Magnetic properties and spin dynamics in the single-molecule paramagnets Cu6Fe and Cu6Co

The magnetic properties and the spin dynamics of two molecular magnets have been investigated by magnetization and dc susceptibility measurements, electron paramagnetic resonance and proton nuclear magnetic resonance NMR over a wide range of temperatures 1.6–300K at applied magnetic fields H=0.5 and 1.5 T. The two molecular magnets consist of (CuI2+(saldmen)(H2O))6(Fe3+(CN)6(ClO4)3 ·8H2O in short Cu6Fe here H saldmen is the Schiff base resulted by reacting salicylaldehyde with N,N-dimethylethylenediamine and the analog compound with cobalt, Cu6Co. It is found that in Cu6Fe, whose magnetic core is constituted by six Cu2+ ions and one Fe3+ ion all with s=1/2, a weak ferromagnetic interaction between Cu2+ moments through the central Fe3+ ion with J=0.14 K is present, while in Cu6Co the Co3+ ion is diamagnetic and the weak interaction is antiferromagnetic with J=−1.12 K. The NMR spectra show the presence of nonequivalent groups of protons with a measurable contact hyperfine interaction consistent with a small admixture of s wave function with the d function of the magnetic ion. The NMR relaxation results are explained in terms of a single-ion Cu2+, Fe3+, Co3+ uncorrelated spin dynamics with an almost temperature-independent correlation time due to the weak magnetic exchange interaction. We conclude that the two molecular magnets studied here behave as single-molecule paramagnets with a very weak intramolecular interaction, almost on the order of the dipolar intermolecular interaction. Thus they represent a separate class of molecular magnets which differ from the single-molecule magnets investigated up to now, where the intramolecular interaction is much larger than the intermolecular one.