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

TL;DR

This study investigates the magnetic properties of the spin liquid Tb2Sn2O7, revealing a complex magnetic transition, a novel magnetic structure, and persistent fluctuations at low temperatures, advancing understanding of frustrated magnetism.

Contribution

It uncovers a new magnetic structure in Tb2Sn2O7 not predicted by existing models, highlighting the role of dipolar interactions and anisotropy in spin liquid degeneracy lifting.

Findings

Magnetic correlations switch from antiferromagnetic to ferromagnetic below 2 K.

A new magnetic structure with mixed ferromagnetic and antiferromagnetic features is observed.

Persistent slow magnetic fluctuations exist down to the lowest temperatures.

Abstract

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.