D. Givord

Federal University of Rio de Janeiro, Rio de Janeiro, Rio de Janeiro, Brazil

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Publications (330)524.86 Total impact

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    Full-text · Dataset · Jan 2016
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    ABSTRACT: The interest of giving active functions to antiferromagnetic (AFM) materials in spintronics devices has been realized recently. Mn2Au is a high-Néel temperature antiferromagnet with large Mn moment, lying in plane of the tetragonal structure. To determine the direction of the moments in Mn2Au, an original approach is demonstrated, which should be generic to planar AFM materials. It involves the rotation of the granular sample around an axis perpendicular to the applied magnetic field. The family of easy moment directions is 〈110〉. For grains prevented from rotating, the dominant magnetization process is AFM domain wall motion. Textured Mn2Au nanoelements could be introduced in spintronics devices, in which the Mn moments would be switched under modest external excitation.
    No preview · Article · Aug 2015 · Journal of Magnetism and Magnetic Materials
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    ABSTRACT: The interaction of high gradient magnetic fields (HGMF) with living cells has recently garnered significant interest owing to a possibility to remotely drive cell machinery without chemical or biological agents. Magnetic gradient forces applied to a whole cell or its organelles from external magnetic fields may offer unique control of cellular endocytosis, motion, proliferation and differentiation as well as new opportunities for promising applications in nanomedicine. Here we present recent results from our study of interactions of static and oscillating HGMFs with different types of cells.
    Full-text · Article · Jul 2015
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    ABSTRACT: The rare-earth crisis, which peaked in the summer of 2011 with the prices of both light and heavy rare earths soaring to unprecedented levels, brought about the widespread realization that the long-term availability and price stability of rare earths could not be guaranteed. This triggered a rapid response from manufacturers involved in rare earths, as well as governments and national and international funding agencies. In the case of rare-earth-containing permanent magnets, three possibilities were given quick and serious consideration: (I) increased recycling of devices containing rare earths; (II) the search for new, mineable, rare-earth resources beyond those in China; and (III) the development of high-energy-product permanent magnets with little or no rare-earth content used in their manufacture. The Replacement and Original Magnet Engineering Options (ROMEO) project addresses the latter challenge using a two-pronged approach. With its basis on work packages that include materials modeling and advanced characterization, the ROMEO project is an attempt to develop a new class of novel permanent magnets that are free of rare earths. Furthermore, the project aims to minimize rare-earth content, particularly heavy-rare-earth (HRE) content, as much as possible in Nd-Fe-B-type magnets. Success has been achieved on both fronts. In terms of new, rare-earth-free magnets, a Heusler alloy database of 236,945 compounds has been narrowed down to approximately 20 new compounds. Of these compounds, Co2MnTi is expected to be a ferromagnet with a high Curie temperature and a high magnetic moment. Regarding the reduction in the amount of rare earths, and more specifically HREs, major progress is seen in electrophoretic deposition as a method for accurately positioning the HRE on the surface prior to its diffusion into the microstructure. This locally increases the coercivity of the rather small Nd-Fe-B-type magnet, thereby substantially reducing the dependence on the HREs Dy and Tb, two of the most critical raw materials identified by the European Commission. Overall, the ROMEO project has demonstrated that rapid progress can be achieved when experts in a specific area are brought together to focus on a particular challenge. With more than half a year of the ROMEO project remaining, further progress and additional breakthroughs can be expected.
    Full-text · Article · Jun 2015 · JOM: the journal of the Minerals, Metals & Materials Society
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    ABSTRACT: Isotropic Sm–Co thin films with different SmxCoy phases (1:7, 1:5, and 2:7) are prepared by triode sputtering of targets of variable composition. A room-temperature coercivity value of 6.8 T is achieved in the film with the SmCo5 phase. Transmission electron microscopy (TEM) and 3D atom probe analyses of films that comprise this compound reveal the presence of Sm-rich 4-nm-sized precipitates within grains and along grain boundaries. Atomic-resolution scanning transmission electron microscopy/high-resolution high-angle annular dark-field (STEM/HAADF) imaging show that stacking faults occur within SmCo5 grains, which correspond to local phase variants including Sm2Co7, Sm5Co19 and SmCo3. The contribution to domain wall pinning of precipitates and stacking faults, as well as grain boundaries between misaligned grains, is discussed semi-quantitatively. Micromagnetic simulations are carried out to evaluate the influence of stacking faults and grain boundaries on magnetization reversal. The results indicate that the high coercivity values achieved can mainly be attributed to the strong pinning of magnetic domains at the grain boundaries of randomly oriented SmCo5 nanograins.
