NMR and $\mu^{+}$SR detection of unconventional spin dynamics in Er(trensal) and Dy(trensal) molecular magnets

TL;DR

This study uses NMR and muon spin relaxation to investigate unconventional spin dynamics in Er(trensal) and Dy(trensal) molecular magnets, revealing complex relaxation processes and potential quantum effects at low temperatures.

Contribution

It provides new insights into the temperature and field dependence of spin relaxation in lanthanide molecular magnets, highlighting the presence of multiple relaxation processes and deviations from standard models.

Findings

Peak in relaxation rates below 30K indicating spin slowing down

Spectral width increases around 50K, higher than relaxation peaks

Evidence of multiple relaxation processes with different correlation times

Abstract

Measurements of proton Nuclear Magnetic Resonance (1H NMR) spectra and relaxation and of Muon Spin Relaxation ($\mu^{+}$SR) have been performed as a function of temperature and external magnetic field on two isostructural lanthanide complexes, Er(trensal) and Dy(trensal) featuring crystallographically imposed trigonal symmetry. Both the nuclear 1/T1 and muon $\lambda$ longitudinal relaxation rates, LRR, exhibit a peak for temperatures T lower than 30K, associated to the slowing down of the spin dynamics, and the width of the NMR absorption spectra starts to increase significantly at T ca. 50K, a temperature sizably higher than the one of the LRR peaks. The LRR peaks have a field and temperature dependence different from those previously reported for all Molecular Nanomagnets. They do not follow the Bloembergen-Purcell-Pound scaling of the amplitude and position in temperature and field and thus cannot be explained in terms of a single dominating correlation time $\tau$c determined by the spin slowing down at low temperature. Further, for T lower than 50K the spectral width does not follow the temperature behavior of the magnetic susceptibility chi. We suggest, using simple qualitative considerations, that the observed behavior is due to a combination of two different relaxation processes characterized by the correlation times $\tau$LT and $\tau$HT, dominating for T lower than 30K and T higher than 50K, respectively. Finally, the observed flattening of LRR for T lower than 5K is suggested to have a quantum origin.