High Field NMR Studies of Static Ordering and SpinEnergy Coupling in ManganeseFluoride
Abstract
The antiferromagnetic to paramagnetic transition in MnF(,2) in a 75 kG field applied along {001} has been studied using MNR. The timeaverage 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 leastsquares 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 linewidth 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 powerlaw 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 sitedifference width (DELTA)(GAMMA) was found to be slowly varying and (TURN).25 x 10('6) sec('1). These results are interpreted using the KuboTomitaMoriya formalism, together with a rudimentary theory incorporating a wave vector dependent extension of the LandauPlaczek ratio.
 Publication:

Ph.D. Thesis
 Pub Date:
 1981
 Bibcode:
 1981PhDT........79D
 Keywords:

 Physics: Condensed Matter