Magnetic Frustration in Lead Pyrochlores

TL;DR

This paper introduces a new family of lead-based pyrochlores, expanding the phase space of frustrated magnetic systems, and investigates their magnetic properties, revealing potential quantum spin ice behavior and diverse magnetic ground states.

Contribution

The study reports on lead pyrochlores with larger B-site cations, exploring their magnetic phases and identifying candidates for quantum spin ice and other exotic states.

Findings

Pr₂Pb₂O₇ shows no long-range order down to 400 mK and has a spin ice-like heat capacity anomaly.

Nd₂Pb₂O₇ transitions to magnetic order at 0.41 K with near-zero Weiss temperature.

Gd₂Pb₂O₇ is a Heisenberg antiferromagnet with order at 0.81 K and a T^{3/2} heat capacity dependence.

Abstract

The rich phase diagrams of magnetically frustrated pyrochlores have maintained a high level of interest over the past 20 years. To experimentally explore these phase diagrams requires a means of tuning the relevant interactions. One approach to achieve this is chemical pressure, that is, varying the size of the non-magnetic cation. Here, we report on a new family of lead-based pyrochlores A$_2$Pb$_2$O$_7$ (A = Pr, Nd, Gd), which we have characterized with magnetic susceptibility and specific heat. Lead is the largest known possible B-site cation for the pyrochlore lattice. Thus, these materials significantly expand the phase space of the frustrated pyrochlores. Pr$_2$Pb$_2$O$_7$ has an absence of long-range magnetic order down to 400 mK and a spin ice-like heat capacity anomaly at 1.2 K. Thus, Pr$_2$Pb$_2$O$_7$ is a candidate for a quantum spin ice state, despite weaker exchange. Nd$_2$Pb$_2$O$_7$ transitions to a magnetically ordered state at 0.41 K. The Weiss temperature for Nd$_2$Pb$_2$O$_7$ is $\theta_{\text{CW}}$ = $-$0.06 K, indicating close competition between ferromagnetic and antiferromagnetic interactions. Gd$_2$Pb$_2$O$_7$ is a Heisenberg antiferromagnet that transitions to long-range magnetic order at 0.81 K, in spite of significant site mixing. Below its ordering transition, we find a $T^{3/2}$ heat capacity dependence in Gd$_2$Pb$_2$O$_7$, confirmation of a ground state that is distinct from other gadolinium pyrochlores. These lead-based pyrochlores provide insight into the effects of weakened exchange on highly frustrated lattices and represent further realizations of several exotic magnetic ground states which can test theoretical models.