Monovacancy paramagnetism in neutron-irradiated graphite probed by $^{13}$C NMR

TL;DR

This study uses $^{13}$C NMR spectroscopy to investigate monovacancy defects in neutron-irradiated graphite, revealing bulk paramagnetism, hyperfine field effects, and thermally activated relaxation behavior consistent with non-interacting defect moments.

Contribution

It provides the first detailed NMR characterization of monovacancy-induced paramagnetism in neutron-irradiated graphite, linking microscopic defect behavior to macroscopic magnetic properties.

Findings

Paramagnetism follows Curie behavior in NMR measurements.

Hyperfine fields enhance nuclear spin-lattice relaxation rate by two orders of magnitude.

Relaxation rate below 10 K fits a thermally activated model with a Zeeman energy of 0.41 meV.

Abstract

We report on the magnetic properties of monovacancy defects in neutron-irradiated graphite, probed by $^{13}$C nuclear magnetic resonance spectroscopy. The bulk paramagnetism of the defect moments is revealed by the temperature dependence of the NMR frequency shift and spectral linewidth, both of which follow a Curie behavior, in agreement with measurements of the macroscopic magnetization. Compared to pristine graphite, the fluctuating hyperfine fields generated by the defect moments lead to an enhancement of the $^{13}$C nuclear spin-lattice relaxation rate $1/T_{1}$ by about two orders of magnitude. With an applied magnetic field of 7.1 T, the temperature dependence of $1/T_{1}$ below about 10 K can well be described by a thermally activated form, $1/T_{1}\propto\exp(-\Delta/k_{B}T)$, yielding a singular Zeeman energy of ($0.41\pm0.01$) meV, in excellent agreement with the sole presence of polarized, non-interacting defect moments.