American Physical Society, Physical review B, 9(80)
DOI: 10.1103/physrevb.80.094413
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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 [{CuII(saldmen)(H2O)}6{FeIII(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.