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

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 (μSR) experiment [S. T. Bramwell et al., Nature (London) 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 μSR experiments in Dy(2)Ti(2)O(7) that such an effect is not observable in a TF μSR 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.

New physics in frustrated magnets: Spin ices, monopoles, etc. (Review Article)

“…Non-Ohmic conductivity, the Wien effect, [199] for a weak "monopole electrolyte" has been studied theoretically and experimentally by the transverse field low-temperature µSR [200,201] (notice, though [202]). Other recent theoretical studies of the dynamical characteristics of monopoles include [203,204,205,206,207,208,209].…”

Evidence of impurity and boundary effects on magnetic monopole dynamics in spin ice

“…Recently, it was realized that the fundamental excitations are magnetic charges, commonly referred to as monopoles 5,6 that are created by overturning a spin in the highly degenerate spin-ice ground state, where two spins point in and two point out of each tetrahedron. The motion of magnetic monopoles has been observed experimentally through the generation of monopole currents by the application of a magnetic field 7 , and muon spin rotation 8 , which is a subject of recent controversy 9 . Monte Carlo simulations of a Coulomb gas of monopoles 10 and the dipolar spin-ice model 11 , equation (1), agree well with experimental results down to 1 K, below which the observed dynamics become much slower in the experiments than in the simulations [11][12][13][14][15][16][17] .…”

Irreversible magnetic-field dependence of low-temperature heat transport of spin-ice compound Dy2Ti2O7in a

“…In this regard, there has been no obvious evidence that DTO could have other magnetic excitations than the magnetic monopoles, like the spin fluctuations caused by non-Ising terms in the Hamiltonian. 21,43 Nevertheless, some drastic fieldinduced changes related to crystal lattice and phonons have been indeed found. In a recent work, the ultrasound measurements also indicated sharp anomalies of the sound velocity and sound attenuation at H c2 , which may share a common origin with the κ(H) transitions.…”

“…However, it should be noted that the experimental investigations on probing the monopole excitations have not yet arrived perfect consistency. 20,21 Studying spin-ice materials by using more different tech-niques is probably an effective way. Low-temperature heat transport is a powerful tool to probe the properties of elementary excitations [22][23][24][25][26][27][28][29] and the magnetic-field-induced magnetic transitions [30][31][32][33][34][35][36][37][38] .…”