Magnetization Hysteresis and Quantum Tunneling in Lanthanide Double-Decker Complexes

We present magnetization measurements on single crystals of lanthanide double-deckers [Pc$_{2}$Ln]$^{-}$ TBA$^{+}$. The 4f$^{9 }$(4f$^{8})$ configuration of the Dy$^{3+}$ (Tb$^{3+})$, ion results in a J = 15/2 (J = 6) ground-state multiplett. In SQUID measurements on single crystal samples, we observed very large axial and a significant transverse anisotropy. Magnetization measurements using 2DEG ballistic Hall probes were carried out in a $^{3}$He cryostat. Hysteresis was observed for both compounds up to blocking temperatures of 4.2 and $>$10 K, respectively. The coercivity increased with decreasing temperatures and increasing sweep rate, as expected for the superparamagnet-like behaviour of a SMM. The hysteresis loops displayed step-like features characteristic for resonant quantum tunnelling of the magnetization (QTM). The step height decreased with increasing sweep rate according the Landau-Zener tunnelling mechanism. In conclusion, the lanthanide double decker molecules are SMM with the highest blocking temperatures observed to date.