J. Cibert

University of Grenoble, Grenoble, Rhône-Alpes, France

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Publications (294)623.13 Total impact

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    ABSTRACT: Combined electric- and magnetic-field control of magnetization orientation and reversal is studied using anomalous Hall effect in an ultrathin ferromagnetic (Ga,Mn)As/(Ga,Mn)(As,P) bilayer. Its anisotropy results from the electrically tunable competition between the in-plane and out-of-plane anisotropies of both layers. The magnetic hysteresis loop shape is sensitive to the bias electric field. In the loop reversible part, an electric-field variation is found to reorient reversibly the magnetization. In this case, the magnetization direction follows the easy anisotropy direction controlled by electric field. In contrast, in the hysteretic part, an almost complete nonreversible magnetization reversal is achieved. This is interpreted as resulting from the electric-field-induced enhancement of domain nucleation and domain-wall propagation.
    Physical Review B 11/2014; 90(17). DOI:10.1103/PhysRevB.90.174418 · 3.74 Impact Factor
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    David Ferrand · Joel Cibert
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    ABSTRACT: The strain configuration induced by the lattice mismatch in a core-shell nanowire is calculated analytically, taking into account the crystal anisotropy and the difference in stiffness constants of the two materials. The method is applied to nanowires with the wurtzite structure or the zinc-blende structure with the hexagonal / trigonal axis along the nanowire, and the results are compared to available numerical calculations and experimental data. It is also applied to multishell nanowires, and to core-shell nanowires grown along the $<001>$ axis of cubic semiconductors.
    The European Physical Journal Applied Physics 03/2014; 67(3). DOI:10.1051/epjap/2014140156 · 0.79 Impact Factor
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    ABSTRACT: X-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra at the L2,3 edges of Mn in (Ge,Mn) compounds have been measured and are compared to the results of first principles calculation. Early ab initio studies show that the Density Functional Theory (DFT) can very well describe the valence band electronic properties but fails to reproduce a characteristic change of sign in the L3 XMCD spectrum of Mn in Ge3Mn5, which is observed in experiments. In this work we demonstrate that this disagreement is partially related to an underestimation of the exchange splitting of Mn 2p core states within the local density approximation. It is shown that the change in sign experimentally observed is reproduced if the exchange splitting is accurately calculated within the Hartree–Fock approximation, while the final states can be still described by the DFT. This approach is further used to calculate the XMCD in different (Ge,Mn) compounds. It demonstrates that the agreement between experimental and theoretical spectra can be improved by combining state of the art calculations for the core and valence states respectively.
    Journal of Magnetism and Magnetic Materials 03/2014; 354:151. DOI:10.1016/j.jmmm.2013.10.037 · 2.00 Impact Factor
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    ABSTRACT: Gold-catalyzed ZnTe nanowires were grown at low-temperature by molecular beam epitaxy on a ZnTe(111) B buffer layer, under different II/VI flux ratios, including with CdTe insertions. High resolution electron microscopy and energy-dispersive x-ray spectroscopy (EDX) gave information about the crystal structure, polarity and growth mechanisms. We observe, under stoichiometric conditions, the simultaneous presence of zinc-blende and wurtzite nanowires spread homogeneously on the same sample. Wurtzite nanowires are cylinder-shaped with a pyramidal-structured base. Zinc-blende nanowires are cone-shaped with a crater at their base. Both nanowires and substrate show a Te-ended polarity. Te-rich conditions favor zinc-blende nanowires, while Zn-rich suppress nanowire growth. Using a diffusion-driven growth model, we present a criterion for the existence of a crater or a pyramid at the base of the nanowires. The difference in nanowire morphology indicates lateral growth only for zinc-blende nanowires. The role of the direct impinging flux on the nanowires sidewall is discussed.
    Nano Letters 02/2014; 14(4). DOI:10.1021/nl4046476 · 13.59 Impact Factor
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    ABSTRACT: The growth of gold catalyzed ZnSe nanowires, with CdSe insertions, by molecular beam epitaxy is investigated. In situ reflection high energy electron diffraction and ex-situ transmission electron diffraction reveal that both during, the gold dewetting and the nanowire growth, the gold particles remain always in the solid phase. The nanowire growth proceeds by ledge flow at the gold/nanowire interface as observed ex-situ by the presence of two monolayers high steps at the interface. The nanowire diameters present a high homogeneity corresponding to the low dispersion of the gold droplets. Finally, a rather abrupt interface, of less than 1 nm thick, is observed between the ZnSe barrier and the CdSe quantum dot allowing a high confinement of the excitons. All the above observations are compatible with a Vapor–Solid–Solid growth mode.
