706 research outputs found
Temperature-induced reversal of magnetic interlayer exchange coupling
For epitaxial trilayers of the magnetic rare-earth metals Gd and Tb, exchange
coupled through a non-magnetic Y spacer layer, element-specific hysteresis
loops were recorded by the x-ray magneto-optical Kerr effect at the rare-earth
thresholds. This allowed us to quantitatively determine the strength of
interlayer exchange coupling (IEC). In addition to the expected oscillatory
behavior as a function of spacer-layer thickness , a temperature-induced
sign reversal of IEC was observed for constant , arising from
magnetization-dependent electron reflectivities at the magnetic interfaces.Comment: 4 pages, 4 figures; accepted version; minor changes and new Figs. 2
and 4 containing more dat
Magnetic and Crystallographic Structure of Y₆Mn₂₃D₂₃
The magnetic behavior of Y6Mn23 is dramatically altered upon hydrogenation (or deuteration). In this study it has been found, by means of high-resolution powder diffraction and Rietveld refinement techniques, that the crystallographic structure is distorted from face-centered cubic (Fm3m) at 295 K to a primitive tetragonal structure at 4 K in which deuterium atoms are atomically ordered. Y6Mn23 is a ferromagnetic compound with Tc=486 K, and bulk magnetization of 13.2 Bf.u. (formula unit). After deuteration of Y6Mn23 to the composition Y6Mn23D23, low-temperature scattering data (T\u3c180 K) show that the b and f2 sites in the Fm3m structure are antiferromagnetic and the d and f1 sites have no spontaneous magnetic moment. © 1984 The American Physical Society
Effects of Capping on the (Ga,Mn)As Magnetic Depth Profile
Annealing can increase the Curie temperature and net magnetization in
uncapped (Ga,Mn)As films, effects that are suppressed when the films are capped
with GaAs. Previous polarized neutron reflectometry (PNR) studies of uncapped
(Ga,Mn)As revealed a pronounced magnetization gradient that was reduced after
annealing. We have extended this study to (Ga,Mn)As capped with GaAs. We
observe no increase in Curie temperature or net magnetization upon annealing.
Furthermore, PNR measurements indicate that annealing produces minimal
differences in the depth-dependent magnetization, as both as-grown and annealed
films feature a significant magnetization gradient. These results suggest that
the GaAs cap inhibits redistribution of interstitial Mn impurities during
annealing.Comment: 12 pages, 3 figures, submitted to Applied Physics Letter
Seasonal shifts in bacterial community responses to phytoplankton-derived dissolved organic matter in the Western Antarctic Peninsula
© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Frontiers in Microbiology 8 (2017): 2117, doi:10.3389/fmicb.2017.02117.Bacterial consumption of dissolved organic matter (DOM) drives much of the movement of carbon through the oceanic food web and the global carbon cycle. Understanding complex interactions between bacteria and marine DOM remains an important challenge. We tested the hypothesis that bacterial growth and community succession would respond differently to DOM additions due to seasonal changes in phytoplankton abundance in the environment. Four mesocosm experiments were conducted that spanned the spring transitional period (August–December 2013) in surface waters of the Western Antarctic Peninsula (WAP). Each mesocosm consisted of nearshore surface seawater (50 L) incubated in the laboratory for 10 days. The addition of DOM, in the form of cell-free exudates extracted from Thalassiosira weissflogii diatom cultures led to changes in bacterial abundance, production, and community composition. The timing of each mesocosm experiment (i.e., late winter vs. late spring) influenced the magnitude and direction of bacterial changes. For example, the same DOM treatment applied at different times during the season resulted in different levels of bacterial production and different bacterial community composition. There was a mid-season shift from Collwelliaceae to Polaribacter having the greatest relative abundance after incubation. This shift corresponded to a modest but significant increase in the initial relative abundance of Polaribacter in the nearshore seawater used to set up experiments. This finding supports a new hypothesis that starting community composition, through priority effects, influenced the trajectory of community succession in response to DOM addition. As strong inter-annual variability and long-term climate change may shift the timing of WAP phytoplankton blooms, and the corresponding production of DOM exudates, this study suggests a mechanism by which different seasonal successional patterns in bacterial communities could occur.CL was partially funded by the Graduate School and the
Department of Ecology and Evolutionary Biology at Brown
University and the Brown University-Marine Biological
Laboratory Joint Graduate Program. This material is based upon
