Effect of impurities on the magnetic ordering in chromium

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´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 δ∝${\mathit{g}}^2$. 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´el temperature.