  Motivated by the recent experiment on kagome-lattice antiferromagnets, we
study the zero-field ordering behavior of the antiferromagnetic classical
Heisenberg model on a uniaxially distorted kagome lattice by Monte Carlo
simulations. A first-order transition, which has no counterpart in the
corresponding undistorted model, takes place at a very low temperature. Origin
of the transition is ascribed to a cooperative proliferation of topological
excitations inherent to the model.