work supported by the National Science Foundation under Grant
Nos. ANT-1142114 to LA-Z, OPP-0823101 and PLR-1440435
to HD, and ANT-1141993 to JR. The Gordon and Betty Moore
Foundation grant 1711 supported work by DR
Tricritical Point and the Doping Dependence of the Order of the Ferromagnetic Phase Transition of La1-xCaxMnO3
We report the doping dependence of the order of the ferromagnetic metal to
paramagnetic insulator phase transition in La1-xCaxMnO3. At x = 0.33,
magnetization and specific heat data show a first order transition, with an
entropy change (2.3 J/molK) accounted for by both volume expansion and the
discontinuity of M ~ 1.7 Bohr magnetons via the Clausius-Clapeyron equation. At
x = 0.4, the data show a continuous transition with tricritical point exponents
alpha = 0.48+/- 0.06, beta = 0.25+/- 0.03, gamma = 1.03+/- 0.05, and delta =
5.0 +/- 0.8. This tricritical point separates first order (x<0.4) from second
order (x>0.4) transitions.Comment: 14 pages, including 4 figures: i.e. 10 pages of text and 4 pages of
figures. to appear in Physical Review Letters (accepted
Studies of the Anomalous Hall Effect and Magnetic Structure of Nd2Mo2O7 -Test of the Chirality Mechanism-
Neutron scattering studies have been carried out under the magnetic fields
H//[0_11] and H//[001] on a single crystal of Nd2Mo2O7, whose Hall
resistivity(rhoH) exhibits quite unusual H- and temperature(T)-dependences.
Material parameters such as the single ion anisotropies of the Mo- and Nd-
moments and exchange coupling constants among the Mo-Mo, Mo-Nd and Nd-Nd
moments, have been determined to reproduce various kinds of experimental data
taken as a function of H and T. For example, the neutron Bragg intensities,
magnetization curves and the magnetic specific heats have been reproduced by
the common parameters. By using the magnetic structure reproduced by these
parameters, the spin chirality (chi) of Mo spins or the fictitious magnetic
flux Phi proportional to chi has been calculated as a function of H and T by
using equation chi=, where the bracket means the statistical
average. (Note that we do not use the equation chi=x, because the
local nature of the chirality should be correctly considered.) Comparing the
calculated results with the observed rhoH, we can conclude that the unusual
behavior of rH cannot be understood consistently only by the chirality
mechanism.Comment: 16 pages, 12 fiures, submitted to J. Phys. Soc. Jp
Theory of interlayer exchange interactions in magnetic multilayers
This paper presents a review of the phenomenon of interlayer exchange
coupling in magnetic multilayers. The emphasis is put on a pedagogical
presentation of the mechanism of the phenomenon, which has been successfully
explained in terms of a spin-dependent quantum confinement effect. The
theoretical predictions are discussed in connection with corresponding
experimental investigations.Comment: 18 pages, 4 PS figures, LaTeX with IOP package; v2: ref. added.
Further (p)reprints available from http://www.mpi-halle.de/~theory
Interlayer Exchange Coupling Mediated by Valence Band Electrons
The interlayer exchange coupling mediated by valence band electrons in
all-semiconductor IV-VI magnetic/nonmagnetic superlattices is studied
theoretically. A 3D tight-binding model, accounting for the band and magnetic
structure of the constituent superlattice components is used to calculate the
spin-dependent part of the total electronic energy. The antiferromagnetic
coupling between ferromagnetic layers in EuS/PbS superlattices is obtained, in
agreement with the experimental evidences. The results obtained for the
coupling between antiferromagnetic layers in EuTe/PbTe superlattices are also
presented.Comment: 8 pages, 6 figures, to be submitted to Phys.Rev.
How U.S. Ocean Policy and Market Power Can Reform the Coral Reef Wildlife Trade
As the world’s largest importer of marine ornamental species for the aquaria, curio, home décor, and jewelry industries, the United States has an opportunity to leverage its considerable market power to promote more sustainable trade and reduce the effects of ornamental trade stress on coral reefs worldwide. Evidence indicates that collection of some coral reef animals for these trades has caused virtual elimination of local populations, major changes in age structure, and promotion of collection practices that destroy reef habitats. Management and enforcement of collection activities in major source countries such as Indonesia and the Philippines remain weak. Strengthening US trade laws and enforcement capabilities combined with increasing consumer and industry demand for responsible conservation can create strong incentives for improving management in source countries. This is particularly important in light of the March 2010 failure of the parties to the Convention on International Trade in Endangered Species (CITES) to take action on key groups of corals
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