muSR evidence of a marked exchange-interaction effect on the local spin dynamics of Tb-based molecular nanomagnets

We report on the spin dynamics of two terbium-based molecular nanomagnets, namely Tb(DTBSQ)(HBPz3)2 (in short Tb-SQ) and Tb(Trp)(HBPz3)2 (in short Tb-Trp), investigated by means of longitudinal muon spin relaxation (μSR) measurements as a function of applied field, flanked by ac susceptibility characterization. In the two molecules the terbium(III) ion has an isostructural coordination sphere, but in the former the terbium(III) is coordinated by an organic paramagnetic ligand (3,5 ditertbutylsemiquinonate, SQ), while in the latter is coordinated by a diamagnetic one (tropolonate, Trp). Thus Tb-SQ presents an exchange interaction between the terbium(III) ion and a radical while Tb-Trp does not. Both samples exhibit a muon spin-lattice relaxation rate λ1(T,BL) peak in the temperature range 10-25 K at all applied longitudinal magnetic fields BL=50,150,300 milli-Tesla (mT). In Tb-SQ, λ1(T,BL) displays a BPP-like behavior led by three different correlations times: the first, dominating for T≥15K, follows a thermally activated law τc=τ0exp(σA/kBT) with energy barrier σA/kB, while the second and third ones, dominating, respectively, for 8<15K and T<8K, follow a power-law-like behavior τc=c0T-α with two different values of c0 and α. On the other hand, the temperature and field behavior of λ1(T,BL) in Tb-Trp strongly deviates from a BPP law, displaying a strongly anomalous character. Our results indicate that, in the absence of an exchange interaction and maintaining all the other relevant interactions constant, the local spin dynamics of single-ion magnets strongly differ from the one observed in the presence of such interaction. The combination of μSR and ac susceptibility measurements allows us to disentangle the different Orbach, Raman, and direct mechanisms, which are the key ingredients that control the spin dynamics in Tb-SQ, and evidence the potentiality of μSR in elucidating complex spin dynamics.