Entropy driven incommensurate structures in the frustrated kagome staircase Co₃V₂O₈

Abstract

Co3V2O8 features spin-3/2 moments arrayed on a kagome staircase lattice. A spin density wave with a continuously evolving propagation vector of k = (0, δ, 0), showing both incommensurate states and multiple commensurate lock-ins, is observed at temperatures above the ferromagnetic ground state. Previous work has suggested that this changing propagation vector could be driven by changes in exchange interactions due to Co atom displacements. We present a straightforward model showing that a Hamiltonian with competing (but temperature independent) interactions can semi-quantitatively reproduce this behavior using a mean field approximation. The simulated spin density wave magnetic structures feature buckled kagome planes that are either ferromagnetically or antiferromagnetically ordered. Propagation vectors that differ from δ = 1/2 will have multiple different ways of arranging these ferromagnetic layers that have very similar energies. This classical stacking entropy appears to be crucial in stabilizing the temperature-dependent propagation vector.

Magnetic exchange interactions and non-Debye relaxation in spin-3/2 frustrated Kagomé magnet Co3V2O8

“…At temperatures above the FM ground state, a spin density wave with a constantly evolving propagation vector of k = (0, δ, 0) is reported, exhibiting multiple commensurate as well as incommensurate lock-ins. The changing propagation vector is also reported to be driven by changes in exchange interactions due to Co atom displacements [26]. We therefore evaluated the exchange constant experimentally using conventional temperature-dependent magnetization data.…”