Structural properties of Co2TiSi films on GaAs(001)

Co2TiSi films were grown by molecular beam epitaxy on GaAs(001) and analyzed using reflection high-energy electron diffraction, and electron microscopy. In addition, X-ray diffraction was combined with lattice parameter calculations by density functional theory comparing the L21 and B2 structures and considering the influence of non-stoichiometry. Columnar growth is found and attributed to inhomogeneous epitaxial strain from non-random alloying. In films with thicknesses up to 13 nm, these columns may be the origin of perpendicular magnetization with the easy axis perpendicular to the sample surface. We found L21 and B2 ordered regions, however the [Co]/[Ti]-ratio is changing in dependence of the position in the film. The resulting columnar structure is leading to anisotropic B2-ordering with the best order parallel to the axes of the columns

Perpendicular magnetic anisotropy in the Heusler alloy Co2TiSi/GaAs(001) hybrid structure

Investigation of the thickness dependence of the magnetic anisotropy in B2-type Co2TiSi films on GaAs(001), shows a pronounced perpendicular magnetic anisotropy at 10 K for thicknesses up to 13.5 nm. We have evidenced that the interfacial anisotropy induced by interface clusters has a strong influence on the perpendicular magnetic anisotropy of this hybrid structure, especially at temperatures lower than the blocking temperature of the clusters (28 K). However, as this influence can be ruled out at higher temperatures, the perpendicular magnetic anisotropy which is found to persist up to room-temperature can be ascribed to the magnetic properties of the Co2TiSi films. For thicknesses larger than 15.0 nm, we observe an alignment of the magnetic easy axis parallel to the sample surface, which is most likely due to the shape anisotropy and the film structure

Epitaxial growth and magnetic properties of Mn5Ge3/Ge and Mn5Ge3Cx/Ge heterostructures for spintronic applications

The development of active spintronic devices, such as spin-transistors and spin-diodes, calls for new materials that are able to efficiently inject the spin-polarized current into group-IV semiconductors (Ge and Si). In this paper we review recent achievements of the synthesis and the magnetic properties of Mn5Ge3 /Ge and carbon-doped Mn5Ge3 /Ge heterostructures. We show that high crystalline quality and threading-dislocation free Mn 5 Ge 3 films can be epitaxially grown on Ge(111) substrates despite the existence of a misfit as high as 3.7% between two materials. We have investigated the effect of carbon doping in epitaxial Mn5Ge3 films and show that incorporation of carbon into interstitial sites of Mn5Ge3 can allow not only enhancement of the magnetic properties but also an increase of the thermal stability of Mn5Ge3. Finally, toward the erspective to realize Ge/Mn5Ge3/Ge multilayers for spintronic applications, we shall show how to use carbon to prevent Mn out-diffusion from Mn5Ge3 during Ge overgrowth on top of Mn5Ge3/Ge heterostructures. The above results open the route to develop spintronic devices based on Mn5Ge3Cx /Ge heterostructures using a Schottky contact without needing an oxide tunnel barrier at the interface

Mn segregation in Ge/Mn5Ge3 heterostructures: The role of surface carbon adsorption

International audienceMn5Ge3 compound, with its room-temperature ferromagnetism and possibility to epitaxially grow on Ge, acts as a potential spin injector into group-IV semiconductors. However, the realization of Ge/Mn5Ge3 multilayers is highly hampered by Mn segregation toward the Ge growing surface. Here, we show that adsorption of some monolayers of carbon on top of the Mn5Ge3 surface prior to Ge deposition allows to greatly reduce Mn segregation. In addition, a fraction of deposited carbon can diffuse down to the underneath Mn5Ge3 layers, resulting in an enhancement of the Curie temperature up to ~360 K. The obtained results will be discussed in terms of the formation of adiffusion barrier by filling interstitial sites of Mn5Ge3 by carbon

Boosting the Curie temperature in carbon-doped Mn5Ge3/Ge heterostructures

We have combined structural and magnetic characterisations toinvestigate the effect of carbon incorporation in epitaxial Mn 5 Ge 3 C x filmsgrown on Ge(111) by Molecular Beam Epitaxy (MBE). It is shown that up to acarbon content of saturation of ~0.6, most of carbon can be incorporated intothe interstitial sites of the Mn 5 Ge 3 lattice. Such a process results in a linearincrease in the Curie temperature (T C ) of the alloy, which can reach a value ashigh as ~430 K. Above this carbon content, T C is found to decrease. Structuralcharacterisations reveal that Mn 5 Ge 3 C x films are in perfect epitaxy when x ≤0.6whereas cluster formation in the grown layers is detected above that threshold.The clusters can be attributed to manganese carbide (MnC) compounds, whichare formed when the carbon content exceeds the saturation value of 0.6 byconsuming previously deposited carbon. In addition, we also show that afterpost-thermal annealing, the carbon-doped Mn 5 Ge 3 C x alloys remainmagnetically and structurally stable up to a temperature as high as 1123 K. Theobtained results are very promising for integrating Mn 5 Ge 3 C x intoferromagnetic/semiconductor heterostructures, the ultimate goal being therealisation of spintronics devices

