Ordered Spin Ice State and Magnetic Fluctuations inTb2Sn2O7

We have studied the spin liquid ${\mathrm{Tb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ by neutron diffraction and specific heat measurements. Below about 2 K, the antiferromagnetic liquidlike correlations mostly change 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, akin to an ordered spin ice, with both ferromagnetic and antiferromagnetic character. It suggests that the ordered ground state results from the influence of dipolar interactions combined with a finite anisotropy along $⟨111⟩$ axes. The moment value of $3.3(3){\mu }_{\mathrm{B}}$ deduced from the specific heat is well below that derived from the neutron diffraction of $5.9(1){\mu }_{\mathrm{B}}$, which is interpreted by the persistence of slow collective magnetic fluctuations down to the lowest temperatures.