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

TLDR

The study calculates how the Curie temperature of Ni and Ni‑Cu alloys changes with pressure by examining the pressure‑induced shift of the pole in the uniform static spin susceptibility. The authors model the d‑electron interactions with a short‑range Hubbard‑Kanamori‑Gutzwiller Hamiltonian, treat them in the t‑approximation, and compute the spin susceptibility via Martin–Schwinger Green’s‑function methods using a paramagnetic Ni density‑of‑states curve, while incorporating inter‑d‑band effects and changes in d‑hole count under compression. The calculations agree well with experimental data for Ni, and for Ni‑Cu alloys the model that forbids d‑hole occupation of Cu sites yields substantially better agreement than the rigid‑band approach.

Abstract

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.