Evidence from thermodynamic measurements for a singlet crystal-field ground state in pyrochloreTb2Sn2O7and

Abstract

The magnetic entropy variation for the ordered spin-ice Tb 2 Sn 2 O 7 and the spin-liquid Tb 2 Ti 2 O 7 measured up to ϳ20 K shows that the crystal-field ground state for both systems is a singlet split from the first excited level by a thermal energy of ϳ2 K. It is the superexchange interactions between the Tb 3+ spins which induce the terbium magnetic moments. Our results are analyzed in terms of a tetragonal perturbation for Tb 2 Ti 2 O 7 and this perturbation plus a molecular field for Tb 2 Sn 2 O 7 .Geometrically frustrated magnetic materials are now intensively studied, both theoretically and experimentally. In particular, the pyrochlore crystal structure R 2 M 2 O 7 ͑R stands for a rare-earth ion and M for a nonmagnetic p or d metal͒ built of corner sharing tetrahedra usually inhibits the formation of a collinear long-range magnetic order, leading to a variety of magnetic behaviors, such as spin ices, spin liquids, and complex noncollinear magnetic orders. 1,2Among the pyrochlore compounds, the insulators Tb 2 Ti 2 O 7 and Tb 2 Sn 2 O 7 are probably the less understood systems. They crystallize at room temperature with the cubic space group Fd3m. The local symmetry at a Tb 3+ ion is D 3d . It is important to note that structural distortions below ϳ20 K have been reported from x-ray measurements for Tb 2 Ti 2 O 7 . 3 These distortions are a signature of a transition from cubic to tetragonal structures. However, the phase transition was not observed. Anyhow, the x-ray data show that the rare-earth local symmetry is reduced at low temperature. Neutron diffraction for Tb 2 Sn 2 O 7 shows that the crystal structure is tetragonal in the magnetically ordered state. 4,5 Tb 2 Sn 2 O 7 is a so-called ordered spin-ice system below T c = 0.88͑1͒ K. 4,5 For a given tetrahedron the four spins are close to the ͗111͘ anisotropy axes in a two-in two-out magnetic configuration. In contrast to the classical spin-ice compounds ͑Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 ͒, the magnetic unit cell, which is the crystallographic unit cell, repeats itself. The ordered spin-ice structure is explained within the phenomenological model of Champion et al. 6 which assumes a ferromagnetic first neighbor exchange interaction combined with a finite anisotropy. In fact, a SR experiment has shown that the magnetic ground state is homogeneous ͑no phase separation͒ and dynamical in nature. 5,7 While this has been hotly debated, 8,9 it seems now to be accepted. 10,11 On the other hand, the sibling compound Tb 2 Ti 2 O 7 is classified as a spin-liquid system because it does not display any magnetic order, at least down to 0.05 K, despite the onset of short-range antiferromagnetic correlations developing at ϳ50 K. 12,13 The absence of a long-range magnetic order is a long-standing mystery. [14][15][16] Based on inelastic neutron-scattering measurements, a crystal-field level scheme for the Tb 3+ ions in the two com-pounds of interest here has been proposed assuming the D 3d point symmetry to be valid. 17 The local threefold axis is the crystallographic ax...

Seeing the light: Experimental signatures of emergent electromagnetism in a quantum spin ice

“…Unfortunately, the interpretation of experiment at these low temperatures is muddied by questions of sample quality, with inconsistent results for spin-freezing obtained by different au-thors 23,24,[91][92][93] . Published thermodynamic data at low temperatures is also less than conclusive, showing hints of a saturation of χ(T ) at low temperatures, but strong sample dependence 23,[92][93][94][95] . And the picture is further complicated by strong fluctuations of the lattice 96,97 , with alternative theories of Tb 2 Ti 2 O 7 building on lattice effects [98][99][100] At present there is no published neutron scattering data for Tb 2 Ti 2 O 7 with the combination of k-resolution, energy resolution and low temperature needed to compare with the predictions in Section II D and Section III C of this paper.…”

Quantum fluctuations in spin-ice-like Pr2Zr2O7

“…1f,g) and inconsistent with a full static Jahn-Teller distortion 34 . The inferred zero-field magnetic component C M shows a decade of activated T-dependence between 2 K and 0.2 K (Fig.…”

Unusual field dependence of spin fluctuations on different timescales in Tb2Ti2O7

“…12 The magnetic Coulomb phase has been shown to exist in classical dipolar spin ice as a magnetic analogy of electrostatics 13 and quantum spin ice should display a magnetic analog of quantum electrodynamics. 14 Recent experimental work has attempted to distinguish between the two theoretical predictions for Tb 2 Ti 2 O 7 with evidence presented for 19,20 and against 21 the proposed singlet ground state. 8 In addition other work has suggested the existence of quantum spin fluctuations at low temperatures.…”