    Journal of Crystal Growth 09/2013; 378:233-237. DOI:10.1016/j.jcrysgro.2012.10.010 · 1.69 Impact Factor
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    ABSTRACT: Optically active gold-catalyzed ZnTe nanowires have been grown by molecular beam epitaxy, on a ZnTe(111) buffer layer, at low temperature 350\degree under Te rich conditions, and at ultra-low density (from 1 to 5 nanowires per micrometer^{2}. The crystalline structure is zinc blende as identified by transmission electron microscopy. All nanowires are tapered and the majority of them are <111> oriented. Low temperature micro-photoluminescence and cathodoluminescence experiments have been performed on single nanowires. We observe a narrow emission line with a blue-shift of 2 or 3 meV with respect to the exciton energy in bulk ZnTe. This shift is attributed to the strain induced by a 5 nm-thick oxide layer covering the nanowires, and this assumption is supported by a quantitative estimation of the strain in the nanowires.
    Applied Physics Letters 06/2013; 103(22). DOI:10.1063/1.4832055 · 3.52 Impact Factor
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    ABSTRACT: We report on the electric control of the magnetic anisotropy in an ultrathin ferromagnetic (Ga,Mn)As/(Ga,Mn)(As,P) bilayer with competing in-plane and out-of-plane anisotropies. The carrier distribution and therefore the strength of the effective anisotropy is controlled by the gate voltage of a field effect device. Anomalous Hall Effect measurements confirm that a depletion of carriers in the upper (Ga,Mn)As layer results in the decrease of the in-plane anisotropy. The uniaxial anisotropy field is found to decrease by a factor ~ 4 over the explored gate-voltage range, so that the transition to an out-of-plane easy-axis configuration is almost reached.
    Applied Physics Letters 03/2013; 102(12):122403. DOI:10.1063/1.4798258 · 3.52 Impact Factor
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    ABSTRACT: Mn-doped wurtzite GaN epilayers have been grown by nitrogen plasmaassisted molecular beam epitaxy. Correlated SIMS, structural and magnetic measurements show that the incorporation of Mn strongly depends on the conditions of the growth. Hysteresis loops which persist at high temperature do not appear to be correlated to the presence of Mn. Samples with up to 2 % Mn are purely substitutional Ga1−xMnxN epilayers, and exhibit paramagnetic properties. At higher Mn contents, precipitates are formed which are identified as GaMn3N clusters by x-ray diffraction and absorption: this induces a decrease of the paramagnetic magnetisation. Samples co-doped with enough Mg exhibit a new feature: a ferromagnetic component is observed up to Tc ∼ 175 K, which cannot be related to superparamagnetism of unresolved magnetic precipitates. Typeset using REVTEX 1 Since the discovery of giant magneto-resistance in ferromagnetic metal multilayers [1]
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    ABSTRACT: We report the growth of Mn-doped wurtzite GaN epilayers by nitrogen plasma-assited molecular beam epitaxy. The incorporation of manganese, achieved up to 18%, is found to be very sensitive to the growth conditions. Below about 2 % of Mn, correlated structural and magnetic measurements of GaN:Mn clearly demonstrate that the coexistence of room-temperature ferromagnetism with paramagnetism is an intrinsic property of the pure diluted magnetic semiconductor Ga1−xMnxN. In particular, high-temperature hysteresis loops reveal inhomogeneous ferromagnetism, which cannot be attributed to superparamagnetism of unresolved magnetic precipitates. At higher Mn contents, GaMn3N precipitates are evidenced by x-ray diffraction and absorption, and their increasing contribution is no more negligible. These lead, first to a slow magnetic relaxation below 10 K, related to the blocking of small clusters with a finite magnetic moment, and then to a limited magnetisation due to large antiferromagnetic clusters.
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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.
    Physical Review B 03/2012; 85(11). DOI:10.1103/PhysRevB.85.115204 · 3.74 Impact Factor
  • 03/2012; DOI:10.1051/refdp/200615301
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    ABSTRACT: This work presents results of near-band gap magnetooptical studies on Zn1-xMnxO epitaxial layers. We observe excitonic transitions in reflectivity and photoluminescence that shift toward higher energies when the Mn concentration increases and split nonlinearly under the magnetic field. Excitonic shifts are determined by the s,p-d exchange coupling to magnetic ions, by the electron-hole s-p exchange, and the spin-orbit interactions. A quantitative description of the magnetoreflectivity findings indicates that the free excitons A and B are associated with the Gamma7 and Gamma9 valence bands, respectively, the order reversed as compared to wurtzite GaN. Furthermore, our results show that the magnitude of the giant exciton splittings, specific to dilute magnetic semiconductors, is unusual: the magnetoreflectivity data are described by an effective exchange energy N0(beta(app)-alpha(app))=+0.2±0.1 eV, which points to small and positive N0beta(app). It is shown that both the increase of the gap with x and the small positive value of the exchange energy N0beta(app) corroborate recent theory describing the exchange splitting of the valence band in a nonperturbative way, suitable for the case of a strong p-d hybridization.
