Relaxation dynamics in the frustrated Cr-9 antiferromagnetic ring probed by NMR

We investigate the magnetic properties and the phonon-induced relaxation dynamics of the first regular Cr-9 antiferromagnetic (AF) ring, which represents a prototype frustrated AF ring. Geometrical frustration in Cr-9 yields an energy spectrum with twofold degenerate low-lying levels and a low-spin ground state. The electronic relaxation dynamics is probed by H-1-NMR through the temperature dependence of the spin-lattice relaxation rate 1/T-1. We develop a microscopic model that reproduces 1/T-1(T) curves, taking also into account the wipeout effect. By interpreting these measurements we determine the spin-phonon coupling strength and we investigate the decay of the cluster magnetization due to the spin-phonon interaction. We find that at very low temperatures, the relaxation is characterized by a single dominating Arrhenius-type relaxation process, whereas several relevant processes emerge at higher temperatures. In addition, we calculate the temperature andmagnetic field dependence of level lifetimes.