Pressure Effect on the Curie Temperatures of Transition Metals and Alloys

Abstract

We determined the variation of the Curie temperature with pressure for iron, cobalt, nickel, six iron-nickel alloys, and five cobalt-nickel alloys. A permeability method in zero magnetic field has been used in a belt-type apparatus at pressures up to 90 kbar and temperatures up to 1400 \ifmmode^\circ\else\textdegree\fi{}K. The pressure shifts measured for the Curie temperatures of iron and cobalt are zero; for nickel it is positive and markedly nonlinear. The Curie temperatures of face-centered-cubic iron-nickel alloys decrease very rapidly with pressure when the nickel content is low and show a progressively smaller decrease when nickel is added. They begin to increase when the nickel concentration is over 68 wt%. The existence of maxima on the curves of the Curie temperatures versus pressure or volume seems to be definitely established. pressure increases the Curie temperature of cobalt-nickel alloys in all cases, but here also in a nonlinear way. For strong ferromagnets with a few carriers a band theory has been previously developed. The shift of the Curie temperature is then given by $\frac{d\ensuremath{\theta}}{\mathrm{dP}}=\frac{5}{3}k\ensuremath{\theta}$ ($k=\mathrm{compressibility}$). For pure nickel and nickel-cobalt alloys with a cobalt content less than about 50 wt% good agreement with experiment is obtained at low pressures, but not at higher pressures. The Curie temperature shift of Invar-type alloys has been previously calculated using a weak-itinerant-electron-ferromagnet band theory. In this case we have been able to predict and observe a parabolic decrease of the Curie temperature with pressure: ${\ensuremath{\theta}}^{2}={\ensuremath{\theta}}_{0}^{2} (1\ensuremath{-}\frac{P}{{P}_{0}})$. In order to explain the shifts for the different fcc alloys of nickel with iron, use is made of a model where $\ensuremath{\gamma}$-iron is supposed to have two electronic levels and which gives account of many properties of these alloys at normal pressure.