    No preview · Article · May 2015
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    ABSTRACT: Based on a critical analysis of the experimental coercive properties, general considerations on the reversal mechanisms in RFeB magnets are recalled. By plotting together the experimental parameters obtained in various magnets, common features of the reversal processes are demonstrated. Modeling provides an almost quantitative description of coercivity in these materials and permits connecting the defect characteristic properties to reversal mechanisms.
    Full-text · Article · Feb 2015
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    ABSTRACT: The coercive field and angular dependence of the coercive field of single-grain Nd2Fe14B permanent magnets are computed using finite element micromagnetics. It is shown that the thickness of surface defects plays a critical role in determining the reversal process. For small defect thicknesses reversal is heavily driven by nucleation, whereas with increasing defect thickness domain wall de-pinning becomes more important. This change results in an observable shift between two well-known behavioral models. A similar trend is observed in experimental measurements of bulk samples, where an Nd-Cu infiltration process has been used to enhance coercivity by modifying the grain boundaries. When account is taken of the imperfect grain alignment of real magnets, the single-grain computed results appears to closely match experimental behaviour.
    Full-text · Article · May 2014 · Applied Physics Letters
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    ABSTRACT: In a number of applications (automotive, wind generators), RFeB magnets are parts of systems which operate at temperatures in the range of 160 °C-180 °C. At these high temperatures, coercivity is preserved by substituting Dy atoms for a part of the Nd ones. The enhanced coercivity obtained may be associated to the high magnetocrystalline anisotropy of Dy atoms, which diffuse into the R2Fe14B phase. The introduction of Dy, however, induces a reduction in the remanent magnetization. Furthermore, Dy is an expensive and strategic material. In this study, we explore a radically different approach to coercivity. A magnetic layer is deposited at the surface of the hard grains, this layer being exchange coupled to the main hard phase in a way that its magnetization is antiparallel to it. Under an applied field that tends to reverse the main phase magnetization, the surface layer, the magnetization of which is along the field, works against reversal. This is the concept of superferrimagnetism. In order to test its impact on the coercivity of real systems, magnetically hard thick films of NdFeB have been sandwiched between thin layers of Gd/Fe, which were transformed to GdFe2 upon annealing. Coercivity enhancement was achieved compared to a reference NdFeB single layer.
    No preview · Article · May 2014 · Journal of Applied Physics
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    ABSTRACT: In this study, Nd(Cu)-Fe-B single layer thick films (up to 5 μm) have been prepared by triode sputtering and the effect of excess Nd and Cu content on the magnetic properties and microstructures have been investigated. The films were deposited at 450 °C and depending on the Nd and Cu content, they are either amorphous or crystalline in the as-deposited state. All films were submitted to an ex-situ rapid thermal annealing (500 °C/10 min). While films with a lower Nd and Cu content are characterized by a homogeneous microstructure, those with a higher Nd-Cu content have a wide distribution in grain size (in some cases, peculiar flower-like structures are formed close to the substrate). The inhomogeneous microstructure, together with direct crystallization in the as-deposited state in high Nd-Cu containing films, is attributed to an exothermic reaction. The highest coercivity (2.75 T) is achieved for the lower Nd (17.5 at. %)-Cu (1.5 at. %) content, higher Nd (up to 35 at. %)-Cu (up to 3 at. %) content leading to a reduction in both coercivity and degree of texture.
    No preview · Article · Apr 2014 · Journal of Applied Physics
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    ABSTRACT: We present a magneto-optical Kerr effect study of the magnetization reversal in a FePt/Al2O3 structure under electric (E) fields generated in a liquid electrolyte environment. The FePt film was partially covered with a thick Al2O3 layer that allowed for the study of a pinned domain wall between two regions of different coercive field. Depinning of the trapped domain wall into the region of higher coercivity was achieved by applying positive gate voltages during the magnetic field ramp and prevented in the presence of negative gate voltages. Moving from positive to negative gate voltages produced, in addition, an increase (decrease) in the number (size) of reverse domains in the high anisotropy region. This effect has been associated to an E-field induced decrease of the saturation field. Using a liquid gate to assist domain wall depinning as presented here can be used for the control of multiple pinning structures in parallel.
