First-order transition in frustrated Yb<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> without long-range order

We have examined the pyrochlore lattice compound Yb2Ti2O7 using 170Yb Mössbauer spectroscopy and neutron diffraction. The Yb3+ ion has a well-isolated S' = 1/2 ground state with planar anisotropy and the net interaction is ferromagnetic. The Mössbauer measurements show that short-range correlations develop below about 2 K and at 0.2 K, where specific heat data evidence a λ-transition (Blöte et al. Physica, 43, 549 (1969)), there is a first-order transition to a state where the Yb3+ moments become blocked on the Mössbauer time scale (10–8 s) with well-defined directions. The neutron diffraction measurements show that below 0.2 K there is no long-range magnetic order. PACS Nos.: 75.40-s, 76.80+y, 75.50Dd