Magnetic Excitations in Nickel

The Fourier transform of the spin correlation function has been measured by means of neutron scattering, and its evolution studied as the temperature is carried through the order-disorder transition point from the low to the high-temperature extreme situations. The correlation is exhibited in detail over the complete Brillouin zone for the range of energies up to 0.11 eV at 0.5 , 0.9 , 1.1 and 1.6 times Tc. At the highest temperature the distribution is well explained by the elementary theory of Stoner paramagnetism if an exchange enhancement calculated in random-phase approximation is introduced. At low temperature the correlation shows a sharp ridge corresponding to spin-wave excitations; below 60 meV the spin-wave peaks all fall close to a parabola extrapolated from the quadratic dispersion law of previous low-energy determinations, but at higher energies they fall away from this parabola in a manner agreeing with the calculation of Thompson and Myers. As the temperature is taken through the phase-transition point, critical enhancement of the susceptibility such as would be predicted using the random-phase approximation is not observed. The approach to criticality from either side is characterized instead by a pronounced anticipation of the features of the susceptibility on the other side of the Curie point. Away from a region closely confined to the origin, the transition through Tc is smooth; indeed the correlation functions at 0.9 Tc and 1.1 Tc are strikingly similar. In the critical region the susceptibility is dominated by spin-wave-like peaks rather than paramagnons; it is suggested that the explanation might lie in the existence of large fluctuations of the band splitting near Tc. (author)