Nuclear magnetic relaxation due to critical effects:35Cl relaxation in NH4Cl and ND4Cl

The dynamics of critical phenomena in the molecular field approximation are discussed and a calculation of the nuclear spin-lattice relaxation time (T 1 ) for 35 Cl nuclei in NH 4 Cl at temperatures close to the lambda transition point is described. The calculation is based on a linearized Ising model treatment and the approach should be fairly widely applicable. The T 1 expression may be expected to fail very close to the critical point. Measurements have been made of the 35 ClT 1 in NH 4 Cl and ND 4 Cl at temperatures both below and above the lambda point. The results for NH 4 Cl are compared with those obtained by previous workers. The data are analysed using the theoretical expression mentioned above and it is shown that the temperature variation of the order parameter extracted from the results leads to an anomalously low exponent value beta =0.15.