Spin dynamics of Fe7M (M=Zn,Mn) heterometallic rings probed by neutron spectroscopy

The spin dynamics of two representative members of a family of Fe-based heterometallic antiferromagnetic wheels have been investigated by inelastic neutron scattering (INS). The INS spectra measured at various temperatures have been reproduced using microscopic spin Hamiltonians that contain all the main interactions present in the molecules (isotropic exchange, intramolecular dipolar interaction and local crystal fields). The positions of the observed excitations provide evidence of weak second-nearest neighbor antiferromagnetic exchange interactions. Information about the symmetry of the molecular eigenstates has been obtained by studying the momentum-transfer dependence of the intensities of the observed transitions. The results have also been exploited to investigate the time autocorrelation of the Neel vector in these antiferromagnetic wheels. We have shown that S mixing has a remarkable effect on the Neel vector dynamics and that the tunneling picture is only very approximately valid for these heterometallic rings.