Phase Competition in the Palmer-Chalker XY Pyrochlore Er$_2$Pt$_2$O$_7$

TL;DR

This study investigates Er$_2$Pt$_2$O$_7$, a new XY pyrochlore magnet, revealing its unique magnetic order, spin dynamics, and the influence of phase competition on its quantum fluctuations and low-temperature behavior.

Contribution

It reports the first observation of the Palmer-Chalker $ ext{Γ}_7$ magnetic order in Er$_2$Pt$_2$O$_7$, contrasting with related compounds, and analyzes the effects of phase competition on its spin excitations.

Findings

Er$_2$Pt$_2$O$_7$ orders into the $ ext{Γ}_7$ state at 0.38 K.

Broad heat capacity anomaly at 1.5 K indicates strong short-range fluctuations.

Flat band of excitations matches classical spin wave theory, but high-energy dispersive branches do not.

Abstract

We report neutron scattering measurements on Er$_2$Pt$_2$O$_7$, a new addition to the XY family of frustrated pyrochlore magnets. Symmetry analysis of our elastic scattering data shows that Er$_2$Pt$_2$O$_7$ is the first XY pyrochlore to order 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 Er$_2$Ti$_2$O$_7$ and Er$_2$Ge$_2$O$_7$, both of which order into $\Gamma_5$ magnetic structures at much higher temperatures, $T_N=1.2$ K and $1.4$ K, respectively. In this temperature range, the magnetic heat capacity of Er$_2$Pt$_2$O$_7$ 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, Er$_2$Pt$_2$O$_7$ 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.