Magnetoelastic effects and random magnetic anisotropy in highly strained ultrathin Ni nanowires epitaxied in a SrTiO3 matrix

International audienceWe analyze the magnetic anisotropy of Ni nanowires with diameters smaller than 5 nm. The nanowires are vertically epitaxied in a SrTiO 3 (001) matrix which generates huge tensile strains up to 3.6% along the nanowire axis. This leads to an unusual anisotropy, characterized by an easy magnetization plane perpendicular to the nanowire axis. Hysteresis cycles M(H) unveil an overall in-plane isotropy, while an opening of the M(H) cycles and thermal activation measurements indicate the presence of local energy barriers inside the nanowires. Surprisingly, the coercive field H c (T) decays exponentially with increasing temperature, for both the easy plane and the hard axis. Based on these findings, we provide an analysis of magnetoelastic effects in the nanowires. By considering global averaging over the anisotropy distribution and local averaging according to the Random Magnetic Anisotropy model, we find that the global anisotropy, with its hard axis and isotropic easy plane, is related to the mean strain, while coercivity arises from local strain variations. We evidence that a thermally activated anisotropy softening occurs in the nanowires, in addition to Sharrock's law of thermal reduction of coercivity. Possible mechanisms responsible for this thermal softening of anisotropy are proposed and discussed. Our study eventually allows to identify two major competing effects at play in the present system: an increasing magnetic anisotropy with increasing strain and a reduction of the anisotropy with increasing local strain fluctuations

Confinement of superconducting fluctuations due to emergent electronic inhomogeneities

The microscopic nature of an insulating state in the vicinity of a superconducting state, in the presence of disorder, is a hotly debated question. While the simplest scenario proposes that Coulomb interactions destroy the Cooper pairs at the transition, leading to localization of single electrons, an alternate possibility supported by experimental observations suggests that Cooper pairs instead directly localize. The question of the homogeneity, granularity, or possibly glassiness of the material on the verge of this transition is intimately related to this fundamental issue. Here, by combining macroscopic and nano-scale studies of superconducting ultrathin NbN films, we reveal nanoscopic electronic inhomogeneities that emerge when the film thickness is reduced. In addition, while thicker films display a purely two-dimensional behaviour in the superconducting fluctuations, we demonstrate a zero-dimensional regime for the thinner samples precisely on the scale of the inhomogeneities. Such behavior is somehow intermediate between the Fermi and Bose insulator paradigms and calls for further investigation to understand the way Cooper pairs continuously evolve from a bound state of fermionic objects into localized bosonic entities.Comment: 29 pages 9 figure

Phase separation and surface segregation in Co – Au – SrTiO3 thin films: Self-assembly of bilayered epitaxial nanocolumnar composites

International audiencePhase separation and surface segregation are powerful levers that allow to synthesize nanocompos-ites via self-assembly. In the present work, we combine these concepts with 3-dimensional vertical epitaxial growth and study Co-Au-SrTiO3 thin films as a model system. We demonstrate that SrTiO3, Co and Au undergo phase separation during sequential pulsed laser deposition, giving rise to a dense array of ultrathin bilayered Co-Au nanowires (NWs) with highly anisotropic optical and magnetic properties. A detailed analysis of the structural properties of the embedded metallic NWs reveals stabilization of a Co fcc phase and pronounced coupling to the matrix, which leads to large magnetoelastic effects. We discuss possible growth mechanisms yielding bilayer phase separation in nanocolumnar composites and show how the present results can be used to estimate a lower bound for the Co/Au interface energy

Mechanism of localization of the magnetization reversal in 3 nm wide Co nanowires

