Transitions and spin dynamics at very low temperature in the pyrochlores Yb2Ti2O7 and Gd2Sn2O7

The very low temperature properties of two pyrochlore compounds, Yb2Ti2O7 and Gd2Sn2O7, were investigated using an ensemble of microscopic and bulk techniques. In both compounds, a first order transition is evidenced, as well as spin dynamics persisting down to the 20 mK range. The transition however has a quite different character in the two materials: whereas that in Gd2Sn2O7 (at 1 K) is a magnetic transition towards long range order, that in Yb2Ti2O7 (at 0.24 K) is reminiscent of the liquid-gas transition, in the sense that it involves a 4 orders of magnitude drop of the spin fluctuation frequency, with no long range order. We attribute these unusual features to the frustration of the antiferromagnetic exchange interaction in the pyrochlore lattice.