Theory of the ordered phase in A-site antiferromagnetic spinels

Insulating spinel materials, with the chemical formula AB(2)X(4), behave as diamond lattice antiferromagnets when only the A-site atom is magnetic. Many exhibit classic signatures of frustration, induced not geometrically but by competing first- and second-neighbor exchange interactions. In this paper, we further develop a theory [D. Bergman , Nat. Phys. 3, 487 (2007)] of the magnetism of these materials, focusing on the physics observable within the ordered state. We derive a phenomenological Landau theory that predicts the orientation of the spins within incommensurate spiral ordered states. It also describes how the spins reorient in a magnetic field and how they may undergo a low-temperature lock-in transition to a commensurate state. We discuss microscopic mechanisms for these magnetic-anisotropy effects. The reduction in the ordered moment by quantum fluctuations is shown to be enhanced due to frustration. Our results are compared to experiments on MnSc2S4, the best characterized of such A-site spinels, and more general implications are discussed. One prediction is that magnetically induced ferroelectricity is generic in these materials, and a detailed description of the relation of the electric polarization to the magnetism is given.