Angle-resolved NMR: quantitative theory of 75As T1 relaxation rate in BaFe2As2

Abstract: While NMR measurements of nuclear energy spectra are routinely used to characterize the static properties of quantum magnets, the dynamical information locked in NMR 1/T1 relaxation rates remains notoriously difficult to interpret. The difficulty arises from the fact that information about all possible low-energy spin excitations of the electrons, and their coupling to the nuclear moments, is folded into a single number, 1/T1. Here we develop a quantitative theory of the NMR 1/T1 relaxation rate in a collinear antiferromagnet, focusing on the specific example of BaFe2As2. One of the most striking features of magnetism in BaFe2As2 is a strong dependence of 1/T1 on the orientation of the applied magnetic field. By careful analysis of the coupling between the nuclear and electronic moments, we show how this anisotropy arises from the "filtering" of spin fluctuations by the form-factor for transferred hyperfine interactions. This allows us to make convincing, quantitative, fits to experimental 1/T1 data for BaFe2As2, for different field orientations. We go on to show how a quantitative, angle-dependent theory for the relaxation rate leads to new ways of measuring the dynamical parameters of magnetic systems, in particular the spin wave velocities.