The mechanism of magnetization reversal has been studied in a model system of self-assembled cobalt nanowires with a 3 nm diameter. The structure, orientation and size of grains within the nanowires could be determined by high resolution transmission electron microscopy. The magnetic properties were probed using static and dynamic magnetization measurements. Micromagnetic modeling based on the structural analysis allows us to correlate the structure and the magnetic behavior of the wires, revealing competition between shape anisotropy, magnetocrystalline anisotropy and exchange in the localized reversal within Co hcp oriented grains. These results provide direct experimental evidence of the link between anisotropy fluctuations and reversal localization in nanowires.Fil: Vidal, F.. Universite de Paris Vi. Institut Des Nanosciences de Paris; FranciaFil: Zheng, Y.. Universite de Paris Vi. Institut Des Nanosciences de Paris; FranciaFil: Schio, P.. Universidade Federal Do Sao Carlos; Brasil. Universite de Paris Vi. Institut Des Nanosciences de Paris; FranciaFil: Bonilla, F. J.. Universite de Paris Vi. Institut Des Nanosciences de Paris; FranciaFil: Barturen, Mariana. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universite de Paris Vi. Institut Des Nanosciences de Paris; Francia. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Universidad Nacional de Cuyo; ArgentinaFil: Milano, Julian. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universite de Paris Vi. Institut Des Nanosciences de Paris; Francia. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Universidad Nacional de Cuyo; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Demaille, D.. Universite de Paris Vi. Institut Des Nanosciences de Paris; FranciaFil: Fonda, E.. L’Orme des Merisiers Saint-Aubin. Synchrotron Soleil; FranciaFil: de Oliveira, A. J. A.. Universidade Federal Do Sao Carlos; BrasilFil: Etgens, V. H.. Universite de Paris Vi. Institut Des Nanosciences de Paris; Francia. Fédération Lavoisier Franklin; Franci

Atomic-Scale Study of Metal–Oxide Interfaces and Magnetoelastic Coupling in Self-Assembled Epitaxial Vertically Aligned Magnetic Nanocomposites

Vertically aligned nanocomposites (VANs) of metal/oxide type have recently emerged as a novel class of heterostructures with great scientific and technological potential in the fields of nanomagnetism, multiferroism, and catalysis. One of the salient features of these hybrid materials is their huge vertical metal/oxide interface, which plays a key role in determining the final magnetic and/or transport properties of the composite structure. However, in contrast to their well‐studied planar counterparts, detailed information on the structural features of vertical interfaces encountered in VANs is scarce. In this work, high resolution scanning transmission electron microscopy (STEM) and electron energy‐loss spectroscopy (EELS) are used to provide an element selective atomic‐scale analysis of the interface in a composite consisting of ultrathin, self‐assembled Ni nanowires, vertically epitaxied in a SrTiO3/SrTiO3(001) matrix. Spectroscopic EELS measurements evidence rather sharp interfaces (6–7 Å) with the creation of metallic NiTi bonds and the absence of nickel oxide formation is confirmed by X‐ray absorption spectroscopy measurements. The presence of these well‐defined phase boundaries, combined with a large lattice mismatch between the oxide and metallic species, gives rise to pronounced magnetoelastic effects. Self‐assembled columnar Ni:SrTiO3 composites thus appear as ideal model systems to explore vertical strain engineering in metal/oxide nanostructures

Thermal enhancement of the antiferromagnetic exchange coupling between Fe epilayers separated by a crystalline ZnSe spacer

We have put into evidence the existence of an antiferromagnetic coupling between iron epilayers separated by a ZnSe crystalline semiconductor. The effect has been observed for ZnSe spacers thinner than 4 nm at room-temperature. The coupling constant increases linearly with temperature with a constant slope of ~5.5x 10-9 J/m2K. The mechanisms that may explain such exchange interaction are discussed in the manuscript. It results that thermally-induced effective exchange coupling mediated by spin-dependent on and off resonant tunnelling of electrons via localized mid-gap defect states in the ZnSe spacer layer appears to be the most plausible mechanism to induce the antiferromagnetic coupling.Comment: 29 pages, 8 figure

Improving survival in recurrent medulloblastoma: earlier detection, better treatment or still an impasse?

