Magnetic Frustration in Lead Pyrochlores

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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 A₂Pb₂O₇ (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. Pr₂Pb₂O₇ 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, Pr₂Pb₂O₇ is a candidate for a quantum spin ice state, despite weaker exchange. Nd₂Pb₂O₇ transitions to a magnetically ordered state at 0.41 K. The Weiss temperature for Nd₂Pb₂O₇ is θCW = -0.06 K, indicating close competition between ferromagnetic and antiferromagnetic interactions. Gd₂Pb₂O₇ 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 T³/² heat capacity dependence in Gd₂Pb₂O₇, 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.