Low-field spin dynamics of Cr7Ni and Cr7Ni−Cu−Cr7Ni molecular rings as detected by μSR

Abstract

Muon spin rotation measurements were used to investigate the spin dynamics of heterometallic ${\mathrm{Cr}}_{7}\mathrm{Ni}$ and ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ molecular clusters. In ${\mathrm{Cr}}_{7}\mathrm{Ni}$ the magnetic ions are arranged in a quasiplanar ring and interact via an antiferromagnetic exchange coupling constant $J$, while ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ is composed of two ${\mathrm{Cr}}_{7}\mathrm{Ni}$ linked by a bridging moiety containing one Cu ion, that induces an inter-ring ferromagnetic interaction ${J}^{\ensuremath{'}}\ensuremath{\ll}J$. The longitudinal muon relaxation rate $\ensuremath{\lambda}$ collected at low magnetic fields ${\ensuremath{\mu}}_{0}Hl0.15$ 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 $T\ensuremath{\sim}10$ K for ${\mathrm{Cr}}_{7}\mathrm{Ni}$ and $T\ensuremath{\sim}8$ K for ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$, the two compounds have distinct additional features: ${\mathrm{Cr}}_{7}\mathrm{Ni}$ shows a shoulder in $\ensuremath{\lambda}(T)$ for $Tl8$ K, while ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ shows a flattening of $\ensuremath{\lambda}(T)$ for $Tl2$ 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 $Tg5$ K, while the shoulder presented by ${\mathrm{Cr}}_{7}\mathrm{Ni}$ can be reproduced by a BPP function that incorporates a single electronic characteristic time theoretically predicted to dominate for $Tl5$ K. The flattening of $\ensuremath{\lambda}(T)$ in ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ 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.