Relief of frustration in the Heisenberg pyrochlore antiferromagnet Gd2Pt2O7

The gadolinium pyrochlores Gd2B2O7 are among the best realizations of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We present a magnetic characterization of Gd2B2O7, a unique member of this family. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements show that Gd2B2O7 undergoes an antiferromagnetic ordering transition at T-N = 1.6 K. This transition is strongly first order, as indicated by the sharpness of the heat capacity anomaly, thermal hysteresis in the magnetic susceptibility, and a nondivergent relaxation rate in mu SR. The form of the heat capacity below T-N suggests that the ground state is an anisotropic collinear antiferromagnet with an excitation spectrum that is gapped by 0.245(1) meV. The ordering temperature in Gd2B2O7, T-N = 1.6 K, is a substantial 160% increase from other gadolinium pyrochlores, which are all known to order at 1 K or lower. We attribute this enhancement in T-N to the B-site cation, platinum. Despite being nonmagnetic, platinum has a filled 5d t(2g) orbital and an empty 5d e(g) orbital that can facilitate superexchange. Thus, the magnetic frustration in Gd2B2O7 is partially "relieved," thereby promoting magnetic order.