Magnetic frustration in lead pyrochlores

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 nonmagnetic cation. Here, we report on the family of lead-based pyrochlores A sub(2)Pb sub(2)O sub(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 sub(2)Pb sub(2)O sub(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 sub(2)Pb sub(2)O sub(7) is a candidate for a quantum spin ice state, despite weaker exchange. Nd sub(2)Pb sub(2)O sub(7) transitions to a magnetically ordered state at 0.41 K. The Weiss temperature for Nd sub(2)Pb sub(2)O sub(7) is [straighttheta] sub(CW) = -0.06 K, indicating close competition between ferromagnetic and antiferromagnetic interactions. Gd sub(2)Pb sub(2)O sub(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 super(3/2) heat capacity dependence in Gd sub(2)Pb sub(2)O sub(7), indication 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.