Magnetic properties and spin dynamics in theCr7Fenanomagnet: A heterometallic antiferromagnetic molecular ring

We present magnetic susceptibility, $^{1}\text{H}$ nuclear magnetic resonance (NMR) spectra, spin-spin- and spin-lattice-relaxation rates data, collected in the temperature range $1.65lTl300\text{ }\text{K}$ at two applied magnetic fields $H=0.35$ and 1.5 T, on a single crystal of ${\text{Cr}}_{7}\text{Fe}$ heterometallic molecular nanomagnet. From a simple analysis of the magnetic susceptibility data, we deduce that the ring has a magnetic total spin ${S}_{T}=1/2$ ground state and Cr-Cr antiferromagnetic (AFM) exchange interactions comparable to similar ${\text{Cr}}_{7}\text{M}$ heterometallic rings. The proton NMR data indicate that the main coupling between nuclei and electronic moments is of dipolar origin. The spin-lattice-relaxation rate behavior is qualitatively explained by a model which assumes a single-correlation time as used previously for homometallic AFM rings. The direct comparison of the relaxation data in ${\text{Cr}}_{7}\text{Fe}$ and ${\text{Cr}}_{8}$ shows a substantial change in the spin dynamics as the result of Fe replacing one Cr ion. The difference is attributed to the change in the elastic properties of the sample with Fe substitution and/or to the multi-Lorentzian behavior of the spin-spin correlation function resulting from the presence of inequivalent ions in the heterometallic ring.