Anomalous magnetic noise in an imperfectly flat landscape in the topological magnet Dy ₂ Ti ₂ O ₇

Abstract

Noise generated by motion of charge and spin provides a unique window into materials at the atomic scale. From temperature of resistors to electrons breaking into fractional quasiparticles, “listening” to the noise spectrum is a powerful way to decode underlying dynamics. Here, we use ultrasensitive superconducting quantum interference device (SQUIDs) to probe the puzzling noise in a frustrated magnet, the spin-ice compound Dy2Ti2O7 (DTO), revealing cooperative and memory effects. DTO is a topological magnet in three dimensions—characterized by emergent magnetostatics and telltale fractionalized magnetic monopole quasiparticles—whose real-time dynamical properties have been an enigma from the very beginning. We show that DTO exhibits highly anomalous noise spectra, differing significantly from the expected Brownian noise of monopole random walks, in three qualitatively different regimes: equilibrium spin ice, a “frozen” regime extending to ultralow temperatures, and a high-temperature “anomalous” paramagnet. We present several distinct mechanisms that give rise to varied colored noise spectra. In addition, we identify the structure of the local spin-flip dynamics as a crucial ingredient for any modeling. Thus, the dynamics of spin ice reflects the interplay of local dynamics with emergent topological degrees of freedom and a frustration-generated imperfectly flat energy landscape, and as such, it points to intriguing cooperative and memory effects for a broad class of magnetic materials.

Machine Learning Methods for Multiscale Physics and Urban Engineering Problems

“…A recent study of the magnetic noise using superconducting interference devices (SQUIDs) [ 27 ], a simpler but similar setup to the cylinder geometry experiments discussed above, sought to compare the magnetic noise with theoretical expectations to determine if the noise is produced intrinsically by the ice rules or whether other unrelated effects are responsible. In essence, this study pursued the general question: can we have a glass purely from the ice rules, or is Dy Ti O another glass-like system where random disorder plays a role?…”

“…This comparison excluded many but not all mechanisms. The researchers concluded, “it is tempting to speculate that here we have, on the one hand, the relatively fast motion of monopoles trapped in their individual surroundings where, however, the confined paths cannot lead to a large change of the magnetization and on the other hand, rare longer-distance excursions of monopoles toward, for example, another local trap” [ 27 ]. This raises the question of whether the rare, longer-distance excursions of monopoles are responsible for glass-like behavior.…”

Dynamical fractal and anomalous noise in a clean magnetic crystal

“…Indeed, the dynamical behaviour of the latter has been an enigma since its discovery [5][6][7]. Most recently, as well as strikingly, ultrasensitive lowtemperature SQUID experiments have thrown up a new puzzle: the magnetic noise spectral density exhibits an anomalous power law as a function of frequency, with the low temperature exponent α ≈ 1.5 deviating strongly from the well-known α = 2 of a paramagnet [8,9] (see also Ref. [10][11][12]).…”

“…[10][11][12]). Within the -generally successfulframework of what we call the 'standard model' of spin ice dynamics [13,14], this behaviour cannot be accounted for using broadly accepted model Hamiltonian parameters [9].…”

Spin ice in a general applied magnetic field: Kasteleyn transition, magnetic torque and rotational magnetocaloric effect

“…Spin ice exemplifies much of what is interesting about frustrated systems [1][2][3][4] . The co-existence of a large quasi-degeneracy of ground states [5][6][7][8] and strong correlations [9][10][11][12][13] sets the stage for the emergence of exotic excitations: namely, magnetic monopoles [14][15][16][17][18][19][20][21] . The discovery of spin ice also served as an early example of a theme which has grown in importance in the years since: the interplay of frustration and anisotropy 22 .…”