Observation of Magnetic Level Repulsion in Fe6:Li Molecular Antiferromagnetic Rings

Heat capacity $(C)$, magnetic torque, and proton NMR relaxation rate (1/T₁) measurements were performed on Fe6:Li single crystals in order to study the crossings between S0ex0ex=0ex0ex0 and S0ex0ex=0ex0ex1 and between S0ex0ex=0ex0ex1 and S0ex0ex=0ex0ex2 magnetic states of the molecular rings, at magnetic fields Bc10ex0ex=0ex0ex11.7T and Bc20ex0ex=0ex0ex22.4T, respectively. C vs B data at 0.78K show that the energy gap between two states remains finite at Bc's ( Δ₁/kB0ex0ex=0ex0ex0.86K and Δ₂/kB0ex0ex=0ex0ex2.36K) thus proving that levels repel each other. The large Δ₁ value may also explain the anomalously large width of the peak in 1/T₁ vs B, around Bc1. This anticrossing, unexpected in a centrosymmetric system, requires a revision of the Hamiltonian.