Determination of the mosaic angle distribution of Grafoil platelets using continuous-wave NMR spectra

TL;DR

This paper presents a method using continuous-wave NMR spectra of helium monolayers to accurately determine the mosaic angle distribution of Grafoil platelets, revealing a Gaussian distribution with a 27.5-degree spread.

Contribution

The paper introduces a novel NMR-based technique to estimate the mosaic angle distribution of Grafoil platelets, improving accuracy over previous neutron scattering methods.

Findings

Mosaic angle distribution fits a Gaussian with 27.5-degree spread.

The NMR method provides more accurate estimates than neutron scattering.

The distribution differs significantly from previous estimates.

Abstract

We described details of a method to estimate with good accuracy the mosaic angle distributions of microcrystallites (platelets) in exfoliated graphite like Grafoil which is commonly used as an adsorption substrate for helium thin films. The method is based on analysis of resonance field shifts in continuous-wave (CW) NMR spectra of $^{3}$He ferromagnetic monolayers making use of the large nuclear polarization of the adsorbate itself. The mosaic angle distribution of a Grafoil substrate analyzed in this way can be well fitted to a gaussian form with a $27.5\pm2.5$ deg spread. This distribution is quite different from the previous estimation based on neutron scattering data which showed an unrealistically large isotropic powder-like component.