Vibrational and magnetic properties of mechanically attrited nickel aluminide nanocrystals

The vibrational and magnetic properties of mechanically attrited nanocrystalline Ni{dollar}sb3{dollar}Fe powders were studied. The as-milled Ni{dollar}sb3{dollar}Fe powders were annealed to create nanocrystalline samples with different grain sizes, RMS strains, and grain boundary atomic structures. The average grain size and RMS strain of the samples were measured using x-ray diffractometry and transmission electron microscopy. From inelastic neutron scattering experiments, the phonon density of states (DOS) of various as-milled and annealed Ni{dollar}sb3{dollar}Fe nanocrystalline powders were determined. At low energies ({dollar}<{dollar}15 meV), nanocrystalline samples compared to bulk Ni{dollar}sb3{dollar}Fe showed an enhancement in the phonon DOS that was proportional to the density of grain boundaries in the powders. A broadening of features in the phonon DOS was also observed for the smallest nanocrystals. The room temperature phonon DOS of nanocrystals with an average grain size of 6 nm and a large grain boundary volume fraction of 20%, was different from the phonon DOS of the same powder after it had been exposed to 10 K. It is believed that upon exposure to 10 K the grain boundary local atomic structure changed affecting the vibrational properties of the sample.; The magnetic properties of nanocrystalline Ni{dollar}sb3{dollar}Fe were studied using Mossbauer spectroscopy, M-H magnetization curves, complex permeability measurements using microwave cavity perturbation technique, and small angle neutron scattering. M-H magnetization curves and cavity perturbation measurements showed that the coercivity and magnetic saturation are related to nanocrystal grain size and RMS strain while the complex permeability is intricately related to grain size and frequency. Both Mossbauer spectroscopy and small angle neutron scattering showed that the grain boundary magnetic moment density of as-milled Ni{dollar}sb3{dollar}Fe nanocrystalline powder was smaller than that of powder annealed at low temperature. This indicates that local atomic structure in the grain boundary affects the magnetic moment density.