NMR relaxation in spin ice due to diffusing emergent monopoles

TL;DR

This paper models how magnetic monopoles in spin ice influence nuclear spin relaxation, deriving formulas for relaxation times and explaining discrepancies with experimental data.

Contribution

It provides a theoretical framework for NMR relaxation caused by diffusing monopoles in spin ice, including explicit formulas for relaxation times.

Findings

Derived formulas for T1 and T2 relaxation times as functions of monopole density.

Predicted stretched-exponential relaxation behavior in spin ice.

Experimental data suggests magnetic impurities, not monopoles, dominate relaxation.

Abstract

At low temperatures, spin dynamics in ideal spin ice is due mainly to dilute, thermally excited magnetic monopole excitations. I consider how these will affect the dynamics of a nuclear spin (the same theory applies to muon spin resonance if implanted muons do not diffuse). Up to the time scale for nearby monopoles to be rearranged, a stretched-exponential form of the relaxation functions is expected. I work out the expected exponent in that exponential and the formulas for the $T_1$ (longitudinal) and $T_2$ (dephasing) relaxations, as a function of the monopole density. Experimental NMR is incompatible with the predictions and I suggest is due to magnetic impurities.