Search

Heusler magnetic alloys offer a wide variety of electronic properties very promising for spintronics and magnonics. Some alloys exhibit a spin gap in their band structure at the Fermi energy, the so-called half-metal magnetic (HMM) behavior. This particular property leads to two very interesting properties for spintronics, i.e., fully polarized current together with ultra-low magnetic damping, two key points for spin-transfer-torque based devices. This Tutorial gives experimental details to grow and characterize Heusler Co2MnZ compounds in thin films (Z = Al, Si, Ga, Ge, Sn, Sb) by using molecular beam epitaxy in order to get the proper predicted electronic properties. A first part of this Tutorial is dedicated to control the stoichiometry as best as possible with some methods to test it. The chemical ordering within the lattice was examined by using electron diffraction during growth, regular x-ray diffraction, and scanning transmission electron microscopy. In particular, standard x-ray diffraction is carefully analyzed depending on the chemical ordering in the cubic cell and shown to be inefficient to distinguish several possible phases, on the contrary to electron microscopy. The electronic properties, i.e., magnetic moment, spin polarization, and magnetic damping were reviewed and discussed according to the stoichiometry of the films and also theoretical predictions. Polycrystalline films were also analyzed, and we show that the peculiar HMM properties are not destroyed, a good news for applications. A clear correlation between the spin polarization and the magnetic damping is experimentally demonstrated. At least, our study highlights the major role of stoichiometry on the expected properties.

The influence of magnetic domain walls (DWs) on magnetoresistance in various exchange-bias ferrimagnetic/(ferrimagnetic or ferromagnetic) bilayer systems (Gd68Fe32/Tb55Fe45 and Fe60Gd40/Fe55Sn45) is presented. These systems allow one to create well-defined DWs of various size and shape. The DW morphology is controlled by the applied magnetic field in the film plane. By changing the amplitude of the field, the DW thickness can be controlled, whereas the rotation of the field in plane allows one to create a θ° Bloch DW (90° DW, 180° DW,…). The electrical resistivity of the sample was measured relative to the applied magnetic field (amplitude and direction). The influence of the shape and size of the DW on magnetoresistance was followed. For the two amorphous samples, it made it possible to fit the variation of resistivity relative to the DW thickness and shape using a model based on anisotropic magnetoresistance.

In this paper, two distinct studies are presented: Sb adsorption on Si 〈111〉 analyzed by both ellipsometry and electron diffraction in real time, and Sb doping in Si molecular‐beam epitaxy (MBE). In the adsorption study, saturation is shown to occur at one monolayer of equivalent silicon. Superstructures do not appear at the same coverages and temperatures during adsorption and desorption. It is also confirmed that the anomalies we have previously observed in the desorption kinetics are due to superstructures. A detailed study of the sticking coefficient explains both the differences in adsorption and desorption phase diagrams and the complex evolution of Sb doping in Si MBE. Variations of the doping level with the growth rate and incident dopant flux support this result.

International audience; Soft x-ray resonant magnetic reflectivity measurements on thin films and multilayers in a transverse geometry using linear polarized photons are presented. Magneto-optic calculations taking into account the layer roughness allows us to reproduce all the experimental features of the angular and energy reflectivity curves as well as the asymmetry ratio in both cases. Application to FexMn1−x alloy films epitaxially grown on Ir(001) brings more insights on the magnetic transition occurring at x=0.75.

It is shown that it is possible to grow FeIr metallic superlattices where the growth of Fe on Ir as well as Ir on Fe is two dimensional. The two‐dimensional growth is shown by the occurrence of reflection high‐energy electron‐diffraction (RHEED) oscillations during the growth of Fe on Ir (100) up to 4–7 planes, and during the Ir growth on the Fe surface for an unlimited thickness. Transmission electron microscopy thus shows that superlattices with flat interfaces on a large scale are obtained. The RHEED oscillations during Fe growth disappear after 4–7 deposited planes and the Fe growth becomes three dimensional. The quality of the superlattices is thus damaged by this three‐dimensional growth.

