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 ${{\mathrm{V}}_{6}},$ where each molecule includes a pair of triangles of exchange-coupled vanadyl $({\mathrm{VO}}^{2+},$ 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 ${}^{1}\mathrm{H}$ and ${}^{23}\mathrm{Na}$ nuclear magnetic resonance spin-lattice relaxation rates ${1/T}_{1}.$ 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 ${H}_{c}\ensuremath{\approx}74\mathrm{T}$ and $S=3/2$ for fields above ${H}_{c}.$