Itinerant-Electron Theory of Pressure Effects on Ferromagnetic Transition Temperatures: Ni and Ni-Cu Alloys

The rate of change of Curie temperature with pressure is calculated for Ni and Ni-Cu alloys by considering the pressure-induced shift of the pole in the uniform static spin susceptibility. The short-range Hamiltonian of Hubbard, Kanamori, and Gutzwiller is employed to describe the interactions among $d$ electrons and, following Kanamori, these interactions are treated in the $t$ approximation. The spin susceptibility is calculated using the Green's-function technique of Martin and Schwinger; the density-of-states curve for paramagnetic Ni computed by Hodges et al. is employed in making numerical evaluations. Account is taken of inter-$d$-band interactions, and of the effect on the number of $d$ holes of changes in the conduction band due to compression. Good agreement with experiment is obtained for Ni. For Ni-Cu alloys, calculations based on the rigid-band model yield poor results; but the use of an almost equally simple model in which a $d$ hole is assumed never to enter a Cu site leads to substantial improvement.