Gd-based single-ion magnets with tunable magnetic anisotropy: Molecular design of spin qubits

We report ac susceptibility and continuous wave and pulsed EPR experiments performed on GdW10and GdW30polyoxometalate clusters, in which a Gd3+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 T200mK, 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. The sign of the magnetic anisotropy, the energy level splittings, and the tunneling rates strongly depend on the molecular structure. We argue that GdW30molecules are also promising spin qubits with a coherence figure of merit QM50. © 2012 American Physical Society