Magnetic structure in the spin liquid Tb₂Ti₂O ₇ induced by a [111] magnetic field: Search for a magnetization plateau

We have studied the field-induced magnetic structures of Tb ₂Ti₂O₇ pyrochlore by single-crystal neutron diffraction under a field applied along the [111] axis, up to H=12 T and down to T=40 mK. We refined the magnetic structures with k=0 propagation vector by performing a symmetry analysis in the space group R3̄m, reducing the number of free parameters to three only. The Tb moments gradually reorient towards the field direction, keeping close to a "3-in, 1-out / 1-in, 3-out" spin structure (magnetic space group R3̄m') in the whole measured field range 0.05-12 T. Our results rule out the "all-in/all-out" structure previously proposed and do not support the existence of a magnetization plateau. We perform a quantitative comparison with mean-field calculations and we propose the presence of a low-temperature dynamic symmetry breaking of the local trigonal symmetry, akin to a dynamic Jahn-Teller effect, i.e., preserving the overall cubic symmetry. We discuss the possible origin of this off-diagonal mixing term in the crystal field Hamiltonian in terms of quadrupole-quadrupole interaction or magnetoelastic effects.