Magnetic Excitations in Nickel

In continuation of our* earlier studies on this topic, the generalized susceptibility of nickel has now been measured and calculated over a wide range of temperatures covering the ferromagnetic, the paramagnetic, and the ``critical'' regions. Experimental results (from thermal neutron inelastic scattering) are expressed by means of contour maps showing the Van Hove correlation function $\mathcal{S}\mathrm{(K,\omega )}$ and the related imaginary part Imχ(K,ω) of the susceptibility. The whole Brillouin zone of wave‐vectors is explored over a range of energies ℏω up to about ⅛ eV from 0.47T<sub>c</sub>–1.9T<sub>c</sub>. An absolute comparison is made with calculations based on direct evaluation of the Lindhard expression for generalized susceptibility followed by exchange enhancement of the spin part in random‐phase approximation as per Izuyama, Kim, and Kubo. Tight‐binding‐approximation band structures have been employed, based on Slater‐Koster interpolation of the bands obtained by Fletcher, by Yamashita and Wakoh, and by Dalton and Hubbard. It has been possible to give a credible account of the observed susceptibility over essentially the whole temperature range from the spin‐wave region upwards. At low temperatures, the collective or spin‐wave mode is the most prominent feature, the Stoner mode states being so strongly mixed with the spin‐wave states by the interaction that little trace is left of the ``band of Stoner modes'' normally drawn in simple accounts of this phenomenon. With increase of temperature, a severe broadening of the spin‐wave and the characteristic evolution of the susceptibility function through the critical temperature are correctly predicted. The agreement with experiment is improved by assuming that the effective Coulomb interaction parameter I_eff(K) falls off some 25% between K=0 and K_max. An attempt to estimate the orbital susceptibility will be described.