Structure and magnetism of orthorhombic epitaxial FeMnAs

The molecular beam epitaxy growth of Fe on MnAs/GaAs(001) leads to the formation of an epitaxial FeMnAs phase at the Fe/MnAs interface. The investigation of the structure by high angle annular dark field imaging in a scanning transmission electron microscope reveals an unusual orthorhombic structure, with vacancy ordering. Ab initio calculations show an antiferromagnetic ground state for this orthorhombic FeMnAs.Fil: Demaille, Dominique. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Universite Pierre et Marie Curie. Institut des Nanosciences de Paris; FranciaFil: Patriarche, Gilles. Centre National de la Recherche Scientifique; FranciaFil: Helman, Christian. Comisión Nacional de Energía Atómica; Argentina. Laboratorio Internacional Franco-Argentino en Nanociencias; FranciaFil: Eddrief, Mahmoud. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Universite Pierre et Marie Curie. Institut des Nanosciences de Paris; FranciaFil: Etgens, Hugo. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Universite Pierre et Marie Curie. Institut des Nanosciences de Paris; FranciaFil: Sacchi, Maurizio. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Universite Pierre et Marie Curie. Institut des Nanosciences de Paris; Francia. L’Orme des merisiers Saint-Aubin. Synchrotron SOLEIL; FranciaFil: Llois, Ana Maria. Comisión Nacional de Energía Atómica; Argentina. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Marangolo, Massimiliano. Laboratorio Internacional Franco-Argentino en Nanociencias; Francia. Universite Pierre et Marie Curie. Institut des Nanosciences de Paris; Franci

Suppression of the thermal hysteresis in magnetocaloric MnAs thin film by highly charged ion bombardment

We present the investigation on the modifications of structural and magnetic properties of MnAs thin film epitaxially grown on GaAs induced by slow highly charged ions bombardment under well-controlled conditions. The ion-induced defects facilitate the nucleation of one phase with respect to the other in the first-order magneto-structural MnAs transition with a consequent suppression of thermal hysteresis without any significant perturbation on the other structural and magnetic properties. In particular, the irradiated film keeps the giant magnetocaloric effect at room temperature opening new perspective on magnetic refrigeration technology for everyday use

Fe-induced spin-polarized electronic states in a realistic semiconductor tunnel barrier

International audienceWhenever iron is grown on Zn-terminated ZnSe(001) substrates, the most stable configurations of Fe/ZnSe(001) magnetic tunnel junctions show Fe-Se bonding and include a Fe-Zn intermixed interface layer, as suggested by spectroscopic measurement and confirmed here by calculations based on the density functional theory. The presence of the intermixed Fe-Zn layer at the junction strongly reduces the spin polarization of both the interface and metal-induced gap states within the ZnSe tunnel barrier

Measurement of thermal properties of bulk materials and thin films by modulated thermoreflectance (MTR)

International audienceThermal conductivity and thermal diffusivity of materials must be known at high accuracy for thermal engineering applications, in order to understand energy dissipation in devices and engines. Thermal conductivity libraries can be unreliable since their reported values may not reflect the complexities of the samples under investigation, and new materials may not yet be listed. Over the past three decades, modulated thermoreflectance (MTR) has emerged and evolved as a reliable, non-contact, and noninvasive technique to measure the thermal properties of a wide range of bulk and thin film materials and their interfaces. This Tutorial discusses the basis of, and recent advances in, the MTR technique, and its applications in the thermal characterization of a variety of materials. MTR experiments use two micrometer-wide laser beams. Experimental data (amplitude and phase vs the offset between the pump and the probe) give information about heat diffusion around the heat source along several tens of micrometers. Heat diffusion equations are used to fit the experimental data and extract the required thermal properties. Importantly, best fit procedures are not always needed because some analytical approximations provide evidence of the required thermal properties. We present many examples (bulk materials, layered sample, etc.) which illustrate this

Magnetocrystalline and magnetoelastic constants determined by magnetization dynamics under static strain

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Surface structures of MnAs grown on GaAs (111)-B substrates.

Atomically flat MnAs epilayers grown on GaAs(111)B substrates by molecular-beam epitaxy present surface structures depending on the sample temperature and on the surface stoichiometry: the known (2×2) reconstruction and two new phases, i.e., a (3×1) reconstruction and a mixed phase of (2×2) and (3×1). By monitoring the reflection high-energy electron-diffraction diagram evolution as a function of the annealing temperature, it was possible to ascribe the (3×1) as being the As richest phase and the (2×2) as the As poorest phase. Two structural models for the observed reconstructions [i.e., (3×1) and (2×2)] are proposed

Production of thick Gd freestanding films for energy conversion applications

International audienceThe use of magnetocaloric material in microelectromechanical energy conversion devices requires freestanding films with smooth surfaces to allow fast and efficient heat transfer. We present and discuss a mechanical method to produce freestanding thick Gadolinium films showing excellent magnetic properties, and high-quality surfaces on both sides. Polycrystalline Gd films have been prepared by sputtering on a silicon substrate with a thermally oxidized layer with different thicknesses. Films have been grown at 543 K. Tantalum and Tungsten have been used as buffer layers because of their corrosion resistance and of their thermal conductivity. The magnetic and surface properties of the freestanding films are presented, with special focus on the role of using different capping layers. Full characterizations of the three freestanding Gd films, including AFM measured surface roughness, XRD patterns, Curie temperature, thermo-magnetic curves, and maximum entropy change, are presented and discussed

Non-symmetric spin-pumping in a multiferroic heterostructure

International audienceWe present spin pumping experiments in Co/Pt bilayers grown on Al2O3(0001) and on ferroelectric Y-cut LiNbO3 substrates. Spin pumping is triggered by resonant ferromagnetic resonance induced by surface acoustic waves. We observe that spin pumping efficiency varies when both the magnetization vector and the acoustic wave vector are reversed in Pt/Co/LiNbO3. This phenomenon is not observed in Pt/Co/Al2O3. We propose that the in-plane electric polarization of LiNbO3 is at the origin of the observed phenomenon. These observations open up the perspective of an electric field control of spin pumping efficiency

Finite-Element Modeling of Thermoelastic Attenuation in Piezoelectric Surface Acoustic Wave Devices

International audienceThe assessment of thermoelastic attenuation is crucial in designing surface acoustic wave (SAW) devices. As irregular structures are more and more involved in modern applications for which efficient numerical tools are required, a multi-physics finite-element model is proposed in this paper, where thermoelastic damping in piezoelectric materials can be accounted for in both coated and uncoated conditions. The coupled equations are solved iteratively in time domain, using the Newmark method. The mechanical, electrical, and thermal degrees of freedom are calculated simultaneously at each time step. An application of the model is presented through the investigation of thermoelastic loss in a lithium niobate SAW device