The color of magnetic monopole noise

Abstract

We relate the anomaly in the noise color of spin ice to the emergent nature of its magnetic monopoles and their non-trivial random walk. Monopoles are quasi-particles, and the spin vacuum in which they wander is not structureless. Rather, the underlying spin ensemble filters the thermal white noise, leading to non-trivial coevolution. Thus, monopoles can be considered as “dressed” random walkers, activated by a non-trivial stochastic force that subsumes mutual interactions and the coevolution of their spin vacuum. From this, we suggest that recent experimental results are interpretable in terms of monopole subdiffusion. We then conjecture relations between the color of the noise and other observables, such as relaxation time, monopole density, the dynamic exponent, and the order of the annihilation reaction, which suggests to us the introduction of spin-ice–specific critical exponents in a neighborhood of the ice manifold criticality.

Dynamical fractal and anomalous noise in a clean magnetic crystal

“…The dynamical behavior of the latter has been an enigma since its discovery (5)(6)(7). Most recently, ultrasensitive, low-temperature superconducting quantum interference device (SQUID) experiments have identified another puzzle: The magnetic noise spectral density exhibits an anomalous power law as a function of frequency, with the low-temperature exponent a ≈ 1.5 deviating strongly from the well-known a = 2 of a paramagnet (8,9) [see also (10)(11)(12)]. Within the generally successful framework of what we refer to as the standard model (SM) of spin ice dynamics (13,14), this behavior cannot be accounted for using broadly accepted model Hamiltonian parameters (9).…”

Deconstructing magnetization noise: Degeneracies, phases, and mobile fractionalized excitations in tetris artificial spin ice

“…The fact that these phenomena are field-tunable may open the door to future functionalities in sensing or beyond-Turing computation ( 37 – 40 ). Future studies will concentrate on the detailed nature of the noise spectra and establish relations between the nontriviality of the disorder of certain phases, tied to their collective behavior, and the “color” of the noise spectrum ( 23 , 24 , 41 , 42 ) at larger frequencies.…”

Complex field reversal dynamics in nanomagnetic systems

“…Holistically, the power spectrum is enveloped by 1/(ω4+ω2), quickly approaching 1/ω4, which is unitless because the natural relaxation frequency of a single nanomagnet is set to 1 in the governing differential equations. A signal like this would be hidden in magnetization noise due to the 1/ω4 envelope, masked by the stronger 1/ω2 signals from Brownian motion at higher temperatures or stronger still subdiffusive motion of monopoles at lower temperatures [30]. The driven motion of the magnetization motion has the same 1/ω4 signature as superdiffusivity [30].…”

“…A signal like this would be hidden in magnetization noise due to the 1/ω4 envelope, masked by the stronger 1/ω2 signals from Brownian motion at higher temperatures or stronger still subdiffusive motion of monopoles at lower temperatures [30]. The driven motion of the magnetization motion has the same 1/ω4 signature as superdiffusivity [30]. The rest of the function is modulated by time constants τ and Rτ, giving it a harmonic structure that is audible when converted to the audio output (see electronic supplementary material) and visible when the spectra are plotted (figure 2).…”