Angular Correlation of Photons from Positron Annihilation in Copper-Nickel Alloys

Angular correlation measurements of radiation from positrons annihilating in copper-nickel alloys have been carried out with crossedslit geometry using single crystals containing less than 73 at % Ni. The neck radius of the Fermi surface decreases linearly with increasing nickel content, but the neck does not detach from the hexagonal zone face even at the highest nickel content. No marked change in the composition dependence of neck radius is observed at the magnetic phase boundary near 70% Ni. The neck radius of paramagnetic nickel is estimated to be about 0.13 in unit of that for copper from the extrapolation of the observed data.