Spin order and dynamics in the diamond-lattice Heisenberg antiferromagnets CuRh2O4 and CoRh2O4

Abstract

Antiferromagnetic insulators on the diamond lattice are candidate materials to host exotic magnetic phenomena ranging from spin-orbital entanglement to degenerate spiral ground-states and topological paramagnetism. Compared to other three-dimensional networks of magnetic ions, such as the geometrically frustrated pyrochlore lattice, the investigation of diamond-lattice magnetism in real materials is less mature. In this work, we characterize the magnetic properties of model A-site spinels CoRh2O4 (cobalt rhodite) and CuRh2O4 (copper rhodite) by means of thermo-magnetic and neutron scattering measurements and perform group theory analysis, Rietveld refinement, meanfield theory, and spin wave theory calculations to analyze the experimental results. Our investigation reveals that cubic CoRh2O4 is a canonical S = 3/2 diamond-lattice Heisenberg antiferromagnet with a nearest neighbor exchange J = 0.63 meV and a Néel ordered ground-state below a temperature of 25 K. In tetragonally distorted CuRh2O4, competiting exchange interactions between up to third nearest-neighbor spins lead to the development of an incommensurate spin helix at 24 K with a magnetic propagation vector km = (0, 0, 0.79). Strong reduction of the ordered moment is observed for the S = 1/2 spins in CuRh2O4 and captured by our 1/S corrections to the staggered magnetization. Our work identifies CoRh2O4 and CuRh2O4 as reference materials to guide future work searching for exotic quantum behavior in diamond-lattice antiferromagnets.

Molecular diamond lattice antiferromagnet as a Dirac semimetal candidate

“…In the counterpart insulating system, the frustrated local moment on the diamond lattice has been extensively studied as a spin liquid candidate [7][8][9]. A typical example is the magnetic spinel (AB 2 C 4 ) with the A site diamond lattice [10][11][12][13][14][15][16][17][18], where the properties of the disordered state are under intense debate. A (topological) Mott transition is expected to occur from a spin disordered phase to a Dirac semimetal phase by tuning the electron correlation [2,7].Organic molecular compounds have provided the platform for investigating the pressure-tuned Mott transition for the soft crystal.…”

“…Here the 13 C nuclear spin experiences a sum of the external field H 0 and the spontaneous local field H loc produced by the magnetic moment M loc : ω is given by ω = γ n H eff = γ n (|H 0 +H loc |) = γ n H 2 0 + H 2 n + 2H 0 H n cosϑ, where γ n is the 13 [35].…”

“…. (a) Static spin susceptibility χ obtained from13 C Knight shift K = Aχ/(Nµ B ) with the hyperfine coupling constant A = 0.83T/µ B determined at 0 GPa. The magnetic field was applied parallel to[110].…”

“…Solid curves are the series expansion of the diamond-lattice Heisenberg model[32]. (b)13 C NMR frequency defined by the spectral peak across T N at 0, 1.0, and 2.0 GPa, where the magnetic field is applied along[110]. (c) The nuclear spin-lattice relaxation rate T −1 1 .…”

“…(d) Pressure-temperature phase diagram of (ET)Ag 4 (CN) 5 based on the NMR measurements. Inset shows the spin configuration determined by13 C NMR.…”