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Magnetic Susceptibility and Spin Dynamics of a Polyoxovanadate Cluster: A Proton NMR Study of a Model Spin Tetramer

We report susceptibility and nuclear magnetic resonance (NMR) measurements in a polyoxovanadate compound with formula (NHEt) 3 [V 8 I V V V 4 As 8 O 4 0 (H 2 O)].H 2 O≡{V12}. The magnetic properties can be described by considering only the central square of localized V 4 + ions and treated by an isotropic Heisenberg Hamiltonian of four intrinsic spins ½ coupled by nearest-neighbor antiferromagnetic interaction with J∼17.6 K. In this simplified description the ground state is nonmagnetic with S T =0. The 1 H NMR linewidth (full width at half maximum) data depend on both the magnetic field and temperature, and are explained by the dipolar interaction between proton nuclei and V 4 + ion spins. The behavior of the nuclear spin-lattice relaxation rate T - 1 1 in the temperature range (4.2-300 K) is similar to that of Χ T vs T and it does not show any peak at low temperatures contrary to previous observations in antiferromagnetic rings with larger intrinsic spins. The results are explained by using the general features of the Moriya formula and by introducing a single T-independent broadening parameter for the electronic spin system. From the exponential T dependence of T - 1 at low T (2.5 K<T<4.2 K) we have obtained a field dependent gap following the linear relation Δ N M R = Δ 0 -gμ B H, with the gap Δ 0 ∼17.6 K in agreement with the susceptibility data. Below 2.5 K the proton T - 1 deviates from the exponential decrease indicating the presence of a small, almost temperature independent, but strongly field dependent, nuclear relaxation contribution, which we will investigate in detail in the near future.