305 research outputs found
In-plane uniaxial anisotropy rotations in (Ga,Mn)As thin films
We show, by SQUID magnetometry, that in (Ga,Mn)As films the in-plane uniaxial
magnetic easy axis is consistently associated with particular crystallographic
directions and that it can be rotated from the [-110] direction to the [110]
direction by low temperature annealing. We show that this behavior is
hole-density-dependent and does not originate from surface anisotropy. The
presence of uniaxial anisotropy as well its dependence on the
hole-concentration and temperature can be explained in terms of the p-d Zener
model of the ferromagnetism assuming a small trigonal distortion.Comment: 4 pages, 6 Postscript figures, uses revtex
Intrinsic and Extrinsic Contributions to the Lattice Parameter of GaMnAs
We report on measurements of the crystal structure and hole density in a
series of as-grown and annealed GaMnAs samples. The measured hole densities are
used to obtain the fraction of incorporated Mn atoms occupying interstitial and
substitutional sites. This allows us to make a direct comparison of the
measured lattice parameters with recent density functional theory (DFT)
predictions. We find that the decrease in lattice constant observed on
annealing is smaller than that predicted due to the out diffusion of
interstitial Mn during annealing. The measured lattice parameters after
annealing are still significantly larger than that of GaAs even in samples with
very low compensation. This indicates that the intrinsic lattice parameter of
GaMnAs is significantly larger than that of GaAs, in contradiction to the DFT
prediction.Comment: To appear in Appl. Phys. Lett.,13 pages,3 figures and 1 tabl
Domain imaging and domain wall propagation in (Ga,Mn)As thin films with tensile strain
We have performed spatially resolved Polar Magneto-Optical Kerr Effect
Microscopy measurements on as-grown and annealed Ga0.95Mn0.05As thin films with
tensile strain. We find that the films exhibit very strong perpendicular
magnetic anisotropy which is increased upon annealing. During magnetic
reversal, the domain walls propagate along the direction of surface ripples for
the as-grown sample at low temperatures and along the [110] direction for the
annealed sample. This indicates that the magnetic domain pattern during
reversal is determined by a combination of magnetocrystalline anisotropy and a
distribution of pinning sites along the surface ripples that can be altered by
annealing. These mechanisms could lead to an effective method of controlling
domain wall propagation.Comment: 8 pages, 3 figures. to appear in Journal of Applied Physic
Tuning perpendicular magnetic anisotropy in (Ga,Mn)(As,P) by thermal annealing
We have investigated the effects of post growth low temperature annealing on
the magnetic, electrical and structural properties of
(Ga_0.94,Mn_0.06)(As_0.9,P_0.1) layers grown by molecular beam epitaxy. By
controlling the annealing time we are able to tune the magnetic anisotropy
between an easy axis in the plane for the as-grown samples, to an easy axis
perpendicular to the plane for fully annealed samples. The increase of the
carrier density, as a result of annealing, is found to be the primary reason
for the change in magnetic anisotropy, in qualitative agreement with
theoretical predictions.Comment: 13 pages, 3 figures, submitted to Applied Physics Letter
The association of cold weather and all-cause and cause-specific mortality in the island of Ireland between 1984 and 2007
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.This article has been made available through the Brunel Open Access Publishing Fund.Background This study aimed to assess the relationship between cold temperature and daily mortality in the Republic of Ireland (ROI) and Northern Ireland (NI), and to explore any differences in the population responses between the two jurisdictions. Methods A time-stratified case-crossover approach was used to examine this relationship in two adult national populations, between 1984 and 2007. Daily mortality risk was examined in association with exposure to daily maximum temperatures on the same day and up to 6 weeks preceding death, during the winter (December-February) and cold period (October-March), using distributed lag models. Model stratification by age and gender assessed for modification of the cold weather-mortality relationship. Results In the ROI, the impact of cold weather in winter persisted up to 35 days, with a cumulative mortality increase for all-causes of 6.4% (95%CI=4.8%-7.9%) in relation to every 1oC drop in daily maximum temperature, similar increases for cardiovascular disease (CVD) and stroke, and twice as much for respiratory causes. In NI, these associations were less pronounced for CVD causes, and overall extended up to 28 days. Effects of cold weather on mortality increased with age in both jurisdictions, and some suggestive gender differences were observed. Conclusions The study findings indicated strong cold weather-mortality associations in the island of Ireland; these effects were less persistent, and for CVD mortality, smaller in NI than in the ROI. Together with suggestive differences in associations by age and gender between the two Irish jurisdictions, the findings suggest potential contribution of underlying societal differences, and require further exploration. The evidence provided here will hope to contribute to the current efforts to modify fuel policy and reduce winter mortality in Ireland
