19F NMR Spin Echo in Antiferromagnetic MnF2

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Abstract
Spin echoes of 19F NMR have been observed, in zero external field, in various samples of antiferromagnetic MnF2. The decay of the echo envelope was always found to be exponential. This corresponds to a Lorentzian line profile, indicating that the Gaussian line shape previously observed with a cw spectrometer and found to be an order of magnitude broader, arises from inhomogeneous broadening. A previous calculation of the indirect nuclear spin‐spin interaction via virtual magnon excitations predicts a 3‐μsec T2 with a Gaussian envelope. However, (1) destructive interference between the 19F dipolar and indirect coupling mechanisms and (2) the appreciable contribution via the 19F−55Mn dipolar interaction of the much larger indirect coupling between 55Mn nuclei to the fourth moment of the line will reduce the 19F relaxation rate, the first, by only 20% but the second by approximately a factor of eight, and will change the line shape from Gaussian to Lorentzian. The derived value T2≅30 μsec is in agreement with the relaxation time T2=29±1 measured for a high‐purity MnF2 sample. T2 is observed to monotonically decrease to a value of 22 μsec at 20.2°K. The temperature dependence of the nuclear spin lattice relaxation T1 in the same region is more dramatic, varying from 130 sec at 4.2°K to 3 msec at 20.2°K.