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Helman C.,Centro Atomico Constituyentes | Helman C.,Laboratorio Internacional Franco Argentino en Nanociencias LIFAN | Milano J.,Laboratorio Internacional Franco Argentino en Nanociencias LIFAN | Milano J.,Bariloche Atomic Center | And 15 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

A detailed investigation of the magnetic behavior of an ultrathin Fe film epitaxially grown onto MnAs/GaAs(001) is performed by means of both ferromagnetic resonance (FMR) and magneto-optical Kerr-effect (MOKE) experiments, as a function of temperature. When the MnAs underlayer is fully in the hexagonal ferromagnetic α phase (at low temperature) or in the orthorhombic non magnetic β one (at high temperature), the magnetic behavior of the Fe film is found to be nearly the same and independent of the substrate. In contrast, at intermediate temperatures, when the MnAs underlayer presents a self-organized phase consisting of α-MnAs and β-MnAs striped domains, the Fe film is strongly influenced by the dipolar (stray) fields arising from the substrate. As a consequence, the Fe film breaks in two distinct families of striped domains. Theoretical analysis of the FMR data in the whole investigated temperature range is performed using a free-energy density model which provides an accurate determination of the temperature-dependent magnetic parameters. Furthermore, the mechanism of magnetization reversal has been analyzed as a function of temperature by MOKE. The evolution of the magnetization curves can be satisfactory interpreted starting from the main achievements of the theoretical analysis of FMR data. © 2010 The American Physical Society. Source


Eddrief M.,CNRS Nanosciences Institute of Paris | Eddrief M.,Laboratorio Internacional Franco Argentino en Nanociencias LIFAN | Zheng Y.,CNRS Nanosciences Institute of Paris | Zheng Y.,Laboratorio Internacional Franco Argentino en Nanociencias LIFAN | And 12 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We investigated by x-ray diffraction the Ga concentration dependences of the structural properties of Fe100-xGax (galfenol) thin films grown on a ZnSe/GaAs(001) substrate, a material known for its high magnetostriction. By molecular beam epitaxy (MBE) we grew a series of (001)-oriented layers without in-plane misorientation, ranging from pure Fe up to x=29.4% Ga. We find a strong Ga-induced tetragonal distortion that conserves the pristine Fe in-plane lattice parameters for all Ga compositions. Supported by theoretical predictions, we attribute this unusual tetragonal distortion to short-range ordering of Ga-Ga pairs along the [001]-growth direction. The low-temperature and out-of-equilibrium MBE growth regime tends to stabilize a strong deformed tetragonal phase (up to c/a∼1.05 for x∼29%). This tetragonal structure is fully released by postgrowth annealing. © 2011 American Physical Society. Source


Demaille D.,University Pierre and Marie Curie | Demaille D.,Laboratorio Internacional Franco Argentino en Nanociencias LIFAN | Patriarche G.,CNRS Laboratory for Photonics and Nanostructures | Helman C.,Laboratorio Internacional Franco Argentino en Nanociencias LIFAN | And 12 more authors.
Crystal Growth and Design | Year: 2013

The molecular beam epitaxy growth of Fe on MnAs/GaAs(001) leads to the formation of a new phase of the FeMnAs compound at the Fe/MnAs interface. We investigated the structural and magnetic properties of this interfacial layer by high angle annular dark field imaging in a scanning transmission electron microscope (HAADF-STEM). We determined that the epitaxial FeMnAs layer presents an unusual orthorhombic structure, with vacancy ordering. We completed our study by ab initio calculations, and we foresee an antiferromagnetic ground state for this structure. © 2013 American Chemical Society. Source


Guller F.,Comision Nacional de la Energia Atomica | Guller F.,CONICET | Guller F.,Laboratorio Internacional Franco Argentino en Nanociencias LIFAN | Llois A.M.,Comision Nacional de la Energia Atomica | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

While MoS2 and WS2 nanostructures gain an increasing importance in a number of recent technological applications, the control of their structure as a function of their size and their environment appears of prominent importance. In the present study which relies on first-principles simulations, we predict the dimerized 1T′ structural phase to be the actual ground state of MoS2, WS2, and MoSe2 zigzag nanoribbons of small width and monolayer thickness. We assign this result to the competition between edge energy - which favors the nonpolar 1T′ edges over the polar 1H edges - and the energy of atoms in the center of the ribbons - which favors the 1H ground state of the infinite monolayers. A metal-to-semiconductor transition accompanies the structural transition. At variance, ZrS2 zigzag ribbons are predicted to display the 1T structure whatever their width. In compounds of major technological importance, such structural and electronic flexibility associated with polarity effects opens the possibility for controlling the ribbon type during synthesis. © 2015 American Physical Society. Source


Guller F.,Comision Nacional de la Energia Atomica | Guller F.,CONICET | Guller F.,Laboratorio Internacional Franco Argentino en Nanociencias LIFAN | Llois A.M.,Comision Nacional de la Energia Atomica | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We analyze the characteristics of polarity in unsupported nanoribbons with zigzag edges, by a combination of analytic models, semiempirical Hartree-Fock simulations, and first-principles approach. We consider two materials with widely different ionic-covalent character, MgO and MoS2, and two polarity healing mechanisms: the so-called electronic compensation in ribbons with pristine edges, and ionic compensation in ribbons with an adequately chosen density of missing edge ions. The general expression of compensating charges, the edge metallization and spin polarization in the electronic mechanism, and the efficiency of the ionic mechanism are similar to those known in thin films and at polar surfaces. Differences, however, exist and are related to the low dimensionality, the atomic structure, and the strong undercoordination of edge atoms in nanoribbons. Polarity signatures are specified and a discussion of the possible origins of metallic edge states in these low dimensional objects is provided. © 2013 American Physical Society. Source

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