Antiferromagnetism in Chromium Alloys. II. Transport Properties

The electrical resistivity and thermoelectric power have been studied as a function of temperature in single crystals of Cr containing small concentrations of V, Mn, Mo, W, and Re. The onset of antiferromagnetism is marked by anomalies in the transport properties, and the Néel temperatures can be deduced from these. The variation of $T_N$ upon alloying is qualitatively in accord with the two-band model of Fedders and Martin, especially in its exponentially rapid dependence on electron concentration. In the incommensurable magnetic structures, the most important factors modifying the transport properties are the mutual annihilation of electron and hole regions of the Fermi surface and the consequent reduction in the probability of electron-phonon scattering. Field-cooling experiments in pure Cr indicate that the effect of magnetic superzones is much smaller than these contributions. The electron-phonon scattering depends sensitively on the relation between the energy gap at the Fermi surface and the maximum phonon energy, and consideration of the magnitude of the anomalies in the transport properties below $T_N$, in particular the variation with electron concentration, leads to an approximate value of $2.2k_B{T}_N$ for the mean energy gap, which is consistent with the theory. In the more concentrated commensurable magnetic structures, on the other hand, superzone effects appear to be dominant, since the change in resistance on ordering is proportional to the ordered moment. When different solutes are added to Cr, the low-temperature thermopower behaves in a very complicated way, but its large magnitude in the more concentrated Cr-Mn alloys can probably be ascribed to magnon drag.