The chromium system, comprising pure Cr and alloys with most transition metals and some nontransition metals, is the archetypical spin-density-wave (SDW) system. This paper supplements, with a brief summary and extension to include recent work, two previous comprehensive reviews on Cr (Fawcett, 1988) and Cr alloys . The magnetic phase diagrams are reviewed. Impurity states in CrFe and CrSi, when suitably doped with V or Mn, produce dramatic effects in the electrical resistivity, including a low-temperature resistance minimum due to impurity-resonance scattering. Curie-Weiss paramagnetism appears just above the Néel temperature in dilute CrV alloys. Recent work on inelastic neutron scattering in pure Cr is reviewed: the apparent absence of dispersion of the spin-wave modes at the wave vectors of the incommensurate SDW where the Bragg satellite peaks occur; the energy-dependent anisotropy of the excitations in the longitudinal-SDW phase; the commensurate magnetic scattering at the centre of the magnetic zone, which at higher energy and temperature dominates the inelastic scattering at the satellites; the Fincher-Burke excitations seen at low-energy in the transverse-SDW phase; and the silent satellites seen in single-Q Cr at off-axis incommensurate points as temperature increases towards the Néel transition. X-ray scattering with synchrotron radiation has illuminated the relation between the SDW in Cr and the incommensurate charge-density wave that accompanies it.

The concept of local impurity states within the energy gap of a spin-density-wave (SDW) system is introduced. It is shown that resonant scattering of conduction electrons at these states may lead to greatly enhanced low-temperature resistivity. This impurity resonance scattering (IRS) model is employed to explain the variation of residual resistivity and temperature dependence of resistivity at low temperatures of Cr-Fe and Cr-Si systems on V and Mn doping and application of high pressure.

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AbsLraeLThe energy difference SE between the local impurity slate levels in lhe spindensity-wave (SDw) alloy$ Cr + 13 al.% Si and Cr + 2.7 at.% Fe, is estimated 'from interel 62 beween the peaks in the residual resistivity as the Fermi level Ea is tuned by doping with low concentrations z 01 V or Mu. The measured variation with doping of the SDw waveyeclor in Cr and the spin-wave velocity in Cr(Mn) are used lo alimaie d E F / d z , and hence obtain SE from 6r. The ratio of SE to the energy gap 2A determined from the infrared rellectivily gives the scattering parameler between the nesting parls of the Fermi surface.

The temperature dependence of the magnetic susceptibility of alloys (x = 0 to 20% V), measured over the range 5 < T < 400 K, is found to fit a Curie-Weiss law, with different parameters above and below the Néel temperature in the SDW alloys. The effective moment in the paramagnetic alloys (x = 5, 10 and 20% V), and in the paramagnetic phase of the spin-density-wave (SDW) alloys, varies with the V content in accordance with an environment model in which the Fe moment is , provided none of its nearest neighbours is a V atom, but is suppressed completely by a single V neighbour.

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Spindensity-wave (SDW) binary Crl-,Mn, (x << 0.14.6% Mu) and ternary (Cr + 1.3% Si)l_,Mn, (x = 0.17 and 0.6% Mn) alloys are found to exhibit spin-glass behaviour, which differs however from that of a conventional spin glass in that the magnetic susceptibility x ( T ) is essentially independent of temperature T between the low-temperature maximum and the Ndel temperature TN, and in that the temperature of the maximum is essentially independent of the Mn concentration. Comparison of binary and ternary alloys having the same Mn content shows that the nature of the maximum in x ( T ) depends on whether or not the SDW is commensurate. A model is proposed that explains the increase in x ( T ) at low temperature as resulting from pinning of the phase of the SDW to the Mn moments, which are frozen be low^ TN, causing frustration on the host Cr moments On the surfaces between the resultant SDW phase domains.