Suppression of geometric frustration by magnetoelastic coupling in AuCrS2
Résumé
We studied the structural, magnetic, and electronic properties of the geometrically frustrated layered AuCrS2 system by means of x-ray and neutron powder diffraction, specific heat, dc magnetization, and dc electrical resistivity measurements. The room-temperature structural refinement is consistent with a hexagonal centrosymmetric R-3m symmetry and with formal valence states Au+ and Cr3+, where the Cr3+ ions form a regular triangular lattice within the hexagonal planes. On cooling, we observe a first-order structural phase transition to a monoclinic C2/m symmetry concomitant to an antiferromagnetic order at TN = 47 K. The atomic displacements associated with this transition stretch the triangular lattice, thus suppressing the geometric frustration. This accounts for the magnetic order observed and gives evidence of a large magnetoelastic coupling. The refined magnetic structure is commensurate and consists of double ferromagnetic chains along the stretching direction with μ = 2.54 μB/Cr3+; the residual frustration stabilizes an elegant pattern of alternate ferromagnetic and antiferromagnetic intra- and interplane couplings between adjacent chains. The electrical transport of our sintered powder samples is found to be semiconducting-like with ρ300K ∼ 157 cm and an activation energy of 0.38 eV.