We report a detailed examination the magnetic structure of anhydrous cupric chloride CuCl 2 carried out by powder neutron diffraction, magnetic susceptibility and specific heat measurements on polycrystalline and single crystal samples as well as an evaluation of the spin exchange interactions by first principles density functional theory (DFT) calculations. Anhydrous CuCl 2 shows one dimensional antiferromagnetic behavior and long range antiferromagnetic ordering below a Néel temperature of 23.9 K. Neutron powder and single crystal diffraction reveal that, below 23.9 K, CuCl 2 undergoes a phase transition into an incommensurate magnetic structure (propagation vector (1,0.2257,0.5) with a spin-spiral propagating along b and the moments confined in the bc crystallographic plane. Our DFT calculations show that the spin-spiral results from competing ferromagnetic nearest neighbor and antiferromagnetic next-nearest neighbor spin-exchange interaction along the spin chains. Implications for possible multiferroic behavior of CuCl 2 are discussed.

The magnetic properties of BiCu 2 PO 6 have been analyzed by means of magnetic-susceptibility and inelastic neutron-scattering measurements on powder samples by evaluating the spin-exchange interactions on the basis of density-functional calculations and by simulating the inelastic neutron scattering in terms of spin-exchange parameters. BiCu 2 PO 6 exhibits magnetic properties described by the two-leg spin ladder with strong spin frustration along each leg chain and has a gapped quantum singlet ground state with excited magnetic states, showing an incommensurate dispersion arising from frustration.

A thorough crystal structure determination at very low temperature of (CuCl)LaNb₂O₇, originally proposed as a spin-1/2 square-lattice antiferromagnet, is reported thanks to the use of single-crystal x-ray diffraction and powder neutron diffraction. State-of-the-art calculations (maximum entropy method) reveal that (CuCl)LaNb₂O₇ is orthorhombic with Pbam symmetry. First-principles calculations demonstrate that the dominant magnetic interactions are antiferromagnetic between fourth nearest neighbors with a Cu-Cl-Cl-Cu exchange path, which lead to the formation of spin singlets. The two strongest interactions between the singlets are ferromagnetic, which makes (CuCl)LaNb₂O₇ the first system of ferromagnetically coupled Shastry-Sutherland quantum spin singlets.

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We report a detailed density functional analysis of the spin exchange interactions and the magnetic structure of the high-temperature multiferroic CuBr 2 and compare the results with magnetic susceptibility measurements. CuBr 2 shows one-dimensional antiferromagnetism and undergoes long-range antiferromagnetic ordering at ∼74 K. Due to the competition between the nearest-and next-nearest-neighbor spin exchanges, each Cu 2+ chain has a cycloidal spin-spiral structure, which is described approximately by a quadrupling of the nuclear cell with spin moment rotation of ∼85 • in the plane of the CuBr 2 ribbon plane.The quest for high performance multiferroic materials is stimulated by the expectation to control magnetic properties by electric fields and, vice versa, electric polarization by magnetic fields. Multiferroics may open routes to tunable magnetooptical/magnetoelectric multifunctional memory devices. (For recent reviews on multiferroics, see Refs. 1-5.) Well-known multiferroics are mostly oxides of transition metals with open d shells; systems with other anions have less intensively been investigated. In a large number of multiferroics the ferroelectricity is induced by spiral magnetic ordering that removes inversion symmetry. 6-8 Spiral magnetic order is often realized in a magnetic chain system with competing nearest-neighbor (NN) and next-nearest-neighbor (NNN) spin exchange interactions, and often gives rise to an incommensurate longrange ordered antiferromagnetic (AFM) cycloidal magnetic structures. 9-11 Such a spiral magnetic order removes inversion symmetry so that, with the resulting noncentrosymmetric spin structure, spin-orbit interaction gives rise to asymmetric charge distribution, thereby inducing a permanent dielectric polarization and ferroelectricity below the Néel temperature T N . 12,13 Multiferroicity has been found in spiral quantum chain systems such as LiCu 2 O 2 and LiCuVO 4 , 14,15 which consist of CuO 2 ribbon chains that are made up of edge-sharing CuO 4 squares. In LiCuVO 4 , for example, the NN spin exchange (i.e., the Cu-O-Cu superexchange) is ferromagnetic (FM) while the NNN spin exchange (i.e., the Cu-O· · ·O-Cu spin exchange) via oxygen anions is AFM exchange. 10,12,16 Our notion that the magnitude of the NNN is about three times greater than the NN spin exchange 10,12,16,17 has recently been questioned. 18 The possibility of favorably switching the ferroelectric polarization of LiCuVO 4 with external electric and magnetic fields has been demonstrated by polarized neutron diffraction experiments by Mourigal et al. 19 Recently, we have shown that anhydrous CuCl 2 , which crystallizes with a structure containing CuCl 2 ribbon chains similar to the aforementioned oxocuprates, also shows a spiralmagnetic ordering below a Néel temperature of ∼23 K. 20 Subsequently, Seki et al. have observed multiferroicity in CuCl 2 . 21 Until recently, the magnetic properties of the structurally very similar CuBr 2 (Ref. 22) have remained largely unknown. A measurement of the magnetic susceptibili...