First-order transition in frustrated Yb2Ti2O7 without long-range order

We have examined the pyrochlore lattice compound Yb2Ti2O7 using 170Yb M?ssbauer spectroscopy and neutron diffraction. The Yb3+ ion has a well-isolated Sʹ=1/2 ground state with planar anisotropy and the net interaction is ferromagnetic. The M?ssbauer measurements show that short-range correlations develop below about 2 K and at 0.2 K, where specific heat data evidence a (lambda)-transition (Bl?te et al. Physica, 43, 549 (1969)), there is a first-order transition to a state where the Yb3+ moments become blocked on the M?ssbauer 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.