N ov 2 01 4 Low temperature crystal structure and local magnetometry for the geometrically frustrated pyrochlore Tb 2 Ti 2 O 7

We report synchrotron radiation diffraction and muon spin rotation (μSR) measurements on the frustrated pyrochlore magnet Tb2Ti2O7. The powder diffraction study of a crushed crystal fragment does not reveal any structural change down to 4 K. The μSR measurements performed at 20 mK on a mosaic of single crystals with an external magnetic field applied along a three-fold axis are consistent with published a.c. magnetic-susceptibility measurements at 16 mK. While an inflection point could be present around an internal field intensity slightly above 0.3 T, the data barely support the presence of a magnetization plateau. The study of geometrically frustrated magnetic systems reveals a large variety of new magnetic phases. Among the frustrated materials, the rare-earth pyrochlore oxides which crystallize in a cubic structure (Fd3̄m space group) form a family for which different exotic ground states have been found [1]. Focusing on the insulators, we mention (i) the spin-ice ground state of Ho2Ti2O7 and Dy2Ti2O7 [2, 3], (ii) the spin-liquid ground state of Yb2Ti2O7 [4] characterized by a pronounced peak in the specific heat for powder samples [5, 6], although an exotic ordered magnetic state has been reported for a single crystal [7, 8], (iii) the unconventional dynamical ground state of Tb2Sn2O7 for which magnetic Bragg reflections are observed by neutron diffraction [9] while no spontaneous magnetic field is found by the zero-field positive muon spin relaxation (μSR) technique [10, 11], (iv) the persistent spin dynamics detected in the ordered states of Gd2Sn2O7, Gd2Ti2O7 and Er2Ti2O7 [12, 13, 14, 15, 16], and (v) the splayed ferromagnet Yb2Sn2O7, i.e. essentially a ferromagnetic compound [17, 18, 19], with an emergent gauge field [20]. The most mysterious compound of the rare-earth pyrochlore oxide family might be Tb2Ti2O7 for which no long-range magnetic order is detected down to 20 mK, far below the absolute value of its Curie-Weiss temperature ΘCW = −19 K [21, 22]. Two theoretical ground states have been suggested: a quantum spin ice ground state proposed in Ref. [23] and a Jahn-Teller like distorted ground state based on specific heat measurements [24]. In analogy with the spin-ice systems, a magnetization plateau is expected at low temperature for an external magnetic field Bext applied along a [111] crystal direction if the former ground state is reached [25]. While static magnetization measurements have not found any signature of the predicted plateau down to 43 mK [26, 27, 28], the presence of a weak magnetization plateau below about 0.05 K has been proposed from a.c. susceptibility experiments for 0.06 < Bext < 0.6 T [29]. Pointing out to the complexity of the physics involved, an anomaly in the static magnetic response has been observed around 0.15 K [30, 22, 26, 27] which strangely enough corresponds to a minimum rather than a maximum in the specific heat [22]. Pinch points in the neutron scattering intensity have been observed [31] with dispersive excitations emerging from them [32, 33], suggesting a strong magnetoelastic coupling in the Coulomb phase of Tb2Ti2O7. In addition, a neutron scattering intensity measured at the (