131 research outputs found
Atomic origin of magnetocrystalline anisotropy in Nd2Fe14B
No full textThe magnetic moment reversal at each of the two inequivalent Nd sites in a single crystal of ferromagnetic Nd2Fe14B is probed by dichroic resonant diffraction of circularly polarized x rays. The results, supported by theory, show that the c-axis intrinsic magnetic stability of this superior permanent magnetic material arises predominately at one of the Nd sites (g). The other site (f) undermines magnetic stability by favoring a magnetic moment orientation in the basal plane.This article is published as Haskel, D., J. C. Lang, Z. Islam, A. Cady, G. Srajer, M. Van Veenendaal, and P. C. Canfield. "Atomic origin of magnetocrystalline anisotropy in Nd 2 Fe 14 B." Physical Review Letters 95, no. 21 (2005): 217207.
DOI: 10.1103/PhysRevLett.95.217207.
Copyright 2005 American Physical Society.
Posted with permission
Resonant x-ray scattering study of magnetic ordering due to Fermi-surface nesting in SmNi2Ge2
No full textWe report the results of x-ray resonant scattering studies of samarium nickel germanide (SmNi2Ge2) compound. We confirm that the magnetic structures of this material are in accordance with Fermi-surface nesting as was hypothesized earlier, SmNi2Ge2 orders in an incommensurate anti ferromagnetic (AF) structure characterized by a single propagation vector q = (0,0,q(z)). The value of q(z) is temperature dependent and approaches similar to0.775 r.l.u. near T-N = 17.7 K. Below T-t = 11.8 K, the AF structure is characterized by q(z) = 0.79 +/- 0.002, indicating a long-period-ordered phase. Ordered moments are confined to the basal plane in both the phases, as evidenced by the Q dependence of the magnetic-peak intensities. The temperature dependence of the magnetic structures is consistent with the "superzone" gap theory. In addition, a strong quadrupolar resonance below the Sm L-III edge was observed and compared to recent theoretical work.This article is published as Islam, Zahirul, J. C. Lang, L. Vasiliu-Doloc, G. Srajer, and P. C. Canfield. "Resonant x-ray scattering study of magnetic ordering due to Fermi-surface nesting in SmNi 2 Ge 2." Physical Review B 65, no. 5 (2001): 054404.
DOI: 10.1103/PhysRevB.65.054404.
Copyright 2002 American Physical Society.
Posted with permission
Investigation of the interaction of water with the calcite {1014} surface using ab-initio simulation
No full textDensity functional theory calculations were employed to explore the interaction between
water and the {1014} surface of calcite. In addition a defective {1014} surface
and stepped surfaces in contact with water were investigated. A series of percentage
water coverages and water con�figurations were explored, including dissociated water
states. Static relaxations found associated water to be favourable on the {1014}
surface, although a metastable dissociated state 1.77eV higher in energy was found.
Molecular dynamics (MD) simulations of low water coverage reveal fluctuations in
the H-O water bond when the H atom is directed towards a surface CO3 ion. Desorption
of an H2O molecule was observed in simulations above 900K. Water was found
to be strongly bound to the perfect {1014} surface, with an adsorption energy of
-0.91eV. MD simulations of a defective {1014} surface found water to favour dissociation
at CO3 vacancies. However, water at Ca vacancies di�used across the surface
to form a bond with the nearest surface Ca ion. Water was also found to favour
an associated state at both acute and obtuse steps. On all these imperfect surfaces
water was found to adsorb strongly to the surface, with adsorption energies ranging
from -0.99eV to -1.60eV
Magnetic structure of GdCo2Ge2
No full textResonant and nonresonant magnetic x-ray scattering studies of GdCo2Ge2 were performed to determine its magnetic structure at low temperature. This compound orders in an incommensurate antiferromagnetic (AF) structure characterized by a propagation wave vector tau=(0 0 tau(z)). The value of tau(z) is temperature dependent and approaches 0.930 reciprocal lattice units well below T-N=33.25 K. A peak corresponding to 3 tau(z) was also observed, indicating either a squaring up of the magnetic structure or the presence of a noncollinear amplitude modulated structure below T-N. Fitting the angular dependence of the magnetic scattering integrated intensities to the relevant resonant and nonresonant scattering cross sections revealed that the moment direction lies primarily in the tetragonal basal plane. Scattering measurements at the Co K- edge failed to detect any resonant signal, consistent with the absence of a magnetic moment on the Co sites.This article is published as Good, W., J. Kim, A. I. Goldman, D. Wermeille, P. C. Canfield, C. Cunningham, Z. Islam, J. C. Lang, G. Srajer, and I. R. Fisher. "Magnetic structure of Gd Co 2 Ge 2." Physical Review B 71, no. 22 (2005): 224427.
DOI: 10.1103/PhysRevB.71.224427.
Copyright 2005 American Physical Society.
