We have studied the spin liquid Tb_2Sn_2O_7 by neutron diffraction and specific heat measurements. Below about 2 K, the magnetic correlations change from antiferromagnetic to ferromagnetic. Magnetic order settles in two steps, with a smeared transition at 1.3(1) K then an abrupt transition at 0.87(2) K. A new magnetic structure is observed, not predicted by current models, with both ferromagnetic and antiferromagnetic character. It suggests that the spin liquid degeneracy is lifted by dipolar interactions combined with a finite anisotropy along <111> axes. In the ground state, the Tb3+ ordered moment is reduced with respect to the free ion moment (9 μ_B). The moment value of 3.3(3) μ_B deduced from the specific heat is much smaller than derived from neutron diffraction of 5.9(1) μ_B. This difference is interpreted by the persistence of slow collective magnetic fluctuations down to the lowest temperatures.
@article{arxiv.cond-mat/0503377,
title = {Magnetic order and spin fluctuations in the spin liquid Tb$\_2$Sn$\_2$O$\_7$},
author = {I. Mirebeau and A. Apeteri and J. Rodriguez-Carvajal and P. Bonville and A. Forget and D. Colson and V. Glazkov and J. P. Sanchez and O. Isnard and E. Suard},
journal= {arXiv preprint arXiv:cond-mat/0503377},
year = {2009}
}