Magnetic-Moment Distribution in NiFe and AuFe Alloys

We have determined the magnetic form factors and the corresponding magnetic-moment distributions for ordered Ni₃Fe and for disordered NiFe and AuFe alloys in an attempt to establish the variation of the individual atomic-moment distributions with composition. The results confirm the individual moments previously determined for NiFe alloys by neutron-diffuse-scattering methods and provide an upper limit of 0.03μB for the Au moment in a 25-at.% Fe in Au alloy. A small negative moment density between atoms was found for all of the alloys studied. The symmetry of the moment distributions was determined at the Ni and Fe sites in ordered Ni₃Fe and for Fe in Au0.75{Fe}0.25$. The Fe moment distribution is nearly spherical in both cases (54% ${T}2g$ in ${Ni}₃$Fe and 56% ${T}2g$ in the AuFe alloy) while the Ni moment distribution is strongly distorted. There seems to be little difference between the individual moment distributions in ordered and disordered ${Ni}₃$Fe. In both, the effective ${T}2g$ character decreases relative to pure Ni. The $Ni d$-function populations in ordered ${Ni}₃$Fe suggest that this decreasing ${T}2g$ character is associated with the larger size of, and the stronger $d{-}d$ overlap with, the Fe atoms. The $Ni d(xz)$ and $d(yz)$ functions that are directed toward nearest-neighbor Ni atoms have essentially the same population as in pure Ni while the $d(xy)$ function directed toward nearest-neighbor Fe atoms loses population to $d({x}²{-}{y}²)$. Thus, there is a repulsion of spin-up electrons away from the regions of strong overlap.