Motivated by a recent experiment on volborthite, a typical spin-$1/2$ antiferromagnet with a kagom{e} lattice structure, we study the magnetization process of a classical Heisenberg model on a spatially distorted kagom{e} 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 $Delta m_z=left|1-alpharight|/3alpha$ 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 $120^{circ}$ structure with a $sqrt{3}timessqrt{3}$ 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.
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