Geometrical Frustration in Magnetic B-Site Spinels

Using inelastic neutron scattering, we have studied the effects of geometrical frustration in insulating and metallic spinels, AB₂O₄, where the B sites form a lattice of corner-sharing tetrahedra. Geometrical frustration leads to macroscopic degeneracies and qualitatively new states of matter. In the insulating spinels, such as ZnCr₂O₄, CdFe₂O₄ and ZnFe₂O₄, we have found that fluctuating spins self-organize into weakly interacting nano-scale spin clusters rather than fluctuating individually. The emergence of composite local spin entities suggests an organizing principle for frustrated systems such that interacting degrees of freedom combine to form rigid clusters if macroscopic condensation is not possible. The metallic spinel, LiV₂O₄, shows heavy fermion (HF) behavior at low temperatures. This is surprising because LiV₂O₄ has only one kind of electron around the Fermi level and therefore the ordinary Kondo screening involving two types of electrons, localized and itinerant, is not likely. In order to understand the origin of the HF behavior in LiV₂O₄, we have investigated how the spin dynamics evolve as a function of Li doping in Zn_1-xLi_xV₂O₄. We have found that the characteristic wave vector associated with dynamic spin correlations decreases gradually as x increases up to 1, which indicates the important role of frustration. However, the relaxation rate changes dramatically at around x = 1, suggesting the possible existence of a quantum critical point near x = 1.