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ABSTRACT: We report on the elaboration of germanium manganese nanostructured thin films
and the measurement of their thermoelectric properties. We investigate the
growth of Ge:Mn layers along with a thorough structural characterization of
this materials at the nanoscale. The room temperature thermoelectric properties
of these layers containing spherical inclusions are discussed regarding their
potential as a model of "electron crystal phonon glass material". We show that
the thermal conductivity can be decreased by a factor of 30, even if the
electronic properties can be conserved as in the bulk. The thermoelectric
performance ZT of such material is as high as 0.15 making them a promising
thermoelectric p-type material for Ge related application.
01/2013;
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Emmanuel Arras,
Frédéric Lançon,
Ivetta Slipukhina,
Éric Prestat,
Mauro Rovezzi,
Samuel Tardif,
Andrey Titov, Pascale Bayle-Guillemaud,
Francesco D'Acapito,
André Barski,
Vincent Favre-Nicolin,
Matthieu Jamet,
Joël Cibert,
Pascal Pochet
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ABSTRACT: We use extensive first principle simulations to show the major role played by
interfaces in the mechanism of phase separation observed in semiconductor
multifunctional materials. We make an analogy with the precipitation sequence
observed in over-saturated AlCu alloys, and replace the Guinier-Preston zones
in this new context. A new class of materials, the $\alpha$ phases, is proposed
to understand the formation of the coherent precipitates observed in the GeMn
system. The interplay between formation and interface energies is analyzed for
these phases and for the structures usually considered in the literature. The
existence of the alpha phases is assessed with both theoretical and
experimental arguments.
03/2012;
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ABSTRACT: A micromagnetic study of epitaxial micron-sized iron dots is reported through the analysis of Fresnel contrast in Lorentz Microscopy. Their use is reviewed and developed through analysis of various magnetic structures in such dots. Simple Landau configuration is used to investigate various aspects of asymmetric Bloch domain walls. The experimental width of such a complex wall is first derived and its value is discussed with the help of micromagnetic simulations. Combination of these two approaches enables us to define what is really extracted when estimating asymmetric wall width in Lorentz Microscopy. Moreover, quantitative data on the magnetization inside the dot is retrieved using phase retrieval as well as new information on the degrees of freedom of such walls. Finally, it is shown how the existence and the propagation of a surface vortex can be characterized and monitored. This demonstrates the ability to reach a magnetic sensitivity a priori hidden in Fresnel contrast, based on an original image treatment and backed-up by the evaluation of contrasts obtained from micromagnetic simulations.
Ultramicroscopy 03/2012; 115:26-34. · 2.47 Impact Factor
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ABSTRACT: Uniform single layer graphene was grown on single-crystal Ir films a few
nanometers thick which were prepared by pulsed laser deposition on sapphire
wafers. These graphene layers have a single crystallographic orientation and a
very low density of defects, as shown by diffraction, scanning tunnelling
microscopy, and Raman spectroscopy. Their structural quality is as high as that
of graphene produced on Ir bulk single crystals, i.e. much higher than on metal
thin films used so far.
03/2011;
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Chi Vo-Van,
Zoukaa Kassir-Bodon,
Hongxin Yang,
Johann Coraux,
Jan Vogel,
Stefania Pizzini, Pascale Bayle-Guillemaud,
Mairbek Chshiev,
Laurent Ranno,
Valérie Guisset,
Philippe David,
Violaine Salvador,
Olivier Fruchart
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ABSTRACT: Graphene is an attractive candidate in spintronics for a number of reasons, among which are its electric-field-controlled conductivity, its expected long spin lifetime and its two-dimensional nature. A number of recent proposals call for the development of high-quality ferromagnetic thin films in contact with graphene, whereas only thick polycrystalline or three-dimensional (nanoclusters) morphologies have been demonstrated so far. We report on the growth of flat, epitaxial ultrathin Co films on graphene using pulsed laser deposition. These display perpendicular magnetic anisotropy (PMA) in the thickness range 0.5–1 nm, in agreement with our first-principles calculations. PMA, epitaxy and ultra-small thickness bring new perspectives on graphene-based spintronic devices making use of the zero-field control of an arbitrary magnetization direction, band matching between electrodes and graphene, and interface phenomena such as the Rashba effect and electric field control of magnetism.
