Orientation dependence of the proton relaxation at Zeeman-tunnel level crossings in (NH4)(2)S2O8

摘要

Proton spin-lattice relaxation time T-1 was studied in a single crystal of (NH4)(2)S2O8 as a function of the orientation of the external magnetic field B-0 in the crystalline a * b plane. Angular dependence was observed at 100 K at the proton frequency f(0) = 8.7 MHz and at 10 K at the Zeeman-tunnel level-crossing frequencies 4.3 and 8.6 MHz. At 8.7 and 4.3 MHz the theoretically calculated angular dependence agrees satisfactorily with the experimental data but at 8.6 MHz (10 K) the theory fails. Various sources of the failure are discussed, the most likely one being the anisotropy of the ammonium motion. (C) 2001 Published by Elsevier Science B.V. [References: 13]