H. Weitzel

Crystal structure and magnetic properties of CuMoO4 at low temperature (γ-phase)

Abstract The crystal structure and magnetic properties of a new modification of copper molybdate (γ-CuMoO 4 ) are reported, which is stable below 200 K. Single-crystal X-ray diffraction revealed triclinic symmetry ( P 1 ) with a = 9.699(9) A , b = 6.299(6) A , c = 7.966(7) A , α = 94.62(4)°, β = 103.36(4)°, γ = 103.17(4)° at 180 K and Z = 6. All Cu- and Mo-ions are surrounded by distorted oxygen octahedra. In contrast to the high-pressure modification CuMoO 4 -II, which is of a triclinic distorted wolframite structure, the octahedra are not closed packed. This is also in contrast to the triclinic ( P 1 ) room-temperature phase (α) at standard pressure with tetrahedrally coordinated Mo-ions and with a = 9.901(3) A b = 6.786(2) A , c = 8.369(3) A , α = 101.13(1)°, β = 96.88(1)° and γ = 107.01(1)°. The new crystal structure of γ-CuMoO 4 is extensively discussed and compared with those of AlNbO 4 and α-CoMoO 4 . The transition between α- and γ-phase is of first order and a wide temperature range of coexistence is observed in powder experiments. The relative amount of both phases depends on both temperature and cooling rate. No long range magnetic ordering was observed down to 1.8 K by SQUID measurements. The magnetic properties of γ-CuMoO 4 can be explained by a chain-like arrangement of copper-moments deduced from the crystal structure. If only interactions within such chains are considered, the calculations based on an Ising-model are in good agreement with experimental data.

Crystal and magnetic structure of α-NiMoO4☆

Abstract The crystal and magnetic structures of α-NiMoO4 were determined by single-crystal X-ray diffraction on the one hand, and elastic neutron powder scattering on the other. The compound was prepared by subsolidus reaction and crystallizes with Ni and Mo ions in oxygen octahedra, space group C2/m. Below 19 K antiferromagnetic ordering is observed. The magnetic unit cell has a doubled c-axis, i.e. k = (0,0, 1 2 ) , and the magnetic space group is C2c2/m′. The Ni ions occupy two different sites, so that two sublattices have to be distinguished. By neglecting the small canting angle between these sublattices, the magnetic structure is collinear and all moments within one chemical cell are parallel. The magnetic structure can be described by a sequence of such cells with alternating signs along the c-direction. At 1.5 K the absolute values of the magnetic moments and their canting angles with the a-axis (in the + a, + c quadrant) are determined to be |μ(Ni2+)| = 1.62(10)μB and 75(10)° for the ions on mirror planes (Ni(1) site) and to |μ(Ni2+)| = 1.45(15)μB and 55(10)° for the ions on screw axes (Ni(2) site). From a Curie-Weiss fit in the paramagnetic region a Curie-Weiss temperature of 22(2) K and a magnetic moment of 3.05(5)μB for each Ni2+ ion are derived. These values are in good agreement with those predicted by Hunds rules.

Crystal structures and magnetic properties of the high-pressure modifications of CoMoO4 and NiMoO4

Abstract The wolframite structure with β 90° is observed for the high-pressure modifications of CoMoO4(II) and NiMoO4(II). Atomic parameters are reported as derived from X-ray powder diffraction with Rietveld refinement. Both compounds were prepared at a pressure of 50 kbar at 873 K from their α-phases. Antiferromagnetic ordering is observed below 68(1) K for CoMoO4(II) and 83(1) K for NiMoO4(II). At higher temperatures both compounds obey a Curie-Weiss law with Θ = − 125(5) K and |μ|(Co2+) = 5.4(1)μB for CoMoO4(II) and Θ = −145(5) K and |μ|(Ni2+) = 3.7(1)μB for NiMoO4(II).

Magnetic phase diagram of CoNb2O6: A neutron diffraction study

Elastic neutron diffraction experiments were performed using both a powder sample and a single crystal of CoNb2O6. An intermediate and in the b-direction incommensurate magnetic structure between 2.95 and 1.97 K was found by a least-squares refinement to be sinusoidally modulated with moments lying in the a-c plane, including an angle of γ = 29.6° with the c-axis. The temperature-dependent propagation vector is k = (0 kγ 0) with 0.37 ≤ kγ ≤ 0.5 for 2.95 K ≥ T ≥ 1.97 K. The magnetic phase diagram for an external field applied in the c-axis direction was determined. With increasing field, two spin-flip phases could be found. Their propagation vectors are k = (0 13 0) at lower fields and k = (0 12 0) at higher fields.

Magnetic phase diagrams of CoNb2O6

The ternary transition metal oxide CoNb2O6 is a quasi-one-dimensional magnetic material build by zig-zag chains with ferromagnetic intrachain interaction. The chains are weakly coupled by competing antiferromagnetic interactions exhibiting a sinusoidally modulated incommensurate phase, an antiferromagnetic ground state and several field induced spin-flip phases with n-fold unit cell. The magnetic phase diagrams obtained by neutron diffraction for several field directions are presented.

Magnetism in and

Results of a neutron diffraction study on powder samples of and at 1.3 K and on a single crystal of flux-grown at 1.5 K are given. The magnetization and susceptibility of single crystals have been measured for both compounds from 2.0 K to room temperature. In addition a Mossbauer spectrum of was taken at 4.2 K, and the specific heat of has been measured from 0.5 K up to 25 K. and exhibit antiferromagnetic order below 4.9 K and 5.7 K, respectively, with a canted magnetic structure. The powder samples reveal two propagation vectors, , and , for as well as for . The magnetization measurements are interpreted in the mean-field approximation by taking the crystal field, spin - orbit coupling, isotropic Heisenberg exchange, magnetic dipole - dipole interaction and an external field into account. Within this model very good agreement between theory and experiment was obtained. By reproducing the antiferromagnetic - paramagnetic phase transitions with an applied external field parallel to the a- or c-direction an estimation of the effective exchange interaction between zigzag chains running along the c-direction is given. The magnetic structures, as derived from model considerations, agree with neutron diffraction results.

Magnon dispersion and magnetic phase diagram of MnWO4

Abstract The magnetic phase diagram of MnWO4 is derived for several field directions from the temperature and field dependence of magnetization, the specific heat in applied fields and by neutron diffraction. Furthermore, we report about magnon dispersion in MnWO4, determined by inelastic neutron scattering and calculated analytically in spin-wave approximation.