Heisenberg spin triangles in (formula presented)-type magnetic molecules: Experiment and theory

We report the results of systematic experimental and theoretical studies of two closely related species of magnetic molecules of the type (formula presented) where each molecule includes a pair of triangles of exchange-coupled vanadyl (formula presented) spin (formula presented) ions. The experimental studies include the temperature dependence of the low-field susceptibility from room temperature down to 2 K, the dependence of the magnetization on magnetic field up to 60 T for several low temperatures, the temperature dependence of the magnetic contribution to the specific heat, and the (formula presented)\ and (formula presented) nuclear magnetic resonance spin-lattice relaxation rates (formula presented) This body of experimental data is accurately reproduced for both compounds by a Heisenberg model for two identical uncoupled triangles of spins; in each triangle, the spins interact via isotropic antiferromagnetic exchange, where two of the three V-V interactions have exchange constants that are equal and an order of magnitude larger than the third; the ground-state eigenfunction has total spin quantum number (formula presented) for magnetic fields below a predicted critical field (formula presented) and (formula presented) for fields above (formula presented).