Transitions and Spin Dynamics at Very Low Temperature in the Pyrochlores Yb_2Ti_2O_7 and Gd_2Sn_2O_7

摘要

The very low temperature properties of two pyrochlore compounds, Yb_2Ti_2O_7 and Gd_2Sn_2O_7, 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 Gd_2Sn_2O_7 (at 1 K) is a magnetic transition towards long range order, that in Yb_2Ti_2O_7 (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.