We report ac susceptibility and continuous wave and pulsed EPR experiments performed on ${\mathrm{GdW}}_{10}$ and ${\mathrm{GdW}}_{30}$ polyoxometalate clusters, in which a ${\mathrm{Gd}}^{3+}$ ion is coordinated to different polyoxometalate moieties. Despite the isotropic character of gadolinium as a free ion, these molecules show slow magnetic relaxation at very low temperatures, characteristic of single molecule magnets. For $T\lesssim 200\mathrm{mK}$, the spin-lattice relaxation becomes dominated by pure quantum tunneling events, with rates that agree quantitatively with those predicted by the Prokof’ev and Stamp model [ Phys. Rev. Lett. 80, 5794 (1998)]. The sign of the magnetic anisotropy, the energy level splittings, and the tunneling rates strongly depend on the molecular structure. We argue that ${\mathrm{GdW}}_{30}$ molecules are also promising spin qubits with a coherence figure of merit $Q_{\mathrm{M}}\gtrsim 50$.

We investigate the mechanism of spin-lattice relaxation of Er ions encapsulated in polyoxometalate clusters, which below 4 K can only reverse its spin via quantum tunneling processes. The temperature-independent rate ${\tau }^{-1}$ is, at zero field, ten orders of magnitude larger than the rates predicted for direct phonon-induced processes. In addition, we observe that ${\tau }^{-1}$ is suppressed by external magnetic bias and hyperfine interactions but enhanced by increasing the concentration of Er ions. The observed relaxation agrees with predictions for pure quantum tunneling, showing that this phenomenon drives the thermalization of electronic spins. A possible link between these two phenomena is discussed, involving the collective emission of phonons from particular spin configurations attained via quantum tunneling.

Continuous-wave, multi-frequency electron paramagnetic resonance (EPR) studies are reported for a series of single-crystal and powder samples containing different dilutions of a recently discovered mononuclear Ho^III (4f¹⁰) single-molecule magnet (SMM) encapsulated in a highly symmetric polyoxometalate (POM) cage. The encapsulation offers the potential for applications in molecular spintronics devices, as it preserves the intrinsic properties of the nanomagnet outside of the crystal. A significant magnetic anisotropy arises due to a splitting of the Hund's coupled total angular momentum (J = L + S = 8) ground state in the POM ligand field. Thus, high-frequency (50.4 GHz) EPR studies reveal a highly anisotropic eight line spectrum corresponding to transitions within the lowest m_J = ±4 doublet, split by a strong hyperfine interaction with the I = 7/2 Ho nucleus (100% natural abundance). X-band EPR studies reveal the presence of an appreciable tunneling gap between the m_J = ±4 doublet states having the same nuclear spin projection, leading to a highly non-linear field-dependence of the spectrum at low-frequencies.