Early detection of relapse has been advocated to improve survival in children with recurrent medulloblastoma. However, the prognostic factors and the longer term outcome of these patients remains unclear. Pattern of recurrences were analysed in three consecutive protocols of the Société Française d'Oncologie Pédiatrique (1985-91). A uniform surveillance programme including repeated lumbar puncture combined with computerized tomography (CT) or magnetic resonance imaging (MRI) scan was applied for all registered patients. Forty-six out of 116 patients had progressive or recurrent disease. The median time from diagnosis to recurrence was 10.5 months and 76% relapses occurred during the first 2 years. Seventeen patients had asymptomatic relapses that were detected by the surveillance protocol. Forty-one patients were treated at time of progression. Twenty-three responded to salvage therapy and 11 achieved a second complete remission. The median survival time after progression was 5 months (<1-41 months), and only two patients remained alive at time of follow-up. Length of survival is primarily related to some specific patterns of relapse (time from diagnosis to recurrence, circumstances of relapse, extent of relapse) and to the response to salvage therapy. No evidence of long-term benefit appeared from any form of treatment

Magnetoresistance in granular magnetic tunnel junctions with Fe nanoparticles embedded in ZnSe semiconducting epilayer

We have investigated transport properties of iron (Fe) nanoparticles embedded in zinc selenide (ZnSe) semiconducting epilayers prepared by molecular beam epitaxy. Both positive and negative tunneling magnetoresistances (TMRs) were measured depending on the applied voltage biases and on the temperature. A slow reduction of the TMR magnitude with temperature was detected and it could be explained in terms of a crossover between direct/resonant tunneling and variable range hopping. The temperature behavior of the magnetoresistance is a clear signature of tunneling and hopping mechanisms mediated by the ZnSe barrier localized states. (C) 2008 American Institute of Physics.1031

Psychometric properties of a German version of the neck pain and disability scale

The aim of this study is to evaluate the validity and the psychometric properties of a German version of the 20-item neck pain and disability scale (NPAD) for use in primary care settings. Four hundred and forty-eight participants from 15 general practices in the area of Göttingen Germany completed a multidimensional questionnaire including a newly developed German version of the NPAD (NPAD-d) and self-reported demographic and clinical information. Reliability was tested using Cronbach’s alpha. Item-to-total score correlations were analysed. Factor structure was explored by using unrestricted principal factor analysis. Construct validity of the NPAD-d was evaluated by simple correlation analyses (Pearson’s rho) with social and clinical characteristics. The discriminative abilities of the NPAD-d were examined by comparing differences between subgroups stratified on non-NPAD-d pain related characteristics using t tests for mean scores. Cronbach’s alpha of NPAD-d was 0.94. Item-to-total scale correlations ranged between 0.414 and 0.829. Exploratory principal factor analysis indicated that the NPAD-d covers one factor with an explained variance of 48%. Correlation analysis showed high correlations with criterion variables. The NAPD-d scores of subgroups of patients were significantly different showing good discriminative validity of the scale. The NPAD-d demonstrated good validity and reliability in this general practice setting. The NPAD-d may be useful in the clinical assessment process and the management of neck pain

Sensitivity to change of the Neck Pain and Disability Scale

The Neck Pain and Disability Scale (NPAD) is a 20-item instrument to measure neck pain and related disability. The aim of this study was to assess sensitivity to change of the NPAD. A total of 411 participants from 15 general practices in the middle of Germany completed a multidimensional questionnaire including the German version of the NPAD and self-reported demographic and clinical information. Sensitivity to change was analysed by linear regression analysis of the NPAD at follow-up and educational level, age class, depression, anxiety, and deficits in social support, respectively, and by Pearson’s correlation analyses between mean change in NPAD at follow-up and mean change in prognostic markers. Those having more than basic education (regression coefficient −7.2, p < 0.001) and/or being in a younger age class (−2.9, p = 0.020) consistently reported significantly lower average NPAD scores at follow-up compared to those with basic education and/or a older age class. In contrast, those who were classified to be depressed (regression coefficient 2.1, p < 0.001), anxious (1.9, p < 0.001), or having deficits in social support (5.5, p = 0.004) reported significantly higher NPAD scores. Change in depression, anxiety, and social support scale between baseline and follow-up was significantly correlated with change in the NPAD score. Hence, these data are in the direction anticipated across all baseline factors investigated. In conclusion, the NPAD seems to be a sensitive measure for use in clinical practice and future studies of neck pain and related disability