arXiv:1110.0877v2 [cond-mat.str-el] 21 Nov 2011 Spin Ice: Magnetic Excitations without Monopole Signatures using Muon Spin Rotation S. R. Dunsiger, 1, * A. A. Aczel, 2 C. Arguello, 3 H. Dabkowska, 4 A. Dabkowski, 4 M.-H. Du, 5 T. Goko, 3, 6 B. Javanparast, 7 T. Lin, 7 F. L. Ning, 3, 8 H. M. L. Noad, 2 D. J. Singh, 5 T. J. Williams, 2 Y. J. Uemura, 3, † M. J. P. Gingras, 7,9, ‡ and G. M. Luke 2, 9, § 1 Physik Department, Technische Universit¨ at M¨ unchen, D-85748 Garching, Germany 2 Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada 3 Department of Physics, Columbia University, New York, New York 10027, USA 4 Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario L8S 4M1, Canada 5 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6114, USA 6 TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3, Canada 7 Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada 8 Department of Physics, Zhejiang University, Hangzhou 310027, China 9 Canadian Institute for Advanced Research, Toronto, Ontario M5G 1Z8, Canada (Dated: November 22, 2011) Theory predicts the low temperature magnetic excitations in spin ices consist of deconfined mag- netic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (μSR) experiment [S T Bramwell et al., Nature 461, 956 (2009)] reports results claiming to be consistent with the tem- perature and magnetic field dependence anticipated for monopole nucleation - the so-called second Wien effect. We demonstrate via a new series of μSR experiments in Dy2Ti2 O7 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. PACS numbers: 75.50.Dd, 75.40.Cx, 75.40.Gb, 76.75.+i Spin ices, such as Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 , are top- ical highly frustrated magnetic systems which exhibit a gamut of very interesting phenomena [1, 2]. In (Ho/Dy) 2 Ti 2 O 7 , the Ho 3+ and Dy 3+ magnetic ions re- side on the vertices of a pyrochlore lattice of corner- sharing tetrahedra. A large single ion anisotropy forces the moment to point strictly along local 〈111〉 crystalline axes, along the line which connects the centers of the two adjoining tetrahedra and their common vertex, making the moments classical “local” Ising spins. Since Ho 3+ and Dy 3+ carry a large magnetic moment of ∼ 10 μ B , the dipolar interaction in these systems is ∼ 1 K at nearest neighbor distance and of similar magnitude as the Curie-Weiss temperature θ CW [2]. The frustration in spin ices stems from the 1/r 3 long-range nature of the magnetic dipolar interaction and its consequential “self-screening” (r is the distance between ions) [3–5]. As a result, spin ices are frustrated ferromagnets with low-energy states characterized by two spins “pointing in” and two spins “pointing out” on each tetrahedron – the 2-in/2-out rule which defines minimum energy spin configurations. These map onto the allowed proton con- figurations in water ice which obey the Bernal-Fowler ice-rules [2], hence the name spin ice. The dipolar spin ice model [6] and its refinement [7] yield an accurate microscopic quantitative description of the equilibrium thermodynamic properties of spin ices, both in zero and nonzero magnetic field. In contrast, the problem of the dynamical response of the moments in spin ices remains much less studied and understood. An exciting recent development in that direction is the real- ization that the “2-in/2-out” spin configurations may be described via a divergence-free coarse-grained magneti- zation density field [5, 8]. A thermal fluctuation causing the flip of an Ising spin from an “in” to an “out” direc- tion amounts to the creation of a nearest-neighbor pair of magnetization source and sink on the two adjoining tetrahedra or, in other words, to the nucleation of “mag- netic monopoles” out of the spin-ice-rule obeying vac- uum [5]. Particularly interesting is the observation that monopoles in dipolar spin ice interact via an emerging Coulomb potential which decays inversely proportional to the distance which separates them and are therefore deconfined [5]. A recent numerical study [9] provides evi- dence that the temperature dependence of the relaxation time determined in ac magnetic susceptibility measure- ments [10] can be rationalized in terms of thermally ac- tivated monopoles, at least above 1 K. The wave vector dependence of the neutron scattering intensity suggests power law spin correlations, which are a prerequisite for monopoles with effective Coulomb interactions [11]. Yet, perhaps the reported direct evidence for the presence of monopoles in spin ice and a determination of their effec- tive charge is the most intriguing recent result [12]. In weak electrolytes, including water ice, characterized by a small dissociation rate constant K, the so-called second Wien effect describes the nonlinear increase of K under an applied electric field. In a recent paper [12], Bramwell and co-workers have drawn further on the anal- ogy between magnetic moments in spin ice and protons