Theory of the ordered phase inA-site antiferromagnetic spinels

Abstract

Insulating spinel materials, with the chemical formula AB2X4, 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 1 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 of 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.

Impurity effects in highly frustrated diamond-lattice antiferromagnets

“…This implies that the degeneracy breaking mechanism is not disorder, and we believe that the selection is due to third-neighbor interactions as suggested in Ref. 21. Such poor sensitivity to disorder is consistent with the large momentum q and stiffness c in MnSc 2 S 4 , as we explain above.…”

“…II A 2). Consider fluctuations in the impurity energy δE imp (q(r),r) = E imp (q(r),r) − E imp (q(r),r) (21) and expand to linear order in φ:…”

Spiral spin-liquid and the emergence of a vortex-like state in MnSc2S4

“…Here, the observed helical structure provides strong evidence for the effect of dipolar interactions, which have been shown to favor the helical structure over the cycloidal one 12 . Previous calculations reveal that a value of J 3 ∼ −0.04 K brings q to (0.75 0.75 0) 11,12 , and the dipolar interaction is ∼ 0.026 K for nearest neighbours. However, our mean-field calculations reveal that long-range order at q = (0.75 0.75 0) occurs even in the case where J 3 equals zero and only the dipolar perturbation exists.…”

“…According to previous theoretical studies 11,12 , the inclusion of an antiferromagnetic J 3 perturbation in the J 1 -J 2 model selects the q position for the spiral but leaves the spiral plane undetermined; whether the ground state is cycloidal or helical depends on further anisotropic perturbations. Here, the observed helical structure provides strong evidence for the effect of dipolar interactions, which have been shown to favor the helical structure over the cycloidal one 12 .…”

“…1a) [11][12][13] . Although the diamond lattice is bipartite, and therefore unfrustrated at the near-neighbour (J 1 ) level, the second-neighbour coupling (J 2 ) can generate strong competition.…”