Stanford GES 205 - Is coral bleaching really adaptive

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and the subsequent lifting off of 1-nm-thicknickel (this thickness was chosen for sim-plicity — in principle, half a monolayer issufficient for selective epitaxial growth).The ultra-thin nickel patterns were then oxidized in air to become ultra-thinnickel oxide templates. The patterned sub-strates were annealed to clean the surfacebefore growing Cu(5 nm)/Ni(5 nm)/Cu(70nm)/Co(1.8 nm) films at room temp-erature. The Cu/Co films are used as anunderlayer5to promote epitaxial nickelgrowth on the substrate, with 5-nm-thicknickel displaying a stable perpendicularmagnetization3,6. Films grown on the NiOtemplates are polycrystalline, whereas thosegrown directly onto the GaAs surface aresingle-crystal films. We examined the sample surface by atomic-force microscopy,which yielded roughness parameters in asimilar range to that of epitaxial continuousCu/Ni/Cu films4.The modulated magnetic structures wereconfirmed by magnetic-force microscopy(MFM) and magneto-optic Kerr-effect(MOKE) magnetometry. Figure 1b showsthe MFM images of the dot and wire sam-ples in the remanent state after saturationwith a perpendicular field. The brightstripes in the images correspond to the out-of-plane nickel magnetization, which has astrong magnetic signal compared with thatof the in-plane magnetization. Figure 1c shows MOKE measurementsin the field-perpendicular-to-film (polar)geometry. The perfectly square hysteresisloop obtained from the unpatterned epitax-ial film indicates that only perpendicularmagnetization exists (Fig. 1c, left). For thewire sample, however, the sharp switch atlow field originates from the regions of per-pendicular magnetization (Fig. 1c, right),whereas the gradual saturation in high fieldis caused by the presence of the in-planemagnetized regions. The spin-engineering of this magneticmedium, which can be seen as the integra-tion of patterned features into chemicallyhomogeneous films, opens up newavenues for controlling the spin structureof magnetic materials. Our simulationresults show that the patterned bit resolu-tion can be smaller than 30 nm. Suchmodulated structures can be used as pla-nar patterned magnetic media7withoutbreaking the homogeneity of the magneticfilm, which is essential for avoiding effectscaused by reduced Curie temperature. Fur-thermore, the anisotropy-constrainedmagnetic walls are static in nature, whichprovides a reproducible switching mecha-nism, unlike that of the natural domainwall. Such a system could also be useful forstudying magnetic dipolar and exchangebrief communicationsNATURE|VOL 415|7 FEBRUARY 2002|www.nature.com 601interactions, for example, and domain-wall resistance in highly controlled geo-metric systems.S. P. Li*, W. S. Lew*, J. A. C. Bland*, L. Lopez-Diaz*, M. Natali†, C. A. F. Vaz*, Y. Chen†*Cavendish Laboratory, University of Cambridge,Madingley Road, Cambridge CB3 0HE, UKe-mail: [email protected]†Laboratoire de Microstructures et deMicroélectronique (CNRS), 196 Avenue HenriRavera, 92225 Bagneux Cedex, France1. Krusin-Elbaum, L., Shibauchi, T., Argyle, B., Gignac, L. &Weller, D. Nature 410, 444–446 (2001).2. Chou, S. Y. Proc. IEEE 85, 652–671 (1997).3. O’Brien, W. L. & Tonner, B. P. Phys. Rev. B 49,15370–15373 (1994).4. Hope, S. et al. Phys. Rev. B 55, 11422–11431 (1997).5. Lew, W. S. et al. J. Appl. Phys. 87, 5947–5949 (2000).6. Bochi, G., Ballentine, C. A., Ingelfield, H. E., Thompson, C. V.& O’Handley, R. C. Phys. Rev. B 53, 1729–1732 (1996).7. Chappert, C. et al. Science 280, 1919–1922 (1998).Competing financial interests: declared none.COMMUNICATIONS ARISINGEcologyIs coral bleaching really adaptive?From an experiment in which corals aretransplanted between two depths on aPanamanian coral reef, Baker1infersthat bleaching may sometimes help reefcorals to survive environmental change.Although Baker’s results hint at furthermechanisms by which reef-building coralsmay acclimatize to changing light condi-tions, we do not consider that the evidencesupports his inference1.Baker’s study attempts to test the ‘adaptive bleaching hypothesis’2(ABH), inwhich stressed corals first lose theirdinoflagellate symbionts (bleach) and thenregain a new mix of symbionts that arebetter suited to the imposed stress regime3.Bleaching in response to increased, andnot decreased, irradiance is well known4,and Baker’s corals suffered more bleachingwhen transferred to the shallow site. However, their mortality was lower andtheir mix of symbiont genotypes wasaltered, unlike those corals that were transplanted to the deeper site, leadingBaker to conclude that the higher mortal-ity of the latter corals was due to their failure to bleach and hence to vary theirsymbiont genotypes. We are concerned not only that Bakeruses undefined stresses, which do notspecifically include temperature (thoughtto be the primary cause of mass coralbleaching5), but also that the corals in histreatments recover under very differentconditions. The corals classified as “chroni-cally stressed” recovered at the relativelylow-light, deep-water site (20–23 m),Figure 1 Selective epitaxy in spin engineering and magnetic characterization. a, Selective epitaxial growth of Cu/Ni/Cu/Co structure.Layer 1, magnetic layer of nickel; symbols: crosses, magnetization in the plane of the film; arrows: magnetization perpendicular to thefilm. Layer 2, Cu/Co underlayer. Ultra-thin NiO was used as the embedded template (layer 3) on a GaAs(001) substrate (layer 4). The wirearrays are 2 mm wide and separated by 4 mm; the square dots are 727 mm2and are separated by 3 mm. b, Magnetic-forcemicroscopy (MFM) images of the dot (I) and wire (II) samples in zero field after perpendicular field saturation. Arrow indicates a switch ofthe perpendicular Ni induced by the external stray field of the MFM tip. c, Polar magneto-optic Kerr-effect (MOKE) hysteresis loops for the unpatterned reference sample (left panel) and the wire sample (right panel). Solid and dotted arrows indicate the magnetization orientation in the epitaxial and polycrystalline regions, respectively, of the Ni film.10–1Normalized MOKE signal–2–1 0 1 2–2–1 0 1 2bacApplied field (kOe)123410 µmIII© 2002 Macmillan Magazines Ltdwhere the only two species that showedincreased mortality at depth (Diploriastrigosa and Acropora cervicornis) were veryrare. Baker’s “acutely stressed” corals, how-ever, recovered under the higher light levelsof a shallow-water site (2–4 m). From thisexperimental design, we cannot


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