75As NMR spin-lattice relaxation (1/T1) and spin-echo decay (1/T2) rate
measurements were performed in a single crystal of Ba(Fe0.93Rh0.07)2As2
superconductor. Below the superconducting transition temperature Tc, when the
magnetic field H is applied along the c axes, a peak in both relaxation rates
is observed. Remarkably that peak is suppressed for H || ab. Those maxima in
1/T1 and 1/T2 have been ascribed to the flux lines lattice motions and the
corresponding correlation times and pinning energy barriers have been derived
on the basis of an heuristic model. Further information on the flux lines
motion was derived from the narrowing of 75As NMR linewidth below Tc and found
to be consistent with that obtained from 1/T2 measurements. All the
experimental results are described in the framework of thermally activated
vortices motions.Comment: 8 pages, 11 figure