Collective Animal Behavior from Bayesian Estimation and Probability Matching
Animals living in groups make movement decisions that depend, among other factors, on social interactions with other group members. Our present understanding of social rules in animal collectives is based on empirical fits to observations and we lack first-principles approaches that allow their derivation. Here we show that patterns of collective decisions can be derived from the basic ability of animals to make probabilistic estimations in the presence of uncertainty. We build a decision-making model with two stages: Bayesian estimation and probabilistic matching.
In the first stage, each animal makes a Bayesian estimation of which behavior is best to perform taking into account personal information about the environment and social information collected by observing the behaviors of other animals. In the probability matching stage, each animal chooses a behavior with a probability given by the Bayesian estimation that this behavior is the most appropriate one. This model derives very simple rules of interaction in animal collectives that depend only on two types of reliability parameters, one that each animal assigns to the other animals and another given by the quality of the non-social information. We test our model by obtaining theoretically a rich set of observed collective patterns of decisions in three-spined sticklebacks, Gasterosteus aculeatus, a shoaling fish species. The quantitative link shown between probabilistic estimation and collective rules of behavior allows a better contact with other fields such as foraging, mate selection, neurobiology and psychology, and gives predictions for experiments directly testing the relationship between estimation and collective behavior
Context-dependent preferences in starlings: linking ecology, foraging and choice
Foraging animals typically encounter opportunities that they either pursue or skip, but occasionally meet several alternatives simultaneously. Behavioural ecologists predict preferences using absolute properties of each option, while decision theorists focus on relative evaluations at the time of choice. We use European starlings (Sturnus vulgaris) to integrate ecological reasoning with decision models, linking and testing hypotheses for value acquisition and choice mechanism. We hypothesise that options' values depend jointly on absolute attributes, learning context, and subject's state. In simultaneous choices, preference could result either from comparing subjective values using deliberation time, or from processing each alternative independently, without relative comparisons. The combination of the value acquisition hypothesis and independent processing at choice time has been called the Sequential Choice Model. We test this model with options equated in absolute properties to exclude the possibility of preference being built at the time of choice. Starlings learned to obtain food by responding to four stimuli in two contexts. In context [AB], they encountered options A5 or B10 in random alternation; in context [CD], they met C10 or D20. Delay to food is denoted, in seconds, by the suffixes. Observed latency to respond (Li) to each option alone (our measure of value) ranked thus: LA≈LC<LB<<LD, consistently with value being sensitive to both delay and learning context. We then introduced simultaneous presentations of A5 vs. C10 and B10 vs. C10, using latencies in no-choice tests to predict sign and strength of preference in pairings. Starlings preferred A5 over C10 and C10 over B10. There was no detectable evaluation time, and preference magnitude was predictable from latency differentials. This implies that value reflects learning rather than choice context, that preferences are not constructed by relative judgements at the time of choice, and that mechanisms adapted for sequential decisions are effective to predict choice behaviour.This work was supported by Biotechnology and Biological Sciences Research Council (BBSRC) Grant BB/G007144/1 to AK www.bbsrc.ac.uk; TM was supported by a Doctoral Grant from the Portuguese Foundation for Science and Technology (FCT) www.fct.pt/index.phtml.en. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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