arXiv subjects

Alain Marty

Publications and source records attributed to Alain Marty.

30 records · Page 2Linked to original sources

arXiv2019

New approach for the molecular beam epitaxy growth of scalable single-crystalline WSe$_2$ monolayers

The search for high-quality transition metal dichalcogenides mono- and multi-layers grown on large areas is still a very active field of investigation nowadays. Here, we use molecular beam epitaxy to grow 15$\times$15 mm large WSe$_2$ on mica in the van der Waals regime. By screening one-step growth conditions, we find that very high temperature ($>$900$^{\circ}$C) and very low deposition rate ($<$0.015 nm/min) are necessary to obtain high quality WSe$_2$ films. The domain size can be larger than 1 $μ$m and the in-plane rotational misorientation less than $\pm$0.5$^{\circ}$. The WSe$_2$ monolayer is also robust against air exposure, can be easily transferred over 1 cm$^2$ on SiN/SiO$_2$ and exhibits strong photoluminescence signal. Moreover, by combining grazing incidence x-ray diffraction and transmission electron microscopy, we could detect the presence of few misoriented grains. A two-dimensional model based on atomic coincidences between the WSe$_2$ and mica crystals allows us to explain the formation of these misoriented grains and gives suggestion to remove them and further improve the crystalline quality of WSe$_2$.

cond-mat.mtrl-sci↗

arXiv2019

The valley Nernst effect in WSe$_2$

The Hall effect can be extended by inducing a temperature gradient in lieu of electric field that is known as the Nernst (-Ettingshausen) effect. The recently discovered spin Nernst effect in heavy metals continues to enrich the picture of Nernst effect-related phenomena. However, the collection would not be complete without mentioning the valley degree of freedom benchmarked by the observation of the valley Hall effect. Here we show the experimental evidence of its missing counterpart, the valley Nernst effect. Using millimeter-sized WSe$_{2}$ mono-multi-layers and the ferromagnetic resonance-spin pumping technique, we are able to apply a temperature gradient by off-centering the sample in the radio frequency cavity and address a single valley through spin-valley coupling. The combination of a temperature gradient and the valley polarization leads to the valley Nernst effect in WSe$_{2}$ that we detect electrically at room temperature. The valley Nernst coefficient is in very good agreement with the predicted value.

cond-mat.mtrl-sci↗

arXiv2018

ARPES and transport studies of the elemental topological insulator $α$-Sn

Gray tin, also known as $α$-Sn, can be turned into a three-dimensional topological insulator (3D-TI) by strain and finite size effects. Such room temperature 3D-TI is peculiarly interesting for spintronics due to the spin-momentum locking along the Dirac cone (linear dispersion) of the surface states. Angle resolved photoemission spectroscopy (ARPES) has been used to investigate the dispersion close to the Fermi level in thin (0\,0\,1)-oriented epitaxially strained films of $α$-Sn, for different film thicknesses as well as for different capping layers (Al, AlO$_x$ and MgO). Indeed a proper capping layer is necessary to be able to use $α$-Sn surface states for spintronics applications. In contrast with free surfaces or surfaces coated with Ag, coating the $α$-Sn surface with Al or AlO$_x$ leads to a drop of the Fermi level below the Dirac point, an important consequence for transport is the presence of bulk states at the Fermi level. $α$-Sn films coated by AlO$_x$ are studied by electrical magnetotransport: despite clear evidence of surface states revealed by Shubnikov-de Haas oscillations, an important part of the magneto-transport properties is governed by "bulk" electronic states attributed to the $Γ8$ band, as suggested by {\it ab-initio} calculations.

cond-mat.mtrl-sci↗

arXiv2018

Millimeter-sized magnetic domains in perpendicularly magnetized ferrimagnetic Mn4N thin films grown on SrTiO3

The use of epitaxial layers for domain wall-based spintronic applications is often hampered by the presence of pinning sites. Here, we show that when depositing Mn4N(10 nm) epitaxial films, the replacement of MgO(001) by SrTiO3(001) substrates allows minimizing the misfit, and to obtain an improved crystalline quality, a sharper switching, a full remanence, a high anisotropy and remarkable millimeter-sized magnetic domains, with straight and smooth domain walls. In a context of rising interest for current-induced domain wall motion in rare

cond-mat.mtrl-sci↗

arXiv2017

Electrical properties of single crystal Yttrium Iron Garnet ultra-thin films at high temperatures

We report a study on the electrical properties of 19 nm thick Yttrium Iron Garnet (YIG) films grown by liquid phase epitaxy. The electrical conductivity and Hall coefficient are measured in the high temperature range [300,400]~K using a Van der Pauw four-point probe technique. We find that the electrical resistivity decreases exponentially with increasing temperature following an activated behavior corresponding to a band-gap of $E_g\approx 2$ eV, indicating that epitaxial YIG ultra-thin films behave as large gap semiconductor, and not as electrical insulator. The resistivity drops to about $5\times 10^3$~$Ω\cdot \text{cm}$ at $T=400$ K. We also infer the Hall mobility, which is found to be positive ($p$-type) at 5 cm$^2$/(V$\cdot$sec) and about independent of temperature. We discuss the consequence for non-local transport experiments performed on YIG at room temperature. These electrical properties are responsible for an offset voltage (independent of the in-plane field direction) whose amplitude, odd in current, grows exponentially with current due to Joule heating. These electrical properties also induce a sensitivity to the perpendicular component of the magnetic field through the Hall effect. In our lateral device, a thermoelectric offset voltage is produced by a temperature gradient along the wire direction proportional to the perpendicular component of the magnetic field (Righi-Leduc effects).

