High-pressure synthesis and characterization of the effective pseudospin S=1/2 XY pyrochlores R2 P t2 O7 (R=Er, Yb)

We report on the high-pressure syntheses and detailed characterizations of two effective pseudospin S=1/2 XY pyrochlores Er2Pt2O7 and Yb2Pt2O7 via x-ray/neutron powder diffraction, dc and ac magnetic susceptibility, and specific-heat measurements down to 70 mK. We found that both compounds undergo long-range magnetic transitions at TN,C≈0.3K, which are ascribed to an antiferromagnetic- and ferromagnetic-type order for Er2Pt2O7 and Yb2Pt2O7, respectively, based on the field dependence of their transition temperatures as well as the systematic comparisons with other similar pyrochlores R2B2O7(R=Er,Yb;B=Sn,Ti,Ge). The observed TN of Er2Pt2O7 is much lower than that expected from the relationship of TN versus the ionic radius of B4+ derived from the series of Er2B2O7, while the TC of Yb2Pt2O7 is the highest among the series of ferromagnetic compounds Yb2B2O7(B=Sn,Pt,Ti). Given the monotonic variation of the lattice constant as a function of the B-cation size across these two series of R2B2O7(R=Er,Yb), the observed anomalous values of TN,C in the Pt-based XY pyrochlores imply that another important factor beyond the nearest-neighbor R-R distance is playing a role. In light of the anisotropic exchange interactions Jex={Jzz,J±,J±±,Jz±} for the S=1/2 XY pyrochlores, we have rationalized these observations by considering a weakened (enhanced) antiferromagnetic planar J± (ferromagnetic Ising-like Jzz) due to strong Pt 5d-O2p hybridization within the plane perpendicular to the local [111] direction.