Partial order from disorder in a classical pyrochlore antiferromagnet

We investigate theoretically the phase diagram of a classical Heisenberg antiferromagnet on the pyrochlore lattice perturbed by a weak second-neighbor interaction J(2). The huge ground-state degeneracy of the nearest-neighbor Heisenberg spins is lifted by J(2) and a magnetically ordered ground state sets in upon approaching zero temperature. We have found a new, partially ordered phase with collinear spins at finite temperatures for a ferromagnetic J(2). In addition to a large nematic order parameter, this intermediate phase also exhibits a layered structure and a bond order that breaks the sublattice symmetry. Thermodynamic phase boundaries separating it from the fully disordered and magnetically ordered states scale as 1.87J(2)S(2) and 0.26J(2)S(2) in the limit of small J(2). The phase transitions are discontinuous. We analytically examine the local stability of the collinear state and obtain a boundary T similar to J(2)(2)/J(1) in agreement with Monte Carlo simulations.