cond-mat.mtrl-sci↗

arXiv2016

Non-local opto-electrical spin injection and detection in germanium at room temperature

Non-local carrier injection/detection schemes lie at the very foundation of information manipulation in integrated systems. This paradigm consists in controlling with an external signal the channel where charge carriers flow between a "source" and a well separated "drain". The next generation electronics may operate on the spin of carriers instead of their charge and germanium appears as the best hosting material to develop such a platform for its compatibility with mainstream silicon technology and the long electron spin lifetime at room temperature. Moreover, the energy proximity between the direct and indirect bandgaps allows for optical spin injection and detection within the telecommunication window. In this letter, we demonstrate injection of pure spin currents (\textit{i.e.} with no associated transport of electric charges) in germanium, combined with non-local spin detection blocks at room temperature. Spin injection is performed either electrically through a magnetic tunnel junction (MTJ) or optically, exploiting the ability of lithographed nanostructures to manipulate the distribution of circularly-polarized light in the semiconductor. Pure spin current detection is achieved using either a MTJ or the inverse spin-Hall effect (ISHE) across a platinum stripe. These results broaden the palette of tools available for the realization of opto-spintronic devices.

cond-mat.mtrl-sci↗

arXiv2009

Asymmetric hysteresis of Néel caps in flux-closure magnetic dots

We investigated with XMCD-PEEM magnetic imaging the magnetization reversal processes of Néel caps inside Bloch walls in self-assembled, micron-sized Fe(110) dots with flux-closure magnetic state. In most cases the magnetic-dependent processes are symmetric in field, as expected. However, some dots show pronounced asymmetric behaviors. Micromagnetic simulations suggest that the geometrical features (and their asymmetry) of the dots strongly affect the switching mechanism of the Néel caps.

cond-mat.mtrl-sci↗

arXiv2009

Magnetic bubbles in FePd thin films near saturation

The structure of domain walls delimiting magnetic bubbles in L10 FePd thin layers is described on the basis of Lorentz transmission electron microscopy (LTEM) and multiscale magnetic simulations. Images obtained by high resolution LTEM show the existence of magnetization reversal areas inside domain walls, called vertical Bloch lines (VBLs). Combining these observations and multiscale simulations on various geometries, we can identify the structure of these VBLs, notably the presence or not of magnetic singularities.

cond-mat.mtrl-sci↗

arXiv2009

Probing punctual magnetic singularities during magnetization process in FePd films

We report the use of Lorentz microscopy to observe the domain wall structure during the magnetization process in FePd thin foils. We have focused on the magnetic structure of domain walls of bubble-shaped magnetic domains near saturation. Regions are found along the domain walls where the magnetization abruptly reverses. Multiscale magnetic simulations shown that these regions are vertical Bloch lines (VBL) and the different bubble shapes observed are then related to the inner structure of the VBLs. We were thus able to probe the presence of magnetic singularities as small as Bloch points in the inner magnetization of the domain walls.

cond-mat.mtrl-sci↗

arXiv2009

Lorentz microscopy mapping for domain walls structure study in L1$_0$ FePd thin films

Thin film alloys with perpendicular anisotropy were studied using Lorentz Transmission Electron Microscopy (LTEM). This work focuses on the configuration of domain walls and demonstrates the suitability and accuracy of LTEM for the magnetic characterisation of perpendicular magnetic anisotropy materials. Thin films of chemically ordered ($\unit{L1_0}$) FePd alloys were investigated by micro-magnetic modelling and LTEM phase retrieval approach. The different components of magnetization described by the modeling were studied on experimental images and confirmed by LTEM contrast simulation. Furthermore, quantitative measurements of magnetic induction inside the domain walls were made by using an original method to separate the electrical and magnetical contributions to the phase information. Irregularities were also observed along the domain walls which could play a major role during the magnetization processes.

physics.ins-det↗

arXiv2009

Controlled switching of Néel caps in flux-closure magnetic dots

While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnetic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called Néel caps. We demonstrated the controlled switching of these caps under a magnetic field, occurring via the propagation of a surface vortex. For this we considered flux-closure states in elongated micron-sized dots, so that only the central domain wall can be addressed, while domains remain unaffected.

cond-mat.other↗

arXiv1999

Instabilities in crystal growth by atomic or molecular beams

The planar front of a growing a crystal is often destroyed by instabilities. In the case of growth from a condensed phase, the most frequent ones are diffusion instabilities, which will be but briefly discussed in simple terms in chapter II. The present review is mainly devoted to instabilities which arise in ballistic growth, especially Molecular Beam Epitaxy (MBE). The reasons of the instabilities can be geometric (shadowing effect), but they are mostly kinetic or thermodynamic. The kinetic instabilities which will be studied in detail in chapters IV and V result from the fact that adatoms diffusing on a surface do not easily cross steps (Ehrlich-Schwoebel or ES effect). When the growth front is a high symmetry surface, the ES effect produces mounds which often coarsen in time according to power laws. When the growth front is a stepped surface, the ES effect initially produces a meandering of the steps, which eventually may also give rise to mounds. Kinetic instabilities can usually be avoided by raising the temperature, but this favours thermodynamic instabilities. Concerning these ones, the attention will be focussed on the instabilities resulting from slightly different lattice constants of the substrate and the adsorbate. They can take the following forms. i) Formation of misfit dislocations (chapter VIII). ii) Formation of isolated epitaxial clusters which, at least in their earliest form, are `coherent' with the substrate, i.e. dislocation-free (chapter X). iii) Wavy deformation of the surface, which is presumably the incipient stage of (ii) (chapter IX). The theories and the experiments are critically reviewed and their comparison is qualitatively satisfactory although some important questions have not yet received a complete answer.

cond-mat.mtrl-sci↗