Dzyaloshinsky-Moriya antisymmetric exchange coupling in cuprates: Oxygen effects

Abstract

We reconsider the conventional Moriya approach to the Dzyaloshinsky-Moriya antisymmetric exchange coupling for a single Cu1-O-Cu2 bond in cuprates using a perturbation scheme that provides an optimal way to account for intra-atomic electron correlations, low-symmetry crystal field, and local spin-orbital contributions with a focus on the oxygen term. The Dzyaloshinsky vector and the corresponding weak ferromagnetic moment are shown to be a superposition of comparable and, sometimes, competing local Cu and O contributions. We predict the effect of oxygen staggered spin polarization in the antiferromagnetic edge-shared CuO2 chains due to the uncompensated oxygen Dzyaloshinsky vectors. The polarization is perpendicular to both the main chain antiferromagnetic vector and the CuO2 chain normal. The intermediate 17O NMR is shown to be an effective tool to inspect the effects of Dzyaloshinsky-Moriya coupling in an external magnetic field. In particular, we argue that the puzzling planar 17O Knight shift anomalies observed in the paramagnetic phase of the generic Dzyaloshinsky-Moriya antiferromagnetic cuprate La2CuO4 can be assigned to the effect of the field-induced staggered magnetization. Finally, we revisit the effects of symmetric spin anisotropy, in particular, those directly induced by the Dzyaloshinsky-Moriya coupling. The perturbation scheme generalizes the well-known Moriya approach and presents a basis for reliable quantitative estimates for the symmetric partner of the Dzyaloshinsky-Moriya coupling. In contrast to the conventional standpoint, the parameters of the effective two-ion spin anisotropy are shown to incorporate the contributions of a single-ion anisotropy for two-hole configurations at both Cu and O sites. © 2007 Pleiades Publishing, Inc.