Antiferromagnetism in Chromium Alloys

Abstract

The antiferromagnetism of Cr and its alloys has recently been extensively studied by neutron-diffraction1–3 and transport-property measurements.2,4 The concentration dependence of the moment, the Néel temperature, and the periodicity of the magnetic ordering can be correlated with the theoretical band structures and the anomalies in the transport properties just below TN. Magnetic ordering in Cr is a consequence of the localized nature of the d-functions, which give rise to a large intra-atomic Coulomb interaction; and the narrow d bands which favor the formation of a magnetic state.5,6 The magnetic form factor7 is in qualitative agreement with the relative occupancy of the d bands of different symmetry.8 Decreasing the electron concentration by the addition of V causes a decrease in magnetic moment and TN and also decreases the Q vector of the magnetization. It has been shown6,9 that in some cases, the magnetic periodicity of such a structure is determined by the extension of a prominent piece of the Fermi surface and it appears that, in this case, the distance between the octahedral faces of the electron jack at Γ and the hole surface at H determines Q.10 This hypothesis is in approximate agreement with the calculated Fermi surface dimensions,11 and the change in periodicity and moment with alloying can also be qualitatively understood. The transport properties provide evidence that the magnetic ordering destroys a significant proportion of the Fermi surface. Increasing the electron concentration by adding Mn causes the moment and TN to increase and, in addition, tends to make the magnetic periodicity commensurate with the lattice. This allows a maximum interaction between the magnetization wave and the d electrons. The addition of Mo or W causes a gradual decrease in the moment and TN, probably due to the less localized nature of the d functions in these metals. In pure Cr there is a transition from a transverse to a longitudinal spin density wave on cooling through 121°K, due to the extra entropy associated with the former spin configuration. This transition is rapidly eliminated by the addition of impurities, for reasons which are not at present understood.