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

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 $1\mathrm{/}{T}_1$ by about two orders of magnitude. With an applied magnetic field of 7.1 T, the temperature dependence of $1\mathrm{/}{T}_1$ below about 10 K can well be described by a thermally activated form, $1\mathrm{/}{T}_1\propto \exp (-\mathrm{\Delta }\mathrm{/}{k}_{B}T)$, yielding a singular Zeeman energy of ($0.41\pm 0.01$) meV, in excellent agreement with the sole presence of polarized, non-interacting defect moments.