Combining recent and new inelastic neutron scattering data for the molecular cyclic cluster Cr8 produces a deep understanding of the low lying excitations in bipartite antiferromagnetic Heisenberg rings. The existence of the L band, the lowest rotational band, and the E band, essentially spin wave excitations, is confirmed spectroscopically. The different significance of these excitations and their physical nature is clearly established by high-energy and Q-dependence data.

Neutron intensity elastic scans on trehalose, maltose, and sucrose/H(2)O mixtures as a function of concentration, temperature, and exchanged wave vector are presented. The experimental findings show a crossover in molecular fluctuations between harmonic and anharmonic dynamical regimes. A new operative definition for the degree of fragility of glass-forming systems is furnished by using explicitly the connection between viscosity and mean-square displacement. The procedure is tested for the investigated mixtures and for a set of glass-forming systems. In this frame, the stronger character of trehalose/H(2)O mixture indicates a better attitude in respect to maltose and sucrose/H(2)O mixtures to encapsulate biostructures in a more rigid matrix.

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The archetype of geometrically frustrated compounds SrCr 9 p Ga 12Ϫ9 p O 19 is a kagomé bilayer of Heisenberg Cr 3ϩ ions (Sϭ3/2) with antiferromagnetic interactions. We present an extensive gallium nuclear magnetic resonance ͑NMR͒ study over a broad Cr-concentration range (0.72рpр0.95). This allows us to probe locally the susceptibility of the kagomé bilayer and separate the intrinsic properties due to geometric frustration from those related to site dilution. Compared to the partial study on one sample, pϭ0.90, presented in Phys. Rev. Lett. 85, 3496 ͑2000͒, we perform here a refined study of the evolution of all the magnetic properties with dilution, with a great emphasis on the lowest diluted pϭ0.95 sample synthesized for this study. Our major findings are the following ͑1͒ The intrinsic kagomé bilayer susceptibility reaches a maximum at a temperature of Ϸ40Ϫ50 K, which we show here to be robust up to a dilution as high as Ϸ20%; this maximum is the signature of the development of short-range antiferromagnetic correlations in the kagomé bilayer. ͑2͒ At low T, a highly dynamical state induces a strong wipeout of the NMR intensity, regardless of dilution. ͑3͒ The low-T upturn of the macroscopic susceptibility is associated with paramagnetic defects, which stem from the dilution of the kagomé bilayer. The low-T analysis of the pϭ0.95 NMR line shape, coupled with a more accurate determination of the nuclear Hamiltonian at high T, allows us to discuss in detail the nature of the defect. Our analysis suggests that the defect can be associated with a staggered spin response to the vacancies of the kagomé bilayer. This, altogether with the maximum in the kagomé bilayer susceptibility, is very similar to what is observed in most low-dimensional antiferromagnetic correlated systems, even those with a short spin-spin correlation length. ͑4͒ The spin-glass-like freezing observed at T g Ϸ2 -4 K is not driven by the dilutioninduced defects.

Inelastic neutron scattering has been applied to the study of the spin dynamics of Cr-based antiferromagnetic octanuclear rings where a finite total spin of the ground state is obtained by substituting one Cr 3+ ion (s = 3/2) with Zn (s = 0), Mn (s = 5/2) or Ni (s = 1) di-cations. Energy and intensity measurements for several intra-multiplet and inter-multiplet magnetic excitations allow us to determine the spin wavefunctions of the investigated clusters. Effects due to the mixing of different spin multiplets have been considered. Such effects proved to be important to correctly reproduce the energy and intensity of magnetic excitations in the neutron spectra. On the contrary to what is observed for the parent homonuclear Cr8 ring, the symmetry of the first excited spin states is such that anticrossing conditions with the ground state can be realized in the presence of an external magnetic field. Heterometallic Cr7M wheels are therefore good candidates for macroscopic observations of quantum effects.