New Journal of Physics 10/2010; 12(10):103040. · 4.18 Impact Factor
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Chi Vo-Van,
Zoukaa Kassir-Bodon,
Hongxing Yang,
Johann Coraux,
Jan Vogel,
Stefania Pizzini, Pascale Bayle-Guillemaud,
Mairbek Chshiev,
Laurent Ranno,
Valérie Santonacci,
Philippe David,
Violaine Salvador,
Olivier Fruchart
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ABSTRACT: Graphene is attractive for spintronics due to its long spin life time and high mobility. So far only thick and polycrystalline slabs have been used as ferromagnetic electrodes. We report the growth of flat, epitaxial ultrathin Co films on graphene. These display perpendicular magnetic anisotropy in the thickness range 0.5-1nm, which is confirmed by theory. PMA, epitaxy and ultrathin thickness bring new perspectives for graphene-based spintronic devices such as the zero-field control of an arbitrary magnetization direction, band matching between electrodes and graphene, and interface effects such as Rashba and electric field control of magnetism.
04/2010;
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Chi Vo-Van,
Zoukaa Kassir-Bodon,
Hongxin Yang,
Johann Coraux,
Jan Vogel,
Stefania Pizzini, Pascale Bayle-Guillemaud,
Mairbek Chshiev,
Laurent Ranno,
Valérie Guisset,
Philippe David,
Violaine Salvador,
Olivier Fruchart
New Journal of Physics 01/2010; 12(12):103040--15. · 4.18 Impact Factor
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Abhinav Jain,
Matthieu Jamet,
André Barski,
Thibaut Devillers,
I -S Yu,
Clément Porret, Pascale Bayle Guillemaud,
Vincent Favre-Nicolin,
S Gambarelli,
V Maurel,
G Desfonds,
J F Jacquot,
Samuel Tardif
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ABSTRACT: In this study, we have systematically investigated the magnetic anisotropy of di⬚erent ferromag- netic (Ge,Mn) nanostructures. Thin Ge1⬚⬚xMnx ⬚lms have been grown by molecular beam epitaxy on Ge(111) and Ge(001) substrates. We have studied the following (Ge,Mn) systems: thin Ge3Mn5 ⬚lms grown on Ge(111) substrate, randomly distributed spherical Ge3Mn5 clusters and ⬚nally crystalline and amorphous (Ge,Mn) nanocolumns grown on Ge(001) substrates. The magnetic anisotropy is probed by complementary experimental techniques: SQUID and VSM magnetometry as well as Electron Paramagnetic Resonance (EPR). The crystalline structure and morphology of these nanos- tructures have been investigated by transmission electron microscopy and X-ray di⬚raction. We evidenced a good correlation between the magnetic properties and the structural morphology. With EPR we could ⬚t the angular dependence of the resonance ⬚eld and extract the anisotropy constants for these nanostructures.
Submitted to Physical Review B. 01/2010;
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ABSTRACT: The structure of domain walls delimiting magnetic bubbles in L1<sub>0</sub> 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.
Journal of Applied Physics 11/2009; · 2.17 Impact Factor
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Olivier Fruchart,
Nicolas Rougemaille,
Azzedine Bendounan,
Jean-Christophe Toussaint,
Rachid Belkhou,
Yuan Tian,
Hyeonseung Yu,
Fabien Cheynis,
Aurélien Masseboeuf, Pascale Bayle-Guillemaud,
Alain Marty
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ABSTRACT: We investigated with XMCD-PEEM magnetic imaging the magnetization reversal processes of N\'eel 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 N\'eel caps. Comment: Proceeding for MMM-Intermag 2010 (Washington)
11/2009;
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ABSTRACT: Changing the morphology of the growing surface and the nature of residual impurities in (Ge,Mn) layers - by using different substrates - dramatically changes the morphology of the ferromagnetic Mn-rich inclusions and the magnetotransport properties. We obtained p-type layers with nanocolumns, either parallel or entangled, and n-type layers with spherical clusters. Holes exhibit an anomalous Hall effect, and electrons exhibit a tunneling magnetoresistance, both with a clear dependence on the magnetization of the Mn-rich inclusions; holes exhibit orbital MR, and electrons show only the normal Hall effect, and an additional component of magnetoresistance due to weak localization, all three being independent of the magnetic state of the Mn rich inclusions. Identified mechanisms point to the position of the Fermi level of the Mn-rich material with respect to the valence band of germanium as a crucial parameter in such hybrid layers.
