Low-field spin dynamics of Cr 7 Ni and Cr 7 Ni-Cu-Cr 7 Ni molecular rings as detected by μsR

Muon spin rotation measurements were used to investigate the spin dynamics of heterometallic Cr 7 Ni and Cr 7 Ni-Cu-Cr 7 Ni molecular clusters. In Cr 7 Ni the magnetic ions are arranged in a quasiplanar ring and interact via an antiferromagnetic exchange coupling constant J, while Cr 7 Ni-Cu-Cr7Ni is composed of two Cr7Ni linked by a bridging moiety containing one Cu ion, that induces an inter-ring ferromagnetic interaction J ≪ J. The longitudinal muon relaxation rate λ collected at low magnetic fields μ0H <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 ∼ 10Kfor Cr 7 Ni and T ∼ 8KforCr 7 Ni-Cu-Cr 7 Ni, the two compounds have distinct additional features: Cr7Ni shows a shoulder in λ(T ) for T <8 K, while Cr 7 Ni-Cu-Cr 7 Ni shows a flattening of λ(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 Cr 7 Ni 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 λ(T ) in Cr 7 Ni-Cu-Cr 7 Ni 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.