Spin Correlations and Excitations in Spin-frustrated Molecular and Molecule-based Magnets

The present thesis provides extensive investigations on the effect of geometrical spin frustration in both molecular-based spin clusters and infinite pyrochlore lattice, using advanced neutron scattering, DC and AC susceptibilities, and specific heat methods. Clear short-range spin correlations have been observed in the spin-frustrated molecular magnet {Mo$_{72}$Fe$_{30}$} by means of polarized neutron scattering. Simulations of the spin correlations were carried out within a frustrated three-sublattice spin model. The simulations are in reasonable agreement with the differential magnetic cross section measured at 1.5 K. The specific heat of {Mo$_{72}$Fe$_{30}$} was measured down to 60 mK. The low-lying magnetic excitations of {Mo$_{72}$Fe$_{30}$} are identified by the Schottky anomalies in the specific heat data, consistent with the theoretical predictions of quantum rotational band model. The nature of the magnetic ground state of {Mo$_{72}$Fe$_{30}$} can therefore be interpreted within the three-sublattice spin model, where the 30 Fe$^{3+}$ spins are divided into three sublattices with equal and coplanar unit vectors each. The low-lying magnetic excitations of single molecular magnet {As$_{6}$V$_{15}$} were determined by means of specific heat measurements down to 60 mK. The resultant energy spectrum agrees with the theoretical predictions of the “three-spin” model and the results of inelastic neutron scattering experiments. Hence the “three-spin” model is a good approach to the magnetic ground state of {As$_{6}$V$_{15}$}. Complicated magnetic behaviors have been observed in the antiferromagnetic pyrochlore Na$_{3}$Co(CO$_{3}$)$_{2}$Cl. The average crystal structure of Na$_{3}$Co(CO$_{3}$)$_{2}$Cl was determined by means of X-ray and neutron powder diffraction measurements. Though there is no site disorder in the Co$^{2+}$ magnetic sublattice, positional disorder may exist in Na and Cl atom sites. The DC and AC susceptibility measurements show a spin-glass-like transition at 4.5 K, and a field- and frequency- independent kink at 17 K. Temperature dependence of the specific heat exhibits a peak at 1.5 K due to a long-range magnetic order, a broad hump at 5 K associated with the spin-glass-like transition, and a very small hump at 17 K. The diffuse neutron scattering with polarization analysis reveals short-range spin correlations dominated by antiferromagnetic coupling over the range of nearest neighbors. Ferromagnetic coupling exists between next-nearest neighbors. Long-range magnetic order below [...