Spin Ice: Magnetic Excitations without Monopole Signatures using Muon Spin Rotation

TL;DR

This study shows that muon spin rotation experiments in spin ice do not detect magnetic monopole signatures, instead revealing temperature-independent spin fluctuations at low temperatures, challenging previous claims of monopole observation.

Contribution

The paper provides new muSR experimental evidence that contradicts earlier claims of monopole detection in spin ice, highlighting the dominance of spin fluctuations.

Findings

No observable second Wien effect in muSR experiments

Spin fluctuations become temperature independent at low temperatures

Contradicts previous monopole nucleation signatures

Abstract

Theory predicts the low-temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (muSR) experiment [S T Bramwell et al, Nature 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation - the so-called second Wien effect. We demonstrate via a new series of muSR experiments in Dy_2Ti_2O_7 that such an effect is not observable in a TF muSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.