Martin Boehm

Formation of a magnetic soliton lattice in copper metaborate.

The magnetic ground state of CuB2O4 is incommensurate at T = 1.8 K and undergoes a continuous phase transition to a noncollinear commensurate antiferromagnetic state at T(small star), filled approximately 10 K. Close to T(small star), filled higher-order magnetic satellites are observed. Coexistence of long- and short-range magnetic order is observed in both magnetic phases. This suggests that the association of the Dzyaloshinskii-Moriya interaction and anisotropy leads to the formation of a magnetic soliton lattice.

Magnetoelastic coupling in the triangular lattice antiferromagnet CuCrS2

, as shown in the inset of Fig.1. The nearest-neighbor intralayer Cr-Cr distance of 3.48 Ais small compared to the interlayer Cr-Cr distance of 6.55 A.The magnetic exchange within the layers over Cr-S-Cr path-ways is expected to be much stronger than the interlayerexchange via Cr-S-Cu-S-Cr bonds. Hence, one would expectto find the crystalline anisotropy to be reflected in the mag-netic system, as described in the dioxides. Nevertheless,early neutron powder diffraction experiments on CuCrS

Electron spin resonance in CuCrS2 chrome-copper disulphides synthesized by different methods

The electron spin resonance (ESR) in CuCrS2 disulphides is found to be strongly dependent on a synthesis method used. At a temperature of 300 K, a polycrystalline CuCrS2 sample is paramagnetic with a g-value of 1.95 at 40 K, it undergoes the magnetic transition. In the temperature range 4.2–290 K, a single-crystal sample prepared by a chemical vapor transport method exhibits the ESR features typical of a ferromagnet. It is shown that these features are related to the presence of a small amount of the single-crystal CuCr2S4 impurity in the CuCrS2 single crystal.

Structural properties of Pb3Mn7O15 determined from high-resolution synchrotron powder diffraction

Abstract We report on the crystallographic structure of the layered compound Pb 3 Mn 7 O 15 . Previous analysis based on laboratory X-ray data at room temperature gave contradictory results in terms of the description of the unit cell. Motivated by recent magnetic bulk measurements of this system [N.V. Volkov, K.A. Sablina, O.A. Bayukov, E.V. Eremin, G.A. Petrakovskii, D.A. Velikanov, A.D. Balaev, A.F. Bovina, P. Boni, E. Clementyev, J. Phys. Condens. Matter 20 (2008) 055217], we re-investigated the chemical structure with high-resolution synchrotron powder diffraction at temperatures between 15 and 295xa0K. Our results show that the crystal structure of stoichiometric Pb 3 Mn 7 O 15 has a pronounced 2-dimensional character and can be described in the orthorhombic space group Pnma.

Structural properties of Pb{sub 3}Mn{sub 7}O{sub 15} determined from high-resolution synchrotron powder diffraction

Abstract We report on the crystallographic structure of the layered compound Pb 3 Mn 7 O 15 . Previous analysis based on laboratory X-ray data at room temperature gave contradictory results in terms of the description of the unit cell. Motivated by recent magnetic bulk measurements of this system [N.V. Volkov, K.A. Sablina, O.A. Bayukov, E.V. Eremin, G.A. Petrakovskii, D.A. Velikanov, A.D. Balaev, A.F. Bovina, P. Boni, E. Clementyev, J. Phys. Condens. Matter 20 (2008) 055217], we re-investigated the chemical structure with high-resolution synchrotron powder diffraction at temperatures between 15 and 295xa0K. Our results show that the crystal structure of stoichiometric Pb 3 Mn 7 O 15 has a pronounced 2-dimensional character and can be described in the orthorhombic space group Pnma.

