A polarised-neutron scattering demonstration of deviations from Stoner-theory behaviour in nickel

Abstract

Constant-energy surveys through the spin-wave dispersion function of a nickel crystal (containing 0.8 at.% molybdenum as impurity) have been taken at 8, 12, 16 and 20 meV by means of the scattering of polarised neutrons. The experimental configuration was so arranged that at low temperatures, with the crystal in its magnetic ground state (all spins parallel or antiparallel to the magnetisation direction) a neutron beam polarised oppositely to the magnetisation could give no spin-wave scattering. By observing the onset of spin-wave scattering in this configuration as the temperature was raised the authors were able to detect the onset of spin deviation in the crystal with approach to the Curie temperature. The authors' results are analysed in terms of two theories, one due to Sokoloff (1978) and to Korenman, Murray and Prange (1977), and one to Liu (1980). The former essentially attributes the magnetic disorder at elevated temperatures to the continuous directional fluctuation of the magnetisation vector across a magnetic continuum. Locally, a Stoner band theory of ferromagnetism would be valid in this theory. If the conditions for the theory be held to apply, the authors' data show that the band splitting in nickel declines only by about a quarter or a third as the Curie temperature is approached, while a local moment appears.