10/2009;
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ABSTRACT: 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.
09/2009;
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ABSTRACT: Data storage relies on the handling of two states, called bits. The market of mass storage is currently still dominated by magnetic technology, hard disk drives for the broad public and tapes for massive archiving. In these devices each bit is stored in the form of the direction of magnetization of nanosized magnetic domains, i.e. areas of ferromagnetic materials displaying a uniform magnetization. While miniaturization is the conventional way to fuel the continuous increase of device density, disruptive solutions are also sought. To these pertain in recent years many fundamental studies no longer considering the magnetic domains themselves, but the manipulation of the domain walls (DWs) that separate such domains. Concepts of storage and logic based on the propagation of DWs along lithographically-patterned stripes have been patented, while many fundamental aspects of DW propagation deeply related to condensed matter physics are still hotly debated. If one now considers magnetic dots of submicrometer dimensions, the magnetization has a tendency to curl along the outer edges of the nanostructure to close its magnetic flux and thereby reduce its magnetostatic energy. Then both domains and DWs of well-defined geometries arise, whose combined manipulation has been proposed as a multilevel magnetic storage scheme... Comment: To appear in Elettra Sincrotrone Highlights 2008-2009
09/2009;
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ABSTRACT: Dimensionality cross-over is a classical topic in physics. Surprisingly it has not been searched in micromagnetism, which deals with objects such as domain walls (2D) and vortices (1D). We predict by simulation a second-order transition between these two objects, with the wall length as the Landau parameter. This was conrmed experimentally based on micron-sized ux-closure dots.
09/2009;
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ABSTRACT: 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 characterization of perpendicular magnetic anisotropy materials. Thin films of chemically ordered (L1(0)) FePd alloys were investigated by micro-magnetic modeling 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.
Ultramicroscopy 09/2009; 110(1):20-5. · 2.47 Impact Factor
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ABSTRACT: 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. Comment: Ultramicroscopy (2009) In press
08/2009;
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ABSTRACT: Off-axis electron holography was used to observe and quantify the magnetic microstructure of a perpendicular magnetic anisotropic (PMA) recording media. Thin foils of PMA materials exhibit an interesting up and down domain configuration. These domains are found to be very stable and were observed at the same time with their stray field, closing magnetic flux in the vacuum. The magnetic moment can thus be determined locally in a volume as small as few tens of cubic nanometers().
Nano Letters 08/2009; 9(8):2803-6. · 13.20 Impact Factor
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ABSTRACT: Lorentz transmission electron microscopy (LTEM) combined with in-situ magnetizing experiments is a powerful tool for the investigation of the magnetization of the reversal process at the micron scale. We have implemented this tool on a conventional transmission electron microscope (TEM) to study the exchange anisotropy of a polycrystalline Co35Fe65/NiMn bilayer. Semi-quantitative maps of the magnetic induction were obtained at different field values by the differential phase contrast (DPC) technique adapted for a TEM (SIDPC). The hysteresis loop of the bilayer has been calculated from the relative intensity of magnetic maps. The curve shows the appearance of an exchange-bias field reveals with two distinct reversal modes of the magnetization: the first path corresponds to a reversal by wall propagation when the applied field is parallel to the anisotropy direction whereas the second is a reversal by coherent rotation of magnetic moments when the field is applied antiparallel to unidirectional anisotropy direction.
07/2009;
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ABSTRACT: While achieving high Curie temperatures (above room temperature) in diluted magnetic semiconductors remains a challenge in the case of well controlled homogeneous alloys, several systems characterized by a strongly inhomogeneous incorporation of the magnetic component appear as promising. Incorporation of manganese into germanium drastically alters the growth conditions, and in certain conditions of low temperature Molecular Beam Epitaxy it leads to the formation of well organized nanocolumns of a Mn-rich material, with a crystalline structure in epitaxial relationship with the Mn-poor germanium matrix. A strong interaction between the Mn atoms in these nanocolums is demonstrated by x-ray absorption spectroscopy, giving rise to a ferromagnetic character as observed through magnetometry and x-ray magnetic circular dichroism. Most interesting, intense magneto-transport features are observed on the whole structure, which strongly depend on the magnetic configuration of the nanocolumns. Comment: SPIE Optics & Photonics Symposium, San Diego : \'Etats-Unis d'Am\'erique (2008)
08/2008;
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ABSTRACT: 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 N\'eel 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.
01/2008;