Posted with permission
Microfluidic simulation of a colonial diatom chain reveals pumping behaviour
No full textDiatoms are single-celled organisms with rigid parts in relative motion at the micrometer scale and below. Some species such as Rutilaria philippinarum form colonies that are several cells long. Inspired by recent studies on the linking structures that connect sibling cells [1, see figure] a two-dimensional finite element model was established. In this model, the cell size is 140.m*34.m, the distance between cells varies from 10.m to 30.m, and the colony has infinite length: the model "unit cell" comprises ten cells with periodic boundary conditions. Undisturbed fluid flow between the single cells is allowed for in this first simple model. The cells do not move actively, and are solely moved by the water. The initial fluid velocity varies between 0.01m/s and 1m/s. Stationary solutions of the model starting from equidistant cells shows pairing of neighbouring cells. On the other hand, starting the calculations from already paired cells shows the opposite effect: the pairs tend to reach the equidistant state again. From this result it is concluded that the alternation between these two stationary states causes an oscillatory movement of the chain. These modelling attempts might have laid a bio-inspired basis for a novel type of pumps in the micrometer length scale [2,3].
[1] Gebeshuber I.C. and Crawford R.M. (2006) "Micromechanics in biogenic hydrated silica: hinges and interlocking devices in diatoms", Proc. IMechE Part J: J. Eng. Tribol. 220(J8), 787-796.
[2] Gebeshuber I.C. (2007) "Biotribology inspires new technologies", invited article, Nano Today 2(5), 30-37.
[3] Gebeshuber I.C. and Drack M. "An attempt to reveal synergies between biology and engineering mechanics", IMechE Part C: J. Mech. Eng. Sci., in press.
Microfluidic simulation of a colonial diatom chain reveals pumping behavior
No full textRutilaria philippinarum is a fossil colonial sea-water diatom that used to live in shallow waters [1]. Inspired by recent studies on the linking structures that connect sibling cells [2] a microfluidic computer model was created. This two-dimensional finite element model has the following assumptions: the cell size is 140 micrometers * 34 micrometers, the distance between cells varies from 10 micrometers to 30 micrometers, there are 10 cells with periodic boundary conditions. Undisturbed fluid flow between the single cells is allowed for in this first simple model. The cells do not move actively, and are solely moved by the water. The initial fluid velocity varies between 0.01m/s and 1m/s. Stationary solutions of the model starting from equidistant cells shows pairing of neighboring cells. On the other hand, starting the calculations from already paired cells shows the opposite effect: the pairs aspire the equidistant state again. From this result it is concluded that the alternation between these two stationary states causes an oscillatory movement of the chain. It still has to be shown if this effect also appears in a chain of living diatoms with the silica frustules covered with organic material. If it does, this "pumping behavior" might facilitate nutrient uptake. In any case, our modeling attempts might have laid a bio-inspired basis for a novel type of pumps in the micrometer length scale [3,4]. The appearance of such vibrations in fan wings is a similar effect on the meter length scale.
References:
[1] Ross, R. (1995) "A revision of Rutilaria Greville (Bacillariophyta)", Bulletin Natural History Museum, Botany series 25: 1-93.
[2] Gebeshuber I.C. and Crawford R.M. (2006) "Micromechanics in biogenic hydrated silica: hinges and interlocking devices in diatoms", Proc. IMechE Part J: J. Eng. Tribol. 220(J8), 787-796.
[3] Gebeshuber I.C. (2007) "Biotribology inspires new technologies", invited article, Nano Today 2(5), 30-37.
[4] Gebeshuber I.C. and Drack M. (2008) invited article, "An attempt to reveal synergies between biology and engineering mechanics", IMechE Part C: J. Mech. Eng. Sci. 222, 1281-1287.
