NMR as a probe of the relaxation of the magnetization in magnetic molecules

TL;DR

This paper demonstrates that NMR effectively probes the relaxation dynamics of magnetization in various magnetic molecules, revealing a universal exponential decay characterized by a single relaxation time, consistent with recent experimental observations.

Contribution

It provides a theoretical explanation for the single-lorentzian spectral density and relaxation time behavior observed in magnetic molecules using NMR.

Findings

Exponential decay of magnetization fluctuations characterized by a single relaxation time.

Spectral density of fluctuations is nearly single-lorentzian.

Theory explains recent experimental NMR results on magnetic molecules.

Abstract

We investigate the time autocorrelation of the molecular magnetization $M(t)$ for three classes of magnetic molecules (antiferromagnetic rings, grids and nanomagnets), in contact with the phonon heat bath. For all three classes, we find that the exponential decay of the fluctuations of $M(t)$, associated with the irreversible exchange of energy with the heat bath, is characterized by a single characteristic time $\tau (T,B)$ for not too high temperature $T$ and field $B$. This is reflected in a nearly single-lorentzian shape of the spectral density of the fluctuations. We show that such fluctuations are effectively probed by NMR, and that our theory explains the recent phenomenological observation by Baek et al. (PRB70, 134434) that the Larmor-frequency dependence of $1/T_1$ data in a large number of AFM rings fits to a single-lorentzian form.