Effect of impurities on the magnetic ordering in chromium.

Abstract

It is well known that impurities profoundly alter the magnetic properties of chromium. For example, doping with as little as 0.2% vanadium changes the phase transition from weakly first order in pure chromium to second order. While vanadium impurities suppress the Ne\ifmmode\acute\else\textasciiacute\fi{}el temperature ${\mathit{T}}_{\mathit{N}}$, doping with manganese dramatically enhances ${\mathit{T}}_{\mathit{N}}$. Impurities also change the wave vector of the spin-density wave, which has the order parameter g. According to the theory of Young and Sokoloff (YS), the first-order transition in pure chromium is caused by a charge-density wave with order parameter \ensuremath{\delta}\ensuremath{\propto}${\mathit{g}}^{2}$. Scattering by impurities suppresses the charge-density wave and drives the transition to second order. In this paper we refine the YS formalism by examining the subtle balance between the spin-density and charge-density terms in the free energy. We find that the first-order transition is destroyed when the concentration of vanadium exceeds about 0.15%, in agreement with experimental measurements. We also study the effect of impurities on the band structure and Ne\ifmmode\acute\else\textasciiacute\fi{}el temperature.