Magnetization, ac-susceptibility, and muon spin-relaxation $\left(\mu \text{SR}\right)$ measurements have been performed in neutral phthalocyaninato lanthanide $\left({\left[Ln{\text{Pc}}_2\right]}^0\right)$ single-molecule magnets in order to determine the low-energy levels structure and to compare the low-frequency spin excitations probed by means of macroscopic techniques, such as ac susceptibility, with the ones explored by means of techniques of microscopic character, such as $\mu \text{SR}$. Both techniques show a high temperature thermally activated regime for the spin dynamics and a low-temperature tunneling one. While in the activated regime the correlation times for the spin fluctuations estimated by ac susceptibility and $\mu \text{SR}$ basically agree, clear discrepancies are found in the tunneling regime. In particular, $\mu \text{SR}$ probes a faster dynamics with respect to ac susceptibility. It is argued that the tunneling dynamics probed by $\mu \text{SR}$ involves fluctuations which do not yield a net change in the macroscopic magnetization probed by ac susceptibiliy. Finally resistivity measurements in ${\left[{\text{TbPc}}_2\right]}^0$ crystals show a high temperature nearly metallic behavior and a low-temperature-activated behavior.

• Received 27 May 2010

DOI:https://doi.org/10.1103/PhysRevB.82.134401