Quantum entanglement in nitrosyl iron complexes

Recent magnetic susceptibility measurements for polycrystalline samples of binuclear nitrosyl iron complexes, [Fe-2(C3H3N2S)(2)(NO)(4)] (I) and [Fe-2(SC3H5N2)(2)(NO)(4)] (II), suggest that quantum-mechanical entanglement of the spin degrees of freedom exists in these compounds. Entanglement E exists below the temperature T (E) that we have estimated for complexes I and II to be 80-90 and 110-120 K, respectively. Using an expression of entanglement in terms of magnetic susceptibility for a Heisenberg dimer, we find the temperature dependence of the entanglement for complex II. Having arisen at the temperature T (E) , the entanglement increases monotonically with decreasing temperature and reaches 90-95% in this complex at T = 25 K, when the side effects are still small.