Magnetic ordering and dynamics in the XY pyrochlore antiferromagnet: a muon-spin relaxation study of Er₂Ti₂O₇and Er₂Sn₂O₇

Abstract

Spin dynamics in Er₂Ti₂O₇ and Er₂Sn₂O₇ have been probed by means of muon-spin relaxation (<em>μ</em>⁺SR) in the temperature range 0.02 ≤ <em>T</em> ≤ 300 K. Both compounds are thought to constitute experimental realizations of the highly frustrated XY antiferromagnet on the pyrochlore lattice, for which theory predicts fluctuation-induced magnetic order. Our results for Er₂Ti₂O₇ are consistent with a transition into an ordered state at <em>T</em>_N ~ 1.1 K, in agreement with previous neutron measurements. Below this temperature, the muon relaxation rate λ(<em>T</em>) remains large (~2 MHz) and temperature independent, in contrast to the behaviour in conventional magnets. The thermal evolution of λ(<em>T</em>) for Er₂Sn₂O₇ is somewhat similar to that of the Ti material. However, the depolarization curves remain exponential over the entire temperature range, suggesting a dense distribution of rapidly fluctuating magnetic moments, and, thus, are compatible with the absence of long-range order at least for <em>T</em> &gt; 0.02 K.