Phase Competition in the Palmer-Chalker XY Pyrochlore Er2Pt2O7

We report neutron scattering measurements on Er2Pt2O7, a new addition to the XY family of frustrated pyrochlore magnets. Symmetry analysis of our elastic scattering data shows that Er2Pt2O7 orders into the k = 0, Gamma(7) magnetic structure (the Palmer-Chalker state), at T-N = 0.38 K. This contrasts with its sister XY pyrochlore antiferromagnets Er2Ti2O7 and Er2Ge2O7, both of which order into Gamma(5) magnetic structures at much higher temperatures, T-N = 1.2 and 1.4 K, respectively. In this temperature range, the magnetic heat capacity of Er2Pt2O7 contains a broad anomaly centered at T* = 1.5 K. Our inelastic neutron scattering measurements reveal that this broad heat capacity anomaly sets the temperature scale for strong short-range spin fluctuations. Below T-N = 0.38 K, Er2Pt2O7 displays a gapped spin-wave spectrum with an intense, flat band of excitations at lower energy and a weak, diffusive band of excitations at higher energy. The flat band is well described by classical spin-wave calculations, but these calculations also predict sharp dispersive branches at higher energy, a striking discrepancy with the experimental data. This, in concert with the strong suppression of T-N, is attributable to enhanced quantum fluctuations due to phase competition between the Gamma(7) and Gamma(5) states that border each other within a classically predicted phase diagram.