Electrical Resistivity and the Depression of the Néel Temperature in Cr-Mo and Cr-Fe

Abstract

Existing data are used to show that Mo and Fe impurities cause the Néel temperature $T_N$ of Cr to decrease at almost exactly the same rate up to 10 at.%. Elements with a higher valence than Cr generally increase $T_N$. It is suggested that two Fe electrons are localized and produce a localized magnetic moment, for which experimental evidence already exists. The effective valence of Fe would then be similar to that of Mo. In order to investigate the similarities and differences of Cr-Fe and Cr-Mo, electrical-resistivity measurements have been made on two Cr alloys with 9.35-at.% Mo and 9.35-at.% Fe, respectively, from 2 to 300°K. For Cr-Mo, $T_N=197\mathrm{}°$K, and for Cr-Fe, $T_N=181\mathrm{}°$K. At 0 and 200°K the resistivity of Cr-Fe is 12.6 and 3.2 times higher, respectively, than that of Cr-Mo. We suggest that localized magnetic moments at Fe sites combined with atomic disorder produce a large, nearly temperature-independent spin-disorder scattering in Cr-Fe. A simple model of electrical conduction is employed to explain the temperature dependence of the electrical resistivity. Below $T_N$ an energy gap with a BCS temperature dependence opens up over a part of the Fermi surface, and conduction takes place in two bands. As a result of the gap, electrons in the magnetic band are thermally frozen out with decreasing temperature, which leads to the rise in resistivity just below $T_N$. The 0°K gap is estimated to be 0.14 eV for Cr-Mo and 0.072 eV for Cr-Fe. Pure Cr and Cr-Mo have nearly the same balance of conduction between the magnetic and nonmagnetic bands. In Cr-Fe the balance is shifted toward conduction in the magnetic band.