High Field NMR Studies of Static Ordering and Spin-Energy Coupling in Manganese-Fluoride

The antiferromagnetic to paramagnetic transition in MnF(,2) in a 75 kG field applied along {001} has been studied using MNR. The time-average staggered and uniform magnetizations N and M were obtained respectively from the difference and sum of the NMR frequencies of the two ('19)F sites (alpha) and (beta), over the range .12(DEGREES)K < T(,c) - T < 6(DEGREES)K. The critical behavior of the order parameter was found to be. N(T) = A (T(,c)(H) - T)('(beta)eff). The usual three parameter least-squares fit gave an effective critical exponent. (beta)(,eff) = 0.333 (+OR-) 0.003. This is very similar to the corresponding low field result, as expected. Of particular interest were the NMR line-width and shape behaviors in the ordered phase, as these bear on the coupling of N and M to the spin system thermal diffusion mode there. The data analysis employed least squares shape fitting. Excellent Lorentizian fits were obtained where expected. The fitted values of (GAMMA) = 1/T(,2) were combined to yield the site average (GAMMA)(' )=(, )((GAMMA)(,(alpha)) + (GAMMA)(,(beta)))/(,2) and site difference (DELTA)(GAMMA) = (GAMMA)(,(beta)) - (GAMMA)(,(alpha)). Here (GAMMA) was found to diverge critically, although not with a strict power-law behavior. In fact, the exponent corresponding to the log log plot slope changes from -0.67 to -0.52 in going from 6(DEGREES)K to 0.14(DEGREES)K below T(,c). The site-difference width (DELTA)(GAMMA) was found to be slowly varying and (TURN).25 x 10('6) sec('-1). These results are interpreted using the Kubo-Tomita-Moriya formalism, together with a rudimentary theory incorporating a wave -vector dependent extension of the Landau-Placzek ratio.