Magnetization Step in Spatially Distorted Heisenberg Kagome Antiferromagnets

Motivated by a recent experiment on volborthite, a typical spin-1/2 antiferromagnet with a kagome lattice structure, we study the magnetization process of a classical Heisenberg model on a spatially distorted kagome lattice using the Monte Carlo (MC) method. We find a distortion-induced magnetization step at low temperatures and low magnetic fields. The magnitude of this step is given by Delta m(z) = vertical bar 1 - alpha vertical bar/3 alpha at zero temperature, where alpha denotes the spatial anisotropy in exchange constants. The magnetization step signals a first-order transition at low temperatures, between two phases distinguished by distinct and well-developed short-range spin correlations, one characterized by spin alignment of a local 120 degrees structure with a root 3 x root 3 period, and the other by a partially spin-flopped structure. We point out the relevance of our results to the unconventional steps observed in volborthite.