The magnetic properties and structure of the quasi-two-dimensional antiferromagnet CoPS3.

The magnetic properties and magnetic structure are presented for CoPS3, a quasi-two-dimensional antiferromagnet on a honeycomb lattice with a Néel temperature of [Formula: see text] K. The compound is shown to have XY-like anisotropy in its susceptibility, and the anisotropy is analysed to extract crystal field parameters. For temperatures between 2 K and 300 K, no phase transitions were observed in the field-dependent magnetization up to 10 Tesla. Single-crystal neutron diffraction shows that the magnetic propagation vector is k = [Formula: see text] with the moments mostly along the [Formula: see text] axis and with a small component along the [Formula: see text] axis, which largely verifies the previously-published magnetic structure for this compound. The magnetic Bragg peak intensity decreases with increasing temperature as a power law with exponent [Formula: see text] for [Formula: see text].

Synthesis and characterisation of new Bi(iii)-containing apatite-type oxide ion conductors: the influence of lone pairs.

Lone-pair cations are known to enhance oxide ion conductivity in fluorite- and Aurivillius-type materials. Among the apatite-type phases, the opposite trend is found for the more widely studied silicate oxide ion conductors, which exhibit a dramatic decrease in conductivity on Bi(iii) incorporation. In this work, the influence of lone-pair cations on the properties of apatite-type germanate oxide ion conductors has been investigated by preparing and characterising seven related compositions with varying Bi(iii) content, by X-ray and neutron powder diffraction and impedance spectroscopy. All materials are very good oxide ion conductors (with conductivities of up to 1.29 × 10-2 S cm-1 at 775 °C). Increasing Bi(iii) content leads to increases in conductivity by up to an order of magnitude, suggesting significant differences in the oxide-ion conduction mechanisms between lone-pair-containing apatite-type germanate and silicate solid electrolytes.

Understanding the Unusual Response to High Pressure in KBe2BO3F2.

Strong anisotropic compression with pressure on the remarkable non-linear optical material KBe2BO3F2 has been observed with the linear compression coefficient along the c axis found to be about 40 times larger than that along the a axis. An unusual non-monotonic pressure response was observed for the a lattice parameter. The derived bulk modulus of 31 ± 1 GPa indicates that KBe2BO3F2 is a very soft oxide material yet with stable structure up to 45 GPa. A combination of high-pressure synchrotron powder X-ray diffraction, high-pressure Raman spectroscopy, and Density Functional Theory calculations points to the mechanism for the unusual pressure response being due to the competition between the K-F bond length and K-F-K bond angle and the coupling between the stretching and twisting vibration modes.

Neutron Laue and X-ray diffraction study of a new crystallographic superspace phase in n-nonadecane-urea.

Aperiodic composite crystals present long-range order without translational symmetry. These materials may be described as the intersection in three dimensions of a crystal which is periodic in a higher-dimensional space. In such materials, symmetry breaking must be described as structural changes within these crystallographic superspaces. The increase in the number of superspace groups with the increase in the dimension of the superspace allows many more structural solutions. This is illustrated in n-nonadecane-urea, revealing a fifth higher-dimensional phase at low temperature.

Rapid neutron-diffraction data collection for hydrogen-bonding studies: application of the Laue diffractometer (LADI) to the case study zinc (tris)thiourea sulfate.

The successful application of the newly developed image-plate neutron Laue diffractometer (LADI) at the Institut Laue-Langevin (ILL), Grenoble, France, for rapid hydrogen-bonding characterization is reported. The case study concerns the promising non-linear optical material zinc (tris)thiourea sulfate (ZTS), which contains 30 atoms in the asymmetric unit and crystallizes in the orthorhombic space group, Pca2(1), a = 11.0616 (9), b = 7.7264 (6), c = 15.558 (1) A [T = 100.0 (1) K]. The results from a 12 h data collection from ZTS on LADI are compared with those obtained over 135 h using the monochromatic four-circle diffractometer D9 at the same reactor source with a crystal 13 times larger in volume. Both studies reveal the extensive hydrogen bonding and other close non-bonded contacts within the material. As expected, the results from D9 are more precise than those obtained from LADI; however, the bond geometry determined from the two experiments is the same within the larger estimated standard deviations. Furthermore, the conclusions drawn from the two studies separately regarding the nature of all supramolecular features are identical. This illustrates that LADI is eminently suitable for rapid characterization of hydrogen-bonded structures by neutron diffraction, with the gain in speed compared with traditional instrumentation being several orders of magnitude.

The antiferromagnetic structures of IrMn3 and their influence on exchange-bias.

We have determined the magnetic structures of single-crystal thin-films of IrMn3 for the crystallographic phases of chemically-ordered L12, and for chemically-disordered face-centred-cubic, which is the phase typically chosen for information-storage devices. For the chemically-ordered L12 thin-film, we find the same triangular magnetic structure as reported for the bulk material. We determine the magnetic structure of the chemically-disordered face-centred-cubic alloy for the first time, which differs from theoretical predictions, with magnetic moments tilted away from the crystal diagonals towards the face-planes. We study the influence of these two antiferromagnetic structures on the exchange-bias properties of an epitaxial body-centred-cubic Fe layer showing that magnetization reversal mechanism and bias-field in the ferromagnetic layer is altered significantly. We report a change of reversal mechanism from in-plane nucleation of 90° domain-walls when coupled to the newly reported cubic structure towards a rotational process, including an out-of-plane magnetization component when coupled to the L12 triangular structure.

Fine-tuning of the spin-density-wave state in Cr/V heterostructures via hydrogen uptake.

