Orbital Arrangements and Magnetic Interactions in the Quasi-One-Dimensional Cuprates ACuMoO₄(OH) (A = Na, K)

Abstract

ABSTRACT:A new spin-1/2 quasi-one-dimensional antiferromagnet KCuMoO4(OH) is prepared by the hydrothermal method. The crystal structures of KCuMoO4(OH) and the already-known Na-analogue, NaCuMoO4(OH), are isotypic, comprising chains of Cu 2+ ions in edge-sharing CuO4(OH)2 octahedra. Despite the structural similarity, their magnetic properties are quite different because of the different arrangements of dx 2 -y 2 orbitals carrying spins. For NaCuMoO4(OH), dx 2 -y 2 orbitals are linked by superexchange couplings via two bridging oxide ions, which gives a ferromagnetic nearestneighbor interaction J1 of -51 K and an antiferromagnetic next-nearest-neighbor interaction J2 of 36 K in the chain. In contrast, a staggered dx 2 -y 2 orbital arrangement in KCuMoO4(OH) results in superexchange couplings via only one bridging oxide ion, which makes J1 antiferromagnetic as large as 238 K and J2 negligible. This comparison between the two isotypic compounds demonstrates an important role of orbital arrangements in determining the magnetic properties of cuprates. IntroductionQuantum spin systems attract much attention because quantum fluctuations may suppress conventional magnetic order and can induce exotic states such as a spin liquid. 1, 2 The most promising candidates are found in compounds that contain Cu 2+ ions with spin 1/2. The Cu 2+ ion has a d 9 electronic configuration with one unpaired electron occupying either the dx 2 -y 2 or dz 2 orbital in the octahedral crystal field. This degeneracy is eventually lifted by lowering symmetry due to the Jahn-Teller effect: when two opposite ligands move away and the other four ligands come closer, the dx 2 -y 2 orbital is selected to carry the spin, whereas the dz 2 orbital is selected in the opposite case. Since this Jahn-Teller energy is quite large, either distortion of octahedra always occurs in actual compounds. Since magnetic interactions between neighboring Cu spins are caused by superexchange interactions via overlapping between Cu d orbitals and O ligand p orbitals, the arrangements of the Cu d orbitals and their connections via ligands are important to determine the magnetic properties of cuprates.Among many cuprates, a rich variety of crystal structures are found in natural minerals and their synthetic analogues. Recently, for example, many copper minerals with Cu 2+ ions in the kagome geometry have been focused on from the viewpoint of quantum magnetism: Volborthite (Cu3V2O7(OH)2•2H2O), [3][4][5] Herbertsmithite (Zn1-xCu3+x(OH)6Cl2), 6 Vesignieite (BaCu3V2O8(OH)2), 7 and so on. Besides, there are a lot of minerals where Cu 2+ forms one-dimensional (1D) chains, such as Chalcanthite (CuSO4•5H2O), 8 Linarite (PbCuSO4(OH)2), 9 and Natrochalcite (NaCu2(SO4)2(OH)•H2O). 10 Natrochalcite also has many synthetic analogues of ACu2(SO4)2(OH) •H2O (A

Anisotropic field-induced gap in the quasi-one-dimensional antiferromagnet KCuMoO4(OH)

“…The J is estimated as J = 238(1) K by using this relation 41 , which agrees well with that estimated from χ. In addition, T N is determined as 1.52 K from a small peak in C m /T , which is also consistent with that expected from M/H.…”

“…Single crystalline samples with a typical size of 0.5 × 0.5 × 0.5 mm 3 were prepared by hydrothermal method 41 . Its magnetization and heat capacity were measured in a SQUID magnetometer (Quantum Design MPMS) and by the relaxation method (Quantum Design PPMS), respectively.…”

“…2 through magnetization, heat capacity, inelastic neutron scattering, and electron spin resonance measurements, as summarized in Table I. KCuMoO 4 (OH) is a new S = 1/2 quasi-onedimensional antiferromagnet with staggered DM interactions 41 . Its local structure around Cu atoms is shown in Fig.…”

“…Due to the crystal symmetry of P nma, a single Cu chain includes two different Cu sites related to each other by a mirror symmetry that bisects a Cu-Cu bond and a twofold screw axis that passes through the Cu atoms. They require the DM vector to align perpendicular to the chain and to alternate along the chain direction, respectively 41 . Thus, extra interactions such as uniform components of the DM vector are absent and the magnetic interactions are very well described by Hamiltonian (1).…”

Spin-orbit interaction driven dimerization in one-dimensional frustrated magnets

“…Known examples include Rb 2 Cu 2 Mo 3 O 12 , 24,25 , LiCuVO 4 , 26,[28][29][30][31][32][33][34][35][36][37] LiCuSbO 4 , 38,39 PbCuSO 4 (OH) 2 , 40 which span a wide range of J 2 /|J 1 | values. However, a direct experimental observation of the predicted nematic ordering is rahter challenging.…”