and Mössbauer spectroscopy were used to determine the nanoparticles' compositions, and transmission and scanning electron microscopy were used to determine the shape and size of the nanoparticles. Magnetometry and susceptometry techniques were used to study the overall magnetism of the systems. The amorphous nanoparticles were found to have a ferromagnetic to paramagnetic transition at 60 K. Annealing and silica coating the nanoparticles were both found to drastically increase the saturation magnetization of the FeCo nanoparticles. The annealed nanoparticles displayed bulk-like properties. Elemental and atomic magnetism were studied using x-ray magnetic circular dichroism and Mössbauer spectroscopy. The amorphous nanoparticles showed evidence of multiple coordination environments characteristic of amorphous systems and annealed particles showed more discrete features characteristic of crystalline systems.
Magnetism, Nanoparticles, Nanomagnetism