We show that the magnetic state in rather thick Cr films can be finely tuned via hydrogen uptake into adjacent vanadium layers at rather low hydrogen pressures. By changing the hydrogen concentration and, hence, the electronic structure in the V layers, it is possible to affect the global properties of spin-density waves (SDWs) in Cr layers, including the SDW period and the Néel temperature. We provide direct experimental evidence that hydrogen uptake into V layers can be used to switch between incommensurate and commensurate SDW states in a reproducible way.

Perpendicular antiferromagnetic ordering of Mn and exchange anisotropy in Fe/Mn multilayers.

Fe/Mn is a model system in which to study exchange bias, since the antiferromagnetic (AF) Mn layers are believed to have uncompensated moments with all spins aligned in the plane and parallel to those of the Fe. We have determined the microscopic AF ordering at the interfaces using single-crystal neutron diffraction. An unexpected magnetic structure is obtained, with out-of-plane Mn moments perpendicular to those of Fe. This explains the low bias field and shows that the simple AF ordering assumed in a variety of exchange-biased systems may well have to be revised.

Long-range ferromagnetism of Mn12 acetate single-molecule magnets under a transverse magnetic field.

We use neutron diffraction to probe the magnetization components of a crystal of Mn12 single-molecule magnets. Each of these molecules behaves, at low temperatures, as a nanomagnet with spin S = 10 and strong anisotropy along the crystallographic c axis. The application of a magnetic field H(perpendicular) perpendicular to c induces quantum tunneling between opposite spin orientations, enabling the spins to attain thermal equilibrium. For T approximately < 0.9(1) K, this equilibrium state shows spontaneous magnetization, indicating the onset of ferromagnetism. These long-range magnetic correlations nearly disappear for mu0H(perpendicular) approximately > 5.5 T, possibly suggesting the existence of a quantum critical point.

Study of ethanol-lysozyme interactions using neutron diffraction.

Single-crystal neutron diffraction has been used to observe the interactions between deuterated ethanol (CD3CD2OH) and lysozyme in triclinic crystals of hen egg white lysozyme soaked in 25% (v/v) ethanol solutions. A total of 6047 observed reflections to a resolution of 2 A were used, and 13 possible ethanol sites were identified. The three highest occupied sites are close to locations for bromoethanol found in an earlier study by Yonath et al. [Yonath, A., Podjarny, A., Honig, B., Traub, W., Sielecki, A., Herzberg, O., & Moult, J. (1978) Biophys. Struct. Mech. 4, 27-36]. Structure refinements including a model for the flat solvent lead to a final crystallographic agreement factor of 0.097. Comparison with earlier neutron studies on triclinic lysozyme showed that neither the molecular structure nor the thermal motions were affected significantly by the ethanol. A detailed analysis of the ethanol-lysozyme contacts showed 61% of these to be with hydrophobic sites, in agreement with the dominant hydrophobic nature of ethanol. This, together with the fact that the molecular structure of lysozyme is not perturbed, suggests a model for denaturation of lysozyme by alcohol, which proceeds via a dehydration of the protein at high alcohol concentration.

Influence of hydrogen bonding on the second harmonic generation effect: neutron diffraction study of 4-nitro-4'-methylbenzylidene aniline.

A neutron diffraction study of the non-linear optical (NLO) material 4-nitro-4'-methylbenzylidene aniline (NMBA) is presented. NMBA exhibits a large macroscopic second-order NLO susceptibility, chi((2)), and this study shows that hydrogen bonding is, in part, responsible for this. No hydrogen bonding was reported in the X-ray study [Ponomarev et al. (1977). Sov. Phys. Crystallogr. 22, 223-225], whereas the present work shows that C-H.X hydrogen bonds (where X = N, O or pi) direct the nature of the three-dimensional lattice. C-H.X (X = N or O) hydrogen bonds are common; however, C-H.pi hydrogen-bond motifs are relatively rare. Such intermolecular interactions help extend the molecular charge transfer into the supramolecular realm, the charge transfer originating as a consequence of the high level of molecular planarity and strong donor-to-acceptor interactions. Molecular planarity, coupled with the favourable nature of the hydrogen bonds, results in parallel stacking of molecules in both the a and c crystallographic directions with extremely close interplanar spacings. Such a combination of influential hydrogen-bonding characteristics accounts, in part, for the large second-order NLO output of the material since the phenomenon is so critically dependent upon the nature of the charge transfer.

Polarized-neutron scattering study of the cooper-pair moment in Sr2RuO4.

We report a study of the magnetization density in the mixed state of the unconventional superconductor Sr2RuO4. On entering the superconducting state we find no change in the magnitude or distribution of the induced moment for a magnetic field of 1 T applied within the Ru O2 planes. Our results are consistent with a spin-triplet Cooper pairing with spins lying in the basal plane. This is in contrast with similar experiments performed on conventional and high- T(c) superconductors.

Pressure-enhanced 3D antiferromagnetic correlations in La(1.4)Sr(1.6)Mn(2)O(7).

Pressure effects on the stability of magnetic phases in La(1.4)Sr(1.6)Mn(2)O(7) have been studied using magnetization measurements and neutron diffraction. At ambient conditions this material is a quasi-two-dimensional ferromagnet. On cooling it becomes ordered three dimensionally: at 90 K La(1.4)Sr(1.6)Mn(2)O(7) it becomes an antiferromagnet, and at 65 K it undergoes a transition into a ferromagnetic phase. Using neutron diffraction techniques on a single crystal of La(1.4)Sr(1.6)Mn(2)O(7) it has been shown that these two magnetic phases belong to a single structural phase and do not coexist at low temperatures. The application of pressure enhances the antiferromagnetic correlations between the Mn(2)O(9) bilayers.