Heisenberg spin triangles in {V₆}-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 {V₆}, where each molecule includes a pair of triangles of exchange-coupled vanadyl (VO²⁺, spin s=1/2) 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 ¹H and ²³Na nuclear magnetic resonance spin-lattice relaxation rates 1/T₁. 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 S=1/2 for magnetic fields below a predicted critical field H_c≈74 T and S=3/2 for fields above H_c.