Spin liquid versus spin solid in A-site spinels

Spinel compounds of stoichiometry AB2X4 with magnetic A-site cations exhibit strong frustration effects. Frustration leads to strongly enhanced spin fluctuations that can easily suppress long range magnetic order (corresponding to a spin solid state) resulting in a dynamic spin liquid ground state. The same argument holds true for the orbital sector if an orbital moment is present. Neutron scattering is ideally suited to investigate the corresponding correlation functions. Here we present neutron diffraction studies on the A-site spinels MAl2O4 (M=Co, Fe, Mn). Whereas the Mn compound representing a spin-only system shows long range magnetic order below 42K, the diffraction patterns of the Fe and Co spinel reveal a characteristic liquid structure factor of the magnetic scattering. This contrasting behavior can be related to the absence/presence of an orbital moment