Structural and magnetic properties of an InGaAs/Fe3_3Si superlattice in cylindrical geometry

The structure and the magnetic properties of an InGaAs/Fe3Si superlattice in a cylindrical geometry are investigated by electron microscopy techniques, x-ray diffraction and magnetometry. To form a radial superlattice, a pseudomorphic InGaAs/Fe3As bilayer has been released from its substrate self-forming into a rolled-up microtube. Oxide-free interfaces as well as areas of crystalline bonding are observed and an overall lattice mismatch between succeeding layers is determined. The cylindrical symmetry of the final radial superlattice shows a significant effect on the magnetization behavior of the rolled-up layers

Search For Spin-lattice Coupling Mediated By Itinerant Electrons: Synchrotron X-ray Diffraction And Raman Scattering From Gd Al3

The coupling among the spin degree of freedom and the atomic displacements in intermetallic Gd Al3 was investigated by means of synchrotron x-ray diffraction and polarized Raman scattering. In this compound, the Gd 4 f7 shell is spherical and the spin-lattice coupling provides a fingerprint of the exchange mechanism and degree of magnetic correlations. X-ray diffraction shows nonresonant symmetry-forbidden charge Bragg peaks below the long-range magnetic ordering temperature TN =18 K, revealing a symmetry-lowering crystal lattice transition associated with Gd displacements, consistent with a Ruderman-Kittel-Kasuya-Yosida mechanism for the magnetic coupling. Raman scattering in fresh broken surfaces shows phonons with conventional frequency behavior, while naturally grown and polished surfaces present frequency anomalies below T* ∼50 K. Such anomalies are possibly due to a modulation of the magnetic energy by the lattice vibrations in a strongly spin-correlated paramagnetic phase. Such interpretation implies that the spin-phonon coupling in metals may depend on the surface conditions. A fully spin-correlated state immediately above TN is inferred from our results in this frustrated system. © 2008 The American Physical Society.772(1999) Physics of Manganites, , edited by T. A. Kaplan and S. D. Mahanti (Springer, New YorkCheong, S.-W., Mostovoy, M., (2007) Nat. Mater., 6, p. 13. , See, for example, NMAACR 1476-1122 10.1038/nmat1804De Campos, A., Rocco, D.L., Carvalho, A.M.G., Caron, L., Coelho, A.A., Gama, S., Da Silva, L.M., De Oliveira, N.A., (2006) Nat. Mater., 5, p. 802. , NMAACR 1476-1122 10.1038/nmat1732Azimonte, C., Cezar, J.C., Granado, E., Huang, Q., Lynn, J.W., Campoy, J.C.P., Gopalakrishnan, J., Ramesha, K., (2007) Phys. Rev. Lett., 98, p. 017204. , PRLTAO 0031-9007 10.1103/PhysRevLett.98.017204Baltensperger, W., Helman, J.S., (1968) Helv. Phys. 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Direct strain and elastic energy evaluation in rolled-up semiconductor tubes by x-ray micro-diffraction

We depict the use of x-ray diffraction as a tool to directly probe the strain status in rolled-up semiconductor tubes. By employing continuum elasticity theory and a simple model we are able to simulate quantitatively the strain relaxation in perfect crystalline III-V semiconductor bi- and multilayers as well as in rolled-up layers with dislocations. The reduction in the local elastic energy is evaluated for each case. Limitations of the technique and theoretical model are discussed in detail.Comment: 32 pages (single column), 9 figures, 39 reference

Concept mapping as a tool to break disciplinary boundaries: isomerism in biological systems

