Angle-Resolved NMR

The simple fact that nuclear and electronic spins interact makes NMR one of the most powerful probes of solid state magnetism. In particular, changes in NMR spectra provide vital information about magnetic order in cases where small sample size or extreme conditions render neutron scattering impossible. However as a probe of magnetic excitations, NMR is famously difficult to interpret, since excitations with many different momenta are mapped onto a single nuclear spin relaxation time.

Here we revisit the existing theory of the NMR T1 relaxation rate in magnetic insulators, and show how this can be extended to take account of the tensor structure of dipolar and transferred hyperfine interactions with nuclear spins. This tensor interaction makes relaxation rates sensitive to the initial orientation of nuclear spins, and as a consequence, both the magnitude and the temperature dependence of the T1 depend on the orientation of the magnetic field used to carry out the experiment.

We demonstrate that this theory is in quantitative agreement with existing data for the collinear antiferromagnets BaFe2As [1] and Li2VOSiO4, and make explicit predictions for the angle-dependence of T1 in the square-lattice antiferromagnets La2CuO4 and YBa2Cu3O6, and the triangular lattice antiferromagnet VCl2. We also explore how these ideas might be used to distinguish uncoventional forms of magnetic order, including spin nematic states which cannot be resolved by their static properties alone.

[1] A. Smerald and Nic Shannon, Europhys Lett 92, 47005 (2010), [2] A. Smerald and Nic Shannon, arXiv:1109.0384 [accepted for publication in Phys. Rev. B]