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

TL;DR

This paper develops a quantitative theory for the angle-dependent NMR 1/T1 relaxation rate in BaFe2As2, revealing how spin fluctuation filtering explains anisotropy and enabling measurement of magnetic dynamical parameters.

Contribution

It introduces a detailed theoretical framework linking NMR relaxation anisotropy to spin fluctuations, providing a method to extract magnetic parameters from experimental data.

Findings

Quantitative fits to experimental 1/T1 data for BaFe2As2.

Identification of hyperfine form-factor filtering as the origin of anisotropy.

Proposal of angle-dependent relaxation measurements to determine spin wave velocities.

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.