Direct-Process NMR Relaxation by Spin Waves in a One-Dimensional Antiferromagnet

Measurements of the proton relaxation time T₁ in (CH₃)₄NMnCl₃ are reported as a function of temperature between 4.0 and 1.2 K. Results are interpreted in terms of a direct relaxation process with spin waves in a one-dimensional antiferromagnet. They represent the first observation of direct-process NMR relaxation by magnons in any system and show that any gap in the spectrum at q=πa has to be less than 0.07 meV.