Low-field spin dynamics of Cr7Ni and Cr7Ni-Cu-Cr7Ni molecular rings as detected by mu SR

Muon spin rotation measurements were used to investigate the spin dynamics of heterometallic C7Ni and Cr7Ni-Cu-Cr7Ni molecular clusters. In Cr7Ni the magnetic ions are arranged in a quasiplanar ring and interact via an antiferromagnetic exchange coupling constant/, while Cr7Ni-Cu-Cr7Ni is composed of two Cr7Ni linked by a bridging moiety containing one Cu ion, that induces an inter-ring ferromagnetic interaction J' DLANGBRAC; J. The longitudinal muon relaxation rate lambda collected at low magnetic fields mu H-0< 0.15 Tesla, shows that the two systems present differences in spin dynamics vs temperature. While both samples exhibit a main peak in the muon relaxation rate vs temperature, at T similar to 10 K for Cr7Ni and T similar to 8 K for Cr7Ni-Cu-Cr7Ni, the two compounds have distinct additional features: Cr7Ni shows a shoulder in lambda(T) for T< 8 K, while Cr7Ni-Cu-Cr7Ni shows a flattening of lambda(T) for T< 2 K down to temperatures as low as T = 20 mK. The main peak of both systems is explained by a Bloembergen-Purcell-Pound (BPP)-like heuristic fitting model that takes into account of a distribution of electronic spin characteristic times for T >5 K, while the shoulder presented by Cr7Ni can be reproduced by a BPP function that incorporates a single electronic characteristic time theoretically predicted to dominate for T< 5 K. The flattening of lambda(T) in Cr7Ni-Cu-Cr7Ni occurring at very low temperature can be tentatively attributed to field-dependent quantum effects and/or to an inelastic term in the spectral density of the electronic spin fluctuations.