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; The strong impact of molecular beam epitaxy growth and Synchrotron Radiation characterization tools in the understanding of fundamental issues in nanomagnetism and spintronics is illustrated through the example of fully epitaxial MgO-based Magnetic Tunnel Junctions (MTJs). If ab initio calculations predict very high tunnel magnetoresistance (TMR) in such devices, some discrepancy between theory and experiments still exists. The influence of imperfections in real systems has thus to be considered like surface contaminations, structural defects, unexpected electronic states, etc. The influence of possible oxygen contamination at the Fe/MgO(001) interface is thus studied, and is shown to be not so detrimental to TMR as predicted by ab initio calculations. On the contrary, the decrease of dislocations density in the MgO barrier of MTJs using Fe1-xVx electrodes is shown to significantly increase TMR. Finally, unexpected transport properties in Fe1-XCox/MgO/Fe1-XCox (001) are presented. With the help of spin and symmetry resolved photoemission and ab initio calculation, the TMR decrease for Co content higher than 25% is shown to come from the existence of an interface state and the shift of the empty Delta 1 minority spin state towards the Fermi level.

We describe an adaptation of picosecond laser ultrasonics tailored for study of GHz-frequency longitudinal and shear acoustic waves in liquids. Time-domain coherent Brillouin scattering is used to detect multicycle acoustic waves after their propagation through variable thickness liquid layers into a solid substrate. A specialized optical pulse shaping method is used to generate sequences of pulses whose repetition rate determines the acoustic frequency. The measurements reveal the viscoelastic liquid properties and also include signatures of the optical and acoustic cavities formed by the multilayer sample assembly. Modeling of the signals allows their features to be distinguished so that liquid properties can be extracted reliably. Longitudinal and shear acoustic wave data from glycerol and from the silicon oil DC704 are presented.

We investigated in detail the structural, chemical, and magnetic properties of Fe at the interface with MgO. The samples were grown by molecular-beam epitaxy and analyzed by using reflection high-energy electron diffraction, scanning tunneling microscopy, Auger electron spectroscopy, x-ray photoemission spectroscopy (XPS), x-ray-absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) measurements. We discuss the presence or absence of C and O contaminants at the surface of the metallic film. We actually show that when growing the first Fe layer on the initial MgO substrate, a carbon contamination at the surface of the annealed Fe layer is observed leading to a C(2×2) surface structure. We propose a method to eliminate this carbon contamination and to get interfaces free of carbon. On the other hand, we observed that the reactivity of the Fe surface to oxygen contamination is greatly influenced by the presence of carbon or absence on the surface. The influence of oxygen adsorption at th...

Double-barrier fully epitaxial crystalline magnetic tunnel junctions employing a class of artificial antiferromagnetic (AAF) subsystem are elaborated by molecular-beam epitaxy. Our specific AAF subsystem is constituted by an Fe(10nm)∕MgO(0.7nm)∕Fe(20nm) trilayer stack where the antiferromagnetic coupling between the two Fe layers occurs by spin-polarized tunneling of electrons across the three, atomic-layer thin MgO insulating barrier. In our junctions, the efficiency of spin filtering is validated by the tunnel magnetoresistance effects of about 140% at room temperature and a high output voltage up to 500 mV at 1.3 V.

In this paper, we present a physical explanation for the dopant surface segregation during two‐dimensional growth in molecular‐beam epitaxy. It is shown that the only way to explain dopant behavior is to assume the climbing of dopant atoms over the steps on the surface, during the growth. An energetical analysis of such phenomenon is presented. Taking into account this mechanism during two‐dimensional growth, two kinetic equations can be written for each layer. Resolution of the system provides analytical solutions, and the growth rate influence on the doping profile is justified. The applicability of this approach to several dopants both in GaAs and Si is discussed. It appears that the origin of this segregation phenomenon lies in the unfavorable energy situation for the incorporation of a dopant instead of a bulk material atom.