Suppression of geometric frustration by magnetoelastic coupling in AuCrS2

Abstract

International audienceWe 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

Grain boundaries in graphene grown by chemical vapor deposition

“…Most of the theoretical work on GBs is focused on that type of GBs which have been found in HOPG and which are built by incorporating non-hexagonal rings between two misoriented, but crystallographically prefect graphene lattices [20]. A recent modeling work considering more amorphous-like GBs has obtained interesting results [21], in good agreement with recent experimental data [22].…”

“…The GB having the largest misorientation (τ = 30 • ) is one in which two graphene halfsheets with zigzag and armchair orientations, respectively, are joined together. This construction (3,2) in the notation of [20], with misorientation angle τ = 46.8 • , shaped into a linear chain of pentagon-heptagon pairs separated by three transverse rows of hexagons.…”

“…The so-called coincidence lattice site method [20] can be used to generate models of ordered GB for all misorientation angles ranging between 0 and 60 • . Geometrically, the idea behind this method is to superimpose exactly two graphene networks, and then to rotate one network relative to the other about a [001] axis issued from an arbitrary common site until a subset of lattice sites of both networks exactly fall back onto each other, thus forming a graphene bilayer with commensurate rotational fault [38].…”

“…The cutting line can then be chosen along a mirror plane of the obtained moiré pattern, which is the case illustrated in figure 3. In these conditions, the GB orientation can be defined by two integer indices (n, m), namely the crystallographic coordinates of a site through which it goes relative to the Bravais vectors of one of the two rotated networks [20]. The misorientation angle then Two initially superimposed graphene sheets have been rotated about a common atom (black circle) in opposite directions and by the same angle θ/2 such that a superlattice of atoms still coincides after the rotation (moiré coincidence).…”

Magnetism of layered chromium sulfidesMCrS2(M=Li, Na, K, Ag, and Au): A first-principles study

“…7,[11][12][13][14]. Cr atoms form a triangular lattice within CrS 2 layers, and the latter are joined by M atoms [ Fig.…”