Spin dynamics in highly frustrated pyrochlore magnets

This paper aims at showing the complementarity between time-of-flight and triple-axis neutron scattering experiments, on the basis of two topical examples in the field of geometrical magnetic frustration. Rare earth pyrochlore magnets R₂Ti₂O₇ (R is a rare earth) play a prominent role in this field, as they form model systems showing a rich variety of ground states, depending on the balance between dipolar, exchange interactions and crystal field. We first review the case of the XY antiferromagnet Er₂Ti₂O₇. Here a transition towards a Neel state is observed, possibly induced by an order-by-disorder mechanism. Effective exchange parameters can be extracted from S(Q, ω). We then examine the case of the spin liquid Tb₂Ti₂O₇. Recent experiments reveal a complex ground state characterized by 'pinch points' and supporting a low energy excitation. These studies demonstrate the existence of a coupling between crystal field transitions and a transverse acoustic phonon mode. (authors)