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.

Measurements are reported of the temperature dependence of the elastic constants and thermal expansion of dilute Cr-Si alloy single crystals containing 0.5, 1.2, 1.6 and 3.0 at.% Si. Well defined anomalies in all elastic constants, and in the thermal expansion, were observed at the Neel points (TN) and at the commensurate-incommensurate spin density wave transition temperatures (TIC) of the alloys. Elastic anomalies were also observed at the longitudinal-transverse incommensurate spin density wave transition temperature of the Cr+0.5 at.% Si crystal, but not in the thermal expansion thereof. The transition at the Neel point of the Cr+0.5 at.% Si crystal is continuous, that of the Cr+1.2 at.% Si and Cr+1.6 at.% Si crystals of first order, while the order of the transition in the Cr+3.0 at.% Si crystal could not be determined unambiguously. For the Cr+1.2 at.% Si and Cr+1.6 at.% Si crystals the discontinuous transitions at TN show hysteresis behaviour. In the 1.2 at.% Si crystal it is also accompanied by structure effects that were absent for 1.6 at.% Si. Hysteresis, accompanied by structure effects, was also observed at TIC for Cr+1.2 at.% Si. The structure effects are ascribed to the possibility of mixed incommensurate-commensurate spin density wave states in the dilute Cr-Si magnetic phase diagram. The magnetovolume and magnetic contributions to the bulk modulus have been found to fit the equation a+bT2+cT4, predicted by theory, rather well up to temperatures (T) close to the Neel points of Cr+0.5 at.% Si and Cr+1.6 at.% Si.

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We have measured the antiferromagnetic spin-density-wave (SDW) order in Cr/Cr(97.5)Mn2.5(001) superlattices. The Mn doping creates a high Néel temperature layer that confines the incommensurate SDW order within the Cr layers. With temperature cycling we observe a transition from commensurate to incommensurate SDW order and discrete changes in the SDW period. We find that these transitions show significant hysteresis (up to 75 K) when the number of SDW nodes within the Cr layer changes by an odd number, while there is no hysteresis for changes of an even number of nodes. This results from the competition between maintaining the spin structure at the interfaces and introducing a spin slip at the nodes of the Cr SDW.

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