Comparative Spectroscopic Study Revealing Why the CO2 Electroreduction Selectivity Switches from CO to HCOO at Cu Sn and Cu In Based Catalysts

To address the challenge of selectivity toward single products in Cu catalyzed electrochemical CO2 reduction, one strategy is to incorporate a second metal with the goal of tuning catalytic activity via synergy effects. In particular, catalysts based on Cu modified with post transition metals Sn or In are known to reduce CO2 selectively to either CO or HCOO depending on their composition. However, it remains unclear exactly which factors induce this switch in reaction pathways and whether these two related bimetal combinations follow similar general structure activity trends. To investigate these questions systematically, Cu In and Cu Sn bimetallic catalysts were synthesized across a range of composition ratios and studied in detail. Compositional and morphological control was achieved via a simple electrochemical synthesis approach. A combination of operando and quasi in situ spectroscopic techniques, including X ray photoelectron, X ray absorption, and Raman spectroscopy, was used to observe the dynamic behaviors of the catalysts surface structure, composition, speciation, and local environment during CO2 electrolysis. The two systems exhibited similar selectivity dependency on their surface composition. Cu rich catalysts produce mainly CO, while Cu poor catalysts were found to mainly produce HCOO . Despite these similarities, the speciation of Sn and In at the surface differed from each other and was found to be strongly dependent on the applied potential and the catalyst composition. For Cu rich compositions optimized for CO production Cu85In15 and Cu85Sn15 , indium was present predominantly in the reduced metallic form In0 , whereas tin mainly existed as an oxidized species Sn2 4 . Meanwhile, for the HCOO selective compositions Cu25In75 and Cu40Sn60 , the indium exclusively exhibited In0 regardless of the applied potential, while the tin was reduced to metallic Sn0 only at the most negative applied potential, which corresponds to the best HCOO selectivity. Furthermore, while Cu40Sn60 enhances HCOO selectivity by inhibiting H2 evolution, Cu25In75 improves the HCOO selectivity at the expense of CO production. Due to these differences, we contend that identical mechanisms cannot be used to explain the behavior of these two bimetallic systems Cu In and Cu Sn . Operando surface enhanced Raman spectroscopy measurements provide direct evidence of the local alkalization and its impact on the dynamic transformation of oxidized Cu surface species Cu2O CuO into a mixture of Cu OH 2 and basic Cu carbonates [Cux OH y CO3 y] rather than metallic Cu under CO2 electrolysis. This study provides unique insights into the origin of the switch in selectivity between CO and HCOO pathways at Cu bimetallic catalysts and the nature of surface active sites and key intermediates for both pathway

Growth competition between semiconducting Ge1-x Mnx nanocolumns and metallic Mn5Ge3 clusters

Structural and magnetic characterizations have been combined to investigate the growthkinetics of Ge 1−x Mn x diluted magnetic semiconductors (DMSs) on Ge(001) substrates bymeans of molecular beam epitaxy (MBE). We have identified the growth process windowallowing stabilization of a high Curie temperature (T C ) nanocolumn phase and provideevidence that the growth of semiconducting Ge 1−x Mn x nanocolumns and metallic Mn 5 Ge 3clusters is a competing process. Due to a continuous increase of the Mn concentration insidenanocolumns, induced by Mn segregation along the growth direction from the interface towardthe film surface, nanocolumns become unstable when the Mn concentration reaches a value of∼40 at.% then transform into Mn 5 Ge 3 clusters. We propose a real-time approach to realizestacked layers consisting of nanocolumns separated by a Ge barrier layer, allowingexploitation of the effect of giant magneto-resistance in multilayer structure

Structural properties of Co2TiSi films on GaAs 001

Co2TiSi films were grown by molecular beam epitaxy on GaAs(001) and analyzed using reflection high-energy electron diffraction, and electron microscopy. In addition, X-ray diffraction was combined with lattice parameter calculations by density functional theory comparing the L21 and B2 structures and considering the influence of non-stoichiometry. Columnar growth is found and attributed to inhomogeneous epitaxial strain from non-random alloying. In films with thicknesses up to 13 nm, these columns may be the origin of perpendicular magnetization with the easy axis perpendicular to the sample surface. We found L21 and B2 ordered regions, however the [Co]/[Ti]-ratio is changing in dependence of the position in the film. The resulting columnar structure is leading to anisotropic B2-ordering with the best order parallel to the axes of the columns

Suppression of Mn segregation in Ge/Mn5Ge3 heterostructures induced by interstitial carbon

Mn 5 Ge 3 compound, with its room-temperature ferromagnetism and possibility to epitaxially grow onGe, acts as a potential spin injector into group-IV semiconductors. It is shown that the realization of Ge/Mn 5 Ge 3 heterostructures is highly hampered by Mn segregation toward the Ge growing surface. The Mnsegregation length can be estimated in-situ and in real time by means of reflection high-energy electron dif-fraction. We present here an approach allowing to greatly reduce or even to prevent the Mn segregation,whose principle is based on filling the Mn 5 Ge 3 lattice with interstitial carbon atoms. In addition, we showthat interstitial carbon in Mn 5 Ge 3 allows to enhance not only the Curie temperature of Mn 5 Ge 3 C x layersbut also in the whole Ge/Mn 5 Ge 3 /Ge heterostructures