Relief of Frustration in the Heisenberg Pyrochlore Antiferromagnet Gd${}_2$Pt${}_2$O${}_7$

Abstract

The gadolinium pyrochlores, Gd${}_2{B}_2$O${}_7$, are amongst the best realizations of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We present a magnetic characterization of Gd${}_2$Pt${}_2$O${}_7$, a unique member of this family. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements show that Gd${}_2$Pt${}_2$O${}_7$ 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 non-divergent 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 Gd${}_2$Pt${}_2$O${}_7$, $T_N=1.6$ K, is a substantial 160% increase from other gadolinium pyrochlores, which have been found to order at 1 K or lower. We attribute this enhancement in $T_N$ to the $B$-site cation, platinum, which, despite being non-magnetic, has a filled $5d$ $t_{2g}$ orbital and an empty $5d$ $e_g$ orbital that can facilitate superexchange. Thus, the magnetic frustration in Gd${}_2$Pt${}_2$O${}_7$ is partially "relieved", thereby promoting magnetic order.

8 pages, 6 figures

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