We report on the magnetic properties of monovacancy defects in neutron-irradiated graphite, probed by 13C 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 13C nuclear spin-lattice relaxation rate 1/T1 by about two orders of magnitude. With an applied magnetic field of 7.1 T, the temperature dependence of 1/T1 below about 10 K can well be described by a thermally activated form, 1/T1∝exp(−Δ/kBT), yielding a singular Zeeman energy of (0.41±0.01) meV, in excellent agreement with the sole presence of polarized, non-interacting defect moments.
@article{arxiv.1707.03748,
title = {Monovacancy paramagnetism in neutron-irradiated graphite probed by $^{13}$C NMR},
author = {Z. T. Zhang and C. Xu and D. Dmytriieva and S. Molatta and J. Wosnitza and Y. T. Wang and M. Helm and Shengqiang Zhou and H. Kühne},
journal= {arXiv preprint arXiv:1707.03748},
year = {2017}
}