Magnetic structure and physical properties of the multiferroic compound PrMn2O5

RMn₂O₅ (R=lanthanide, Bi, Y) multiferroic compounds are intensively studied for their potential application in the spintronic field. In these systems, the key issue is to understand the origin of the strong coupling between the ferroelectric and magnetic orders and to investigate the influence of the nature of the R ions in this coupling. While the phase diagram of RMn₂O₅ compounds with small R size is well established, this of large R size compounds is missing due to the lack of samples originating with difficulties of synthesis. We present in this paper the first investigation of the thermodynamic, structural and magnetic properties of high quality polycrystalline PrMn₂O₅ samples. Our work shows that PrMn₂O₅ presents two magnetic transitions corresponding to commensurate magnetic orderings. We also evidence a weak lattice effect coupled to the magnetic order. Our results point out that the physical properties of PrMn₂O₅ differ from those of the parent compounds with magnetic R ions.     

Neutron diffraction study of the magnetic ordering in the series R 2 BaNiO 5 (R = Rare Earth)

A neutron diffraction study, as a function of temperature, of the title compounds is presented. The whole family (space group Immm, a ≈ 3.8?, b ≈ 5.8?, c ≈ 11.3?) is structurally characterised by the presence of flattened NiO₆ octahedra that form chains along the a-axis, giving rise to a strong Ni-O-Ni antiferromagnetic interaction. Whereas for Y-compound only strong 1D correlations exist above 1.5 K, presenting the Haldane gap characteristic of 1D AF chain with integer spin, 3D AF ordering is established simultaneously for both R and Ni sublattices at temperatures depending on the rare earth size and magnetic moment. The magnetic structures of R₂BaNiO₅ ( R = Nd, Tb, Dy, Ho, Er and Tm) have been determined and refined as a function of temperature. The whole family orders with a magnetic structure characterised by the temperature-independent propagation vector = (1/2, 0, 1/2). At 1.5 K the directions of the magnetic moments differ because of the different anisotropy of the rare earth ions. Except for Tm and Yb (which does not order above 1.5 K), the magnetic moment of the R³⁺ cations are close to the free-ion value. The magnetic moment of Ni²⁺ is around 1.4 , the strong reduction with respect to the free-ion value is probably due to a combination of low-dimensional quantum effects and covalency. The thermal evolution of the magnetic structures from T _N down to 1.5 K is studied in detail. A smooth re-orientation, governed by the magnetic anisotropy of R³⁺, seems to occur below and very close to T _N in some of these compounds: the Ni moment rotates from nearly parallel to the a-axis toward the c-axis following the R moments. We demonstrate that for setting up the 3D magnetic ordering the R-R exchange interactions cannot be neglected. Received 19 July 2001     

Trifold arrangements and cevian dissections

We determine precisely how many triple points can be formed in the plane by an arrangement of n lines lying in three parallel families of p, q, r lines, respectively. Using this result we solve the Euclidean realization problem for such arrangements. We apply our results to solve an analogous problem in which a triangle is dissected by three families of cevians. We conclude by mentioning some related unsolved problems. Received 19 August 1999.     

Moment-volume instabilities in alloys with compositions around the C14 laves phase

We measured the thermal expansion and the specific heat of Ti_xFe_100-x alloys with x = 30.5, 32.5 and 35, all with hexagonal C14 laves phase structure (MgZn₂) like TiFe₂, and determine the temperature dependence of the magnetic contributions to the thermal expansion and the specific heat c_mag. For fixed composition and c mag ( T ) show the same type of behavior, demonstrating that both anomalies are of the same microscopic nature. They originate from moment-volume fluctuations (antiferromagnetic Invar-effect) as a comparison with total energy calculations as a function of atomic volume and moment for TiFe₂ reveals. Received: 26 January 1998 / Accepted: 17 April 1998     

Spin density in Co65Pd35 alloy

Summary The spin density in Co₆₅Pd₃₅ disordered alloy has been determined by means of polarized neutron scattering. The asphericity of spin distribution has been analysed and the individual contributions of both Co and Pd have been deduced.

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Riassunto Si è determinata la densità di spin nella lega disordinata Co₆₅Pd₃₅ per mezzo della diffusione dei neutroni polarizzati. L'asfericità della distribuzione dello spin è stata analizzata deducendo i contributi separati di Co e Pd.

     

Strategic planning decisions in an R & D setting

Strategic planning, by nature, is concerned with the organization's interaction with its environment and is normative because it deals with organizational motives and self image. The salient problem confronting strategic planning is to reflect the abstract and implicit value system held by management within a disciplined and informed analytical methodology.The Gas Research Institute has developed a strategic planning process, centered upon the annual development of a projection of the U.S. energy/economic situation. The process, which requires collaboration of top management and analytical staff, brings current environmental data and sophisticated methodology to bear on the outlook for the gas industry and gas-related technologies, but it ensures that analytical judgements will be influenced and, if necessary, overriden by current management values. Contradictions between managament perceptions and objective analysis are raised and resolved in an explicit process.     

Spontaneous surface magnetisation of single crystal MnF2 in the antiferromagnetic state

Spontaneous magnetisation of (100) and (010) surfaces of single crystal MnF₂ in the antiferromagnetic state has been discovered. The sign of the surface magnetisation is determined by the difference in dielectric constants of MnF₂ and ambient matter: magnetisation is directed to the substance with smaller . Received 28 August 1998 and Received in final form 15 December 1998     

Metal–organic supramolecular compounds self-assembled from [Mg(H2O)6]2+ clusters and ferrocene-containing carboxylic acids to inverse lipid-like layered structures with good pH response

Two metal–organic supramolecular compounds, [Mg(H₂O)₆]·2FcCO₂·2H₂O (1) and [Mg(H₂O)₆]·2FcCOC₆H₄CO₂·2H₂O (2), were prepared by solvothermal synthesis. The solids exhibit non-covalent self-assembly and layered structures with unprecedented inverse lipid-like bilayer arrangements. Further investigations revealed that the layered structures were sensitive to pH changes. The proposed synthesis method in this report would allow development of new types of pH-responsive materials.