Spin-wave contributions to nuclear magnetic relaxation in magnetic metals

TL;DR

This paper calculates nuclear magnetic relaxation rates in 2D and 3D magnetic metals, revealing significant spin-wave contributions that surpass standard models, with divergences and large effects in certain cases.

Contribution

It provides a detailed analysis of spin-wave contributions to nuclear relaxation, including divergences and the importance of two-magnon processes in low-dimensional systems.

Findings

One-magnon decay processes dominate over Korringa relaxation in certain regimes.

Divergences in relaxation rates are cut by magnetic anisotropy or temperature effects.

Two-magnon contributions are significant, especially in 2D ferromagnets.

Abstract

The longitudinal and transverse nuclear magnetic relaxation rates $1/T_1(T)$ and $1/T_2(T)$ are calculated for three- and two-dimensional (3D and 2D) metallic ferro- and antiferromagnets (FM and AFM) with localized magnetic moments in the spin-wave temperature region. The contribution of the one-magnon decay processes is strongly enhanced in comparison with the standard $T$-linear Korringa term, especially for the FM case. For the 3D AFM case this contribution diverges logarithmically, the divergence being cut at the magnon gap $\omega_0$ due to magnetic anisotropy, and for the 2D AFM case as $\omega_0^{-1}$. The electron-magnon scattering processes yield $T^2\ln (T/\omega_0)$ and $T^2/\omega_0^{1/2}$-terms in $1/T_1$ for the 3D AFM and 2D FM cases, respectively. The two-magnon (``Raman'') contributions are investigated and demonstrated to be large in the 2D FM case. Peculiarities of the isotropic 2D limit (where the correlation length is very large) are analyzed.