Magnetization step in spatially distorted Heisenberg kagomé antiferromagnets

Motivated by a recent experiment on volborthite, a typical spin-1/2 antiferromagnet with a kagomé 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 Δmz - | 1 - α|/3α at zero temperature, where α 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° structure with a √3 × √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.