Neutron diffraction studies of nuclear and magnetic structures in theS=1∕2square Heisenberg antiferromagnets(d6−

Abstract

We report the neutron scattering studies of the nuclear and magnetic structures of deuterated samples of the model two-dimensional S =1/2 Heisenberg antiferromagnets on a square lattice, ͑d 6-5CAP͒ 2 CuCl 4 and ͑d 6-5CAP͒ 2 CuBr 4 ͑where 5CAP is 2-amino-5-chloropyridinium͒. Interest in these materials stems from the fact that they have relatively weak exchange between the magnetic ions, and it is therefore possible to perturb their magnetic structures and excitations significantly in experimentally accessible magnetic fields, and thereby access new quantum disordered states. We succeeded in growing fully deuterated single crystals and determined the nuclear and magnetic structures of the bromide at 10 and 1.8 K, respectively, confirming the four-sublattice spin structure expected for systems, where both inter-and intraplane exchange interactions are antiferromagnetic. The determination of the full crystal structure of the bromide highlights the possibility that interlayer exchange may also propagate via hydrogen bonds to and through the 5CAP molecule. We also determined the critical exponents for the sublattice magnetization of the bromide and mapped out the H-T phase diagram of the chloride up to 5 T.

Dynamical structure factor of quasi-two-dimensional antiferromagnet in high fields

“…[1][2][3][4][5][6] A better understanding of many phenomena, from superfluidity to complex quantum phases may also be achieved through comparative investigations of systems of strongly interacting bosons and frustrated antiferromagnets. [5][6][7] Recent developments in the synthesis of molecular based antiferromagnets with moderate exchange constants [8][9][10][11] has opened previously unreachable high magnetic field regime to experimental investigations. 12,13 Furthermore, the recent completion of neutron scattering instruments with enhanced resolution [14][15][16] provides an opportunity to study the dynamics of quantum antiferromagnets in a much wider momentum-energy space and under the influence of applied magnetic field.…”

Field-induced decay dynamics in square-lattice antiferromagnets

“…The strong-field regime is now reachable for a number of layered square-lattice materials with moderate strength of exchange coupling between spins. [2][3][4][5] In addition, new field-induced dynamical effects can be present in the antiferromagnets with other lattice geometries 6,7 as well as in the gapped quantum spin systems near the condensation field for triplet excitations. [8][9][10][11][12][13][14] The ground-state properties of the square-lattice antiferromagnet in a finite field conform with the semiclassical picture of spins gradually tilted towards the field direction, 15 see Fig.…”

“…[16][17][18] Following the prediction of the field-induced spontaneous magnon decays in Ref. 16, there is an ongoing search for suitable spin-1/2 square-lattice materials [2][3][4][5] to investigate such an effect. The existence of substantial damping in the magnon spectrum of the square-lattice Heisenberg model in a field has been recently verified by the Quantum Monte Carlo 17 (QMC) and the Exact Diagonalization 18 numerical study.…”

Interplay of Spin and Spatial Anisotropy in Low-Dimensional Quantum Magnets with Spin 1/2

“…On the other hand, the strong J" will smear even this feature characterizing the ideal 2D magnets and a conventional decrease of the transition temperature will be observed in all magnetic fields. Such behavior was observed in (5CAP) 2 CuCl 4 , the quasi-2D S = 1/2 HAF on the square lattice with J"/J ≈ 0.25 [64].…”

“…Magneto-structural investigations [61][62][63] of monoclinic compounds (5MAP) 2 CuBr 4 and (5BAP) 2 CuBr 4 (5MAP = 5-methyl-2-aminopyridinium, 5BAP = 5-bromo-2-aminopyridinium) revealed that the magnetic interaction occurs between Cu(II) sites with four equivalent nearest neighbors through Br ··· Br contacts forming 2D square layers with exchange coupling J/k B ≈ 7 K. The layers of CuBr 4 tetrahedrons are separated by the bulk of organic cations which stabilize 3D structure. Systematic study of the compounds from the series A 2 CuX 4 [A = 5MAP, 5BAP, 5-chloro-2-aminopyridinium ≡ 5CAP, 5-cyano-2-aminopyridinium ≡ 5CNAP, etc., X = Br, Cl] found that the increasing size of the A cation improves the isolation of individual magnetic square layers but at the same time it reduces the strength of the intra-layer exchange coupling [62][63][64]. Apparently, the chemical modification of the structure can control magnetic properties demonstrating the flexibility of molecular magnetism.…”