    Physical review. B, Condensed matter 07/2011; 84(3). DOI:10.1103/PhysRevB.84.035214 · 3.66 Impact Factor
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    ABSTRACT: We have performed numerical simulations using the finite elements method in order to investigate magnetotransport in manganese doped germanium thin films. Up to now, several groups have reported similar transport measurements on (Ge,Mn) films obtained either by low temperature molecular beam epitaxy or by manganese implantation of germanium wafers. However, different physical interpretations have been proposed, including the existence of a diluted carrier-mediated ferromagnetic phase. Here, by carefully taking into account the morphology of (Ge,Mn) films (Mn-rich ferromagnetic inclusions in a highly diluted germanium matrix), we can reproduce some experimental findings. In particular, in order to observe high positive magnetoresistance and Hall angles, there are two requirements: (i) a strong anomalous Hall effect within Mn-rich inclusions and (ii) a conductivity contrast between these inclusions and the germanium matrix. Finally, we conclude that (Ge,Mn) films behave as granular ferromagnetic systems.
    Journal of Applied Physics 06/2011; 109(12):123906-123906-9. DOI:10.1063/1.3596575 · 2.19 Impact Factor
  • MRS Online Proceeding Library 01/2011; 406. DOI:10.1557/PROC-406-121
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    MRS Online Proceeding Library 01/2011; 148. DOI:10.1557/PROC-148-389
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    ABSTRACT: We theoretically describe the spin excitation spectrum of a two dimensional electron gas embedded in a quantum well with localized magnetic impurities. Compared to the previous work, we introduce equations that allow to consider the interplay between the Coulomb interaction of delocalized electrons and the $sd$ exchange coupling between electrons and magnetic impurities. Strong qualitative changes are found : mixed waves propagate below the single particle continuum, an anticrossing gap is open at a specific wavevector and the kinetic damping due to the electron motion strongly influences the coupling strength between electrons and impurities spins.
    Physical review. B, Condensed matter 11/2010; 83(7). DOI:10.1103/PhysRevB.83.075311 · 3.66 Impact Factor
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    ABSTRACT: We report on the exchange biasing of self-assembled ferromagnetic GeMn nanocolumns by GeMn-oxide caps. The x-ray absorption spectroscopy analysis of this surface oxide shows a multiplet fine structure that is typical of the Mn2+ valence state in MnO. A magnetization hysteresis shift |HE| ∼ 100 Oe and a coercivity enhancement ΔHc ∼ 70 Oe have been obtained upon cooling (300–5 K) in a magnetic field as low as 0.25 T. This exchange bias is attributed to the interface coupling between the ferromagnetic nanocolumns and the antiferromagnetic MnO-like caps. The effect enhancement is achieved by depositing a MnO layer on the GeMn nanocolumns.
    Applied Physics Letters 08/2010; 97(6):062501-062501-3. DOI:10.1063/1.3476343 · 3.52 Impact Factor
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    ABSTRACT: Time-resolved Kerr rotation experiments in CdMnTe quantum wells provide the evidence of mixed spin excitations of the two-dimensional electron gas and magnetic ions. The onset of strong coupling between electron and Mn spin modes reveals the collective (spin-wave) nature of electronic spin excitations probed by this method. We show that resonant exchange coupling between electron-spin waves and magnetic ions spin-flip excitations provide insights in the many-body physics of the two-dimensional electron gas.
    Physical review. B, Condensed matter 08/2010; 82(7). DOI:10.1103/PhysRevB.82.075306 · 3.66 Impact Factor
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    ABSTRACT: Changing the morphology of the growing surface and the nature of residual impurities in (Ge,Mn) layers dramatically changes nanospinodal decomposition, i.e., the morphology of ferromagnetic Mn-rich inclusions. By this way, we are able to control the magnetotransport properties of (Ge,Mn) films. By using different substrates and substrate preparation, we have indeed 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 for the control and the optimization of magnetotransport in such hybrid layers.
    Physical review. B, Condensed matter 07/2010; 82(3). DOI:10.1103/PhysRevB.82.035308 · 3.66 Impact Factor
  • ChemInform 06/2010; 33(22). DOI:10.1002/chin.200222246

Publication Stats

7k Citations
623.13 Total Impact Points

Institutions

  • 1993–2014
    • University of Grenoble
      Grenoble, Rhône-Alpes, France
  • 1981–2014
    • French National Centre for Scientific Research
      • Institut Néel
      Lutetia Parisorum, Île-de-France, France
  • 1989–2013
    • University Joseph Fourier - Grenoble 1
      • Institut Néel
      Grenoble, Rhône-Alpes, France
  • 2008–2011
    • Institut Néel
      Grenoble, Rhône-Alpes, France
  • 2005
    • University of Warsaw
      • Institute of Experimental Physics
      Warszawa, Masovian Voivodeship, Poland
  • 2003–2004
    • Abou Bakr Belkaid University of Tlemcen
      • Département de Physique
      Tlemsen, Tlemcen, Algeria
  • 2000
    • National Institute of Applied Science
      Strasburg, Alsace, France
    • Université Paris 13 Nord
      • Galilee Institute
      Вильтанез, Île-de-France, France
  • 1994
    • Cea Leti
      Grenoble, Rhône-Alpes, France