Inelastic neutron scattering measurements axe described in pure chromium and in dilute alloys of Cr with V and Re that show the role of critical fluctuations in determining the nature of the Neel transition near the triple point between the paramagnetic phase and incommensurate and commensurate spin-density-wave (SDW) phases. In pure chromium, field-cooled so as to produce a single-Q_χ state, critical fluctuations grow rapidly at Q_y and Q_z, as well as at Q_χ, as the Neel temperature T_N is approached from below, so that at the Neel transition the entry to the cubic paramagnetic phase is almost continuous. Doping with V rapidly reduces the amplitude of the critical fluctuations in the paramagnetic phase, as the system moves away from the triple point, and the weak first-order transition seen in pure chromium becomes continuous with the addition of about 0.1%V. Adding 0.18%Re moves the system towards the triple point and increases the amplitude of the critical fluctuations. Extrapolation of the parameters of the Sato-Maki model, which fits the inelastic scattering data rather well, indicates that there is a mode-softening transition that occurs at a finite value, about 0.017 rlu, of the incommensurability parameter, where the SDW becomes commensurate, with a first-order Neel transition.
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