Spin dynamics and coherent tunnelling in the molecular magnetic rings Fe6 and Fe8

We present detailed calculations of low-energy spin dynamics in the "ferric wheel" systems Na:Fe-6 and Cs:Fe-8 in a magnetic field. We compute by exact diagonalisation the low-energy spectra and matrix elements for total-spin and Neel-vector components, and thus the time-dependent correlation functions of these operators. Comparison of our results with the semiclassical theory of coherent quantum tunnelling of the Neel vector demonstrates the validity of a two-state description for the low-energy dynamics of ferric wheels. We discuss the implications of our results for mesoscopic quantum coherent phenomena, and for the experimental techniques to observe them, in molecular magnetic rings.