Monitoring Electron Spin Fluctuations with Paramagnetic Relaxation Enhancement

TL;DR

This paper presents a method to map electron spin fluctuations using NMR relaxation times, enabling better understanding of magnetic interactions and improving dynamic nuclear polarization (DNP) techniques.

Contribution

It introduces a novel analysis based on T1 and T2 relaxation times to quantify electron spin fluctuations, applicable to complex paramagnetic systems.

Findings

Mapped electron spin fluctuations in specific materials.

Demonstrated temperature-dependent DNP enhancement.

Observed linear relation between DNP enhancement and electron relaxation time.

Abstract

The magnetic interactions between the spin of an unpaired electron and the surrounding nuclear spins can be exploited to gain structural information, to reduce nuclear relaxation times as well as to create nuclear hyperpolarization via dynamic nuclear polarization (DNP). A central aspect that determines how these interactions manifest from the point of view of NMR is the timescale of the fluctuations of the magnetic moment of the electron spins. These fluctuations, however, are elusive, particularly when electron relaxation times are short or interactions among electronic spins are strong. Here we map the fluctuations by analyzing the ratio between longitudinal and transverse nuclear relaxation times T1 and T2, a quantity which depends uniquely on the rate of the electron fluctuations and the Larmor frequency of the involved nuclei. This analysis enables rationalizing the evolution of NMR lineshapes, signal quenching as well as DNP enhancements as a function of the concentration of the paramagnetic species and the temperature, demonstrated here for LiMgMnPO4 and Fe(3+) doped Li4Ti5O12, respectively. For the latter, we observe a linear dependence of the DNP enhancement and the electron relaxation time within a temperature range between 100 and 300K.