Magnetic order and spin fluctuations in the spin liquid Tb$_2$Sn$_2$O$_7$.

I. Mirebeau (1) , A. Apeteri (1) , J. Rodriguez-Carvajal (1) , P. Bonville (2) , A. Forget (2) , D. Colson (2) , V. Glazkov (3) , J. P. Sanchez (3) , Olivier Isnard (4) , E. Suard (5)

1 LLB - UMR 12 - Laboratoire Léon Brillouin
2 SPEC - UMR3680 - Service de physique de l'état condensé
3 SPSMS - UMR 9001 - Service de Physique Statistique, Magnétisme et Supraconductivité
4 Laboratoire de Cristallographie
5 ILL - Institut Laue-Langevin

We have studied the spin liquid Tb$_2$Sn$_2$O$_7$ 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 Tb$^{3+}$ ordered moment is reduced with respect to the free ion moment (9 $\mu_{\rm B}$). The moment value of 3.3(3) $\mu_{\rm B}$ deduced from the specific heat is much smaller than derived from neutron diffraction of 5.9(1) $\mu_{\rm B}$. This difference is interpreted by the persistence of slow collective magnetic fluctuations down to the lowest temperatures.

Keywords: specific heat; spin liquid; geometrical frustration; neutron diffraction

Domains: Strongly Correlated Electrons [cond-mat.str-el]

I. Mirebeau, A. Apeteri, J. Rodriguez-Carvajal, P. Bonville, A. Forget, et al.. Magnetic order and spin fluctuations in the spin liquid Tb$_2$Sn$_2$O$_7$.. 2005. ⟨hal-00004478⟩