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 J₂. The huge ground-state degeneracy of the nearest-neighbor Heisenberg spins is lifted by J₂ 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 J₂. 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 1.87J₂S² and 0.26J₂S² in the limit of small J₂. The phase transitions are discontinuous. We analytically examine the local stability of the collinear state and obtain a boundary T~J₂²/J₁ in agreement with Monte Carlo simulations.