O mapeamento conceitual foi utilizado como uma ferramenta para verificar as mudanças conceituais de estudantes de Ensino Médio após a realização de atividades didáticas desenvolvidas durante as aulas de Química. O objetivo pedagógico a ser atingido foi romper as fronteiras que segregam o conhecimento científico em disciplinas isoladas. Os estudantes foram intencionalmente provocados a relacionar conceitos de Química e de Biologia, a fim de compreender melhor e explicar as conseqüências biológicas da isomeria. Os mapas conceituais elaborados pelos estudantes, antes e após as atividades propostas, evidenciaram o aparecimento de relações entre conceitos químicos e biológicos, que foram avaliadas qualitativamente. Este trabalho mostra que os mapas conceituais podem ser utilizados como ferramentas para auxiliar o professor na realização de práticas didáticas interdisciplinares na escola, bem como para acompanhar o progresso dos estudantes em direção à interdisciplinaridade.Concept mapping was used as a tool for checking the conceptual changes caused by didactic activities implemented during chemistry classes in high school. Its pedagogical aim was to break down the boundaries, which segregate scientific knowledge into isolated disciplines. The students were intentionally provoked to merge concepts from chemistry and biology, in order to better understand and explain the biological consequences of isomerism. The concept maps produced by the students before and after the activities confirmed the appearance of relationships among chemical and biological concepts, which were qualitatively evaluated. This work shows that concept maps can be used to follow the students' progress towards interdisciplinarity, and to help the teacher to devise future classroom activities to reinforce and to expand interdisciplinary relationships.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) - PIBI

Magnetic structure and critical behavior of GdRhIn5_{5}: resonant x-ray diffraction and renormalization group analysis

The magnetic structure and fluctuations of tetragonal GdRhIn5 were studied by resonant x-ray diffraction at the Gd LII and LIII edges, followed by a renormalization group analysis for this and other related Gd-based compounds, namely Gd2IrIn8 and GdIn3. These compounds are spin-only analogs of the isostructural Ce-based heavy-fermion superconductors. The ground state of GdRhIn5 shows a commensurate antiferromagnetic spin structure with propagation vector tau = (0,1/2, 1/2), corresponding to a parallel spin alignment along the a-direction and antiparallel alignment along b and c. A comparison between this magnetic structure and those of other members of the Rm(Co,Rh,Ir)n In3m+2n family (R =rare earth, n = 0, 1; m = 1, 2) indicates that, in general, tau is determined by a competition between first-(J1) and second-neighbor(J2) antiferromagnetic (AFM) interactions. While a large J1 /J2 ratio favors an antiparallel alignment along the three directions (the so-called G-AFM structure), a smaller ratio favors the magnetic structure of GdRhIn5 (C-AFM). In particular, it is inferred that the heavy-fermion superconductor CeRhIn5 is in a frontier between these two ground states, which may explain its non-collinear spiral magnetic structure. The critical behavior of GdRhIn5 close to the paramagnetic transition at TN = 39 K was also studied in detail. A typical second-order transition with the ordered magnetization critical parameter beta = 0.35 was experimentally found, and theoretically investigated by means of a renormalization group analysis.Comment: 22 pages, 4 figure

Search for spin-lattice coupling mediated by itinerant electrons: Synchrotron x-ray diffraction and Raman scattering from GdAl(3)

The coupling among the spin degree of freedom and the atomic displacements in intermetallic GdAl(3) was investigated by means of synchrotron x-ray diffraction and polarized Raman scattering. In this compound, the Gd 4f(7) shell is spherical and the spin-lattice coupling provides a fingerprint of the exchange mechanism and degree of magnetic correlations. X-ray diffraction shows nonresonant symmetry-forbidden charge Bragg peaks below the long-range magnetic ordering temperature T(N)=18 K, revealing a symmetry-lowering crystal lattice transition associated with Gd displacements, consistent with a Ruderman-Kittel-Kasuya-Yosida mechanism for the magnetic coupling. Raman scattering in fresh broken surfaces shows phonons with conventional frequency behavior, while naturally grown and polished surfaces present frequency anomalies below T*similar to 50 K. Such anomalies are possibly due to a modulation of the magnetic energy by the lattice vibrations in a strongly spin-correlated paramagnetic phase. Such interpretation implies that the spin-phonon coupling in metals may depend on the surface conditions. A fully spin-correlated state immediately above T(N) is inferred from our results in this frustrated system.77