Dynamics of the two-dimensional S=1/2 dimer system (C5H6N2F)2CuCl4

Inelastic neutron scattering was used to study a quantum S=1/2 antiferromagnetic Heisenberg system Bis(2-amino-5-fluoropyridinium) Tetrachlorocuprate(II). The magnetic excitation spectrum was shown to be dominated by long-lived excitations with an energy gap 1.07(3) meV. The measured dispersion relation is consistent with a simple two-dimensional square lattice of weakly-coupled spin dimers. Comparing the data to a random phase approximation treatment of this model gives the intra-dimer and inter-dimer exchange constants J=1.45(2) meV and J =0.31(3) meV, respectively.

Magnetic Structure and Excitations in CeCuxAl4–x System

CeCuAl3 crystallizing in the tetragonal BaNiSn3-type structure and CeCuxAl4-x solid solutions were investigated by means of elastic and inelastic neutron scattering. Powder neutron diffraction brought information on both temperature evolution of crystallographic parameters and magnetic order at low temperatures. No structural change was observed in the investigated temperature range from 1.5 to 300 K. Weak magnetic peaks outside nuclear Bragg positions observed in solid solutions with 0.90 ≤ x ≤ 1.10 were described by the propagation vector k = (0.40 + δx, 0.60 + δy, 0), where δx ≈ 0.02 and δy ≈ 0.01. The magnetic structure of CeCu0.75Al3.25 consists of two components: an anti-ferromagnetic one described by the same k and a ferromagnetic one with k0 = (0, 0, 0) and magnetic moments lying within the tetragonal basal plane. The evolution of magnetic excitations as a function of Cu-Al concentration in CeCuxAl4-x was studied by inelastic neutron scattering. The measured spectra of CeCuAl3 and the solution with x = 0.95 point to a three-magnetic-peak energy scheme, while only two excitations are expected from the local symmetry conditions on Ce atoms. The standard two-peak spectrum of crystal electric field excitations was observed for Cu-Al substitutions further from the 1:1:3 stoichiometry (x = 0.75 and 1.10). The intermediate concentrations (x = 0.90 and 1.05) exhibit spectra on the border between the former cases with a less clear pronounced first inelastic magnetic peak. The observed behavior is discussed considering the evolution of structural parameters in the CeCuxAl4-x system and the coupling between the lattice vibrations and the crystal electric field excitations.

Bound states and field-polarized Haldane modes in a quantum spin ladder

The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin-ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modeling of the dynamical response demonstrates our complete quantitative understanding of these states.

Single-crystal and powder neutron diffraction study of the FeXMn1 XS solid solutions

FeXMn1-XS (0 <x< 0.3) synthesized on the basis of {alpha}-MnS are the novel Mott-type substances with the rock salt structure. Neutron diffraction study shows that the shift of the Neel temperature of these materials from 150 (x = 0) to 200 K (x = 0.29) under the action of chemical pressure (x) is accompanied by a decrease in the cubic NaCl lattice parameters. The structural transition with a change in crystal symmetry and a decrease in resistance before the magnetic transition at x = 0.25 is found. Therefore, FeXMn1-XS are interesting materials for both fundamental study of the interrelation between the magnetic, electrical, and structural properties in systems with the strong electron correlations of a MnO-type and application.

EFFECT OF MAGNETIC FIELD ON THE MAGNETIC STATE OF COPPER METABORATE

The influence of the external magnetic field on the magnetic state of copper metaborate CuB2O4 was investigated by means of magnetization, heat capacity and neutron diffraction measurements. It was found that the temperature of the magnetic phase transition from the commensurate to the incommensurate magnetic structure depends strongly on the value of the magnetic field. For temperatures below 10 K the magnetization of the crystal increases sharply at the critical magnetic field. A theory based on a symmetry analysis and using a phenomenological approach is developed to describe the field dependence of copper metaborate. The model uses two order parameters corresponding to ferromagnetic and antiferromagnetic moments directed in the tetragonal plane of the crystal. For the description of the incommensurate magnetic structure we introduced the modified Lifshits invariant for the case of two twocomponent order parameters. This theory gives a satisfactory description of the experimental data including the magnetic field-temperature phase diagram of copper metaborate.