Magnetic reconfiguration of MnAs∕GaAs(001) observed by magnetic force microscopy and resonant soft x-ray scattering
No full textWe investigated the thermal evolution of the magnetic properties of MnAs epitaxial films grown on GaAs(001) during the coexistence of hexagonal/orthorhombic phases using polarized resonant (magnetic) soft x-ray scattering and magnetic force microscopy. The results of the diffuse satellite x-ray peaks were compared to those obtained by magnetic force microscopy and suggest a reorientation of ferromagnetic terraces as temperature rises. By measuring hysteresis loops at these peaks we show that this reorientation is common to all ferromagnetic terraces. The reorientation is explained by a simple model based on the shape anisotropy energy. Demagnetizing factors were calculated for different configurations suggested by the magnetic images. We noted that the magnetic moments flip from an in-plane monodomain orientation at lower temperatures to a three-domain out-of-plane configuration at higher temperatures. The transition was observed when the ferromagnetic stripe width L is equal to 2.9 times the film thickness d. This is in good agreement with the expected theoretical value of L=2.6d. © 2006 American Institute of Physics.1008Wolf, S.A., Awschalom, D.D., Buhrman, R.A., Daughton, J.M., Von Molnas, S., Roukes, M.L., Chtchelkanova, A.Y., Treger, D.M., (1994) Science, 294, p. 1488Ramsteiner, M., (2002) Phys. Rev. B, 66, p. 081304Schippan, F., Trampert, A., Däweritz, L., Ploog, K.H., (1999) J. Vac. Sci. Technol. B, 17, p. 1716Ploog, K.H., (2002) J. Appl. Phys., 91, p. 7256Bean, C.P., Rodbell, D.S., (1962) Phys. Rev., 126, p. 104Kastner, M., Herrmann, C., Däweritz, L., Ploog, K.H., (2002) J. Appl. Phys., 92, p. 5711Kaganer, V.M., Jenichen, B., Schippan, F., Braun, W., Däweritz, L., Ploog, K.H., (2002) Phys. Rev. B, 66, p. 045305Kaganer, V.M., Jenichen, B., Schippan, F., Braun, W., Däweritz, L., Ploog, K.H., (2000) Phys. Rev. Lett., 85, p. 341Jenichen, B., Kaganer, V.M., Herrmann, C., Wan, L., Däweritz, L., Ploog, K.H., (2004) Z. Kristallogr., 219, p. 201Plake, T., Hesjedal, T., Mohanty, J., Kastner, M., Däweritz, L., Ploog, K.H., (2003) Appl. Phys. Lett., 82, p. 2308Ney, A., Hesjedal, T., Pampuch, C., Mohanty, J., Das, A.K., Däweritz, L., Koch, R., Ploog, K.H., (2003) Appl. Phys. Lett., 83, p. 2850Engel-Herbert, R., Mohanty, J., Ney, A., Hesjedal, T., Däweritz, L., Ploog, K.H., (2004) Appl. Phys. Lett., 84, p. 1132Jenichen, B., Kaganer, V.M., Kastner, M., Herrmann, C., Däweritz, L., Ploog, K.H., Darowski, N., Zizak, I., (2003) Phys. Rev. B, 68, p. 132301Lindner, J., (2004) J. Magn. Magn. Mater., 277, p. 159Tonnerre, J.-M., Seve, L., Raoux, D., Soullie, G., Rodmacq, B., Wolfers, P., (1995) Phys. Rev. Lett., 75, p. 740Trade Mark, Digital InstrumentsBabcock, K., Elings, V., Dugas, M., Loper, S., (1994) IEEE Trans. Magn., 30, p. 4503Sarid, D., (1991) Scanning Force Microscopy with Applications to Electric, Magnetic and Atomic Forces, , Oxford University Press, New YorkHoly, V., (2001) Phys. Rev. B, 63, p. 205318Magalhaes-Paniago, R., Coelho, L.N., Neves, B.R.A., Westfahl, H., Iikawa, F., Däweritz, L., Spezzani, C., Sacchi, M., (2005) Appl. Phys. Lett., 86, p. 053112Lee, D.R., Sinha, S.K., Nelson, C.S., Lang, J.C., Venkataraman, C.T., Srajer, G., Osgood Iii, R.M., (2003) Phys. Rev. B, 68, p. 224409. , 0163-1829 10.1103/PhysRevB.68.224409Lee, D.R., Sinha, S.K., Nelson, C.S., Lang, J.C., Venkataraman, C.T., Srajer, G., Osgood Iii, R.M., (2003) Phys. Rev. B, 68, p. 224410Schippan, F., Behme, G., Däweritz, L., Ploog, K.H., Dennis, B., Neumann, K.-U., Ziebeck, K.R.A., (2000) J. Appl. Phys., 88, p. 2766Fernandes, R.M., Westfahl Jr., H., Magalhães-Paniago, R., Coelho, L.N., J. Magn. Magn. Mater.Engel-Herbert, R., Hesjedal, T., Mohanty, J., Schaadt, D.M., Ploog, K.H., (2006) Phys. Rev. B, 73, p. 104441Stoner, E.C., Wohlfarth, E.P., (1948) Philos. Trans. R. Soc. London, Ser. A, 240, p. 599. , 0962-8428Stoner, E.C., Wohlfarth, E.P., (1991) IEEE Trans. Magn., 27, p. 3475Aharoni, A., (1998) Introduction to the Theory of Ferromagnetism, , Claredon, Oxfor
Is Postnatal Acetaminophen Therapy Problematic for Preterm Infants?
No full textIn this issue, Srajer et al. report on the neurodevelopmental outcomes at 18–21 months of preterm infants treated with acetaminophen in the NICU.1 Their study shows no evidence of neurological harm from acetaminophen, after adjusting for known risk factors for neurodevelopmental impairment (NDI). Temporal trends in both pain and patent ductus arteriosus (PDA) management have led to an increase in acetaminophen use in the NICU in recent years, resulting in a crucial need for long-term safety data in this vulnerable population
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