    Full-text · Article · Feb 2014 · Applied Physics Letters
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    ABSTRACT: This chapter presents a very sensitive AC calorimeter based on a new concept using a parylene membrane as a mechanical support. The heater and thermometer are lithographed on both sides of the polymer membrane. The thermal insulation has been deliberately set to enable the sensor to work at low temperatures. The performance of the calorimeter is only limited by the Johnson noise thermometer. This sensor has been used to measure the heat capacity of a GdAl2 microcrystal and specific heat jump has been detected correspondent at the ferromagnetic phase transition, reflecting qualities of this new device. This sensor will be used in the future for the study of phase transitions in low dimension magnetic systems. Due to the high resolution and sensitivity of the microdevice, new thermal and thermodynamic phenomena can be detected. Moreover, suspended membranes suggest the broader applicability.
    No preview · Article · Jan 2014
  • V.M.T.S. Barthem · C.V. Colin · H Mayaffre · M-H Julien · Dominique Givord
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    ABSTRACT: The continuous reduction in size of spintronic devices requires the development of structures, which are insensitive to parasitic external magnetic fields, while preserving the magnetoresistive signals of existing systems based on giant or tunnel magnetoresistance. This could be obtained in tunnel anisotropic magnetoresistance structures incorporating an antiferromagnetic, instead of a ferromagnetic, material. To turn this promising concept into real devices, new magnetic materials with large spin-orbit effects must be identified. Here we demonstrate that Mn2Au is not a Pauli paramagnet as hitherto believed but an antiferromagnet with Mn moments of ~4 μB. The particularly large strength of the exchange interactions leads to an extrapolated Néel temperature well above 1,000 K, so that ground-state magnetic properties are essentially preserved up to room temperature and above. Combined with the existence of a significant in-plane anisotropy, this makes Mn2Au the most promising material for antiferromagnetic spintronics identified so far.
    No preview · Article · Dec 2013 · Nature Communications

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  • No preview · Article · Sep 2013
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    ABSTRACT: The magnetic properties and microstructures of two Nd–Fe–B thick films with different Nd contents have been studied. The films were deposited in the amorphous state and were crystallized by post-deposition annealing. Both films show a strong 〈0 0 1〉 fibre texture out-of-plane. The film with the higher Nd content has a large room temperature coercivity of 2.7 T, while the one with the lower Nd content has a room temperature coercivity of only 0.7 T. The difference in coercivity may be explained by the fact that the film with the higher Nd content exhibits a continuous Nd-rich grain boundary phase, giving better isolation of the Nd2Fe14B grains with respect to magnetic exchange interactions. The extrusion of Nd-rich liquid to the top surface of the film with high Nd content during post-deposition annealing led to the formation of ripples in the Ta capping layer, indicating that the films are under compressive stress. This stress-induced flow of the Nd-rich material up through the film explains the excellent distribution of the Nd-rich grain boundary phase. Atom probe tomography has revealed the presence of Cu in the Nd-rich grain boundary phase, explaining the formation of the liquid phase at the relatively low temperature of 550 °C due to the eutectic reaction of Nd and Cu.
    No preview · Article · Aug 2013 · Acta Materialia
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    ABSTRACT: The electric (E) field control of magnetic properties opens the prospects of an alternative to magnetic field or electric current activation to control magnetization. Multilayers with perpendicular magnetic anisotropy (PMA) have proven to be particularly sensitive to the influence of an E-field due to the interfacial origin of their anisotropy. In these systems, E-field effects have been recently applied to assist magnetization switching and control domain wall (DW) velocity. Here we report on two new applications of the E-field in a similar material : controlling DW nucleation and stopping DW propagation at the edge of the electrode.
    Full-text · Article · Jan 2013 · Applied Physics Letters
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    ABSTRACT: We have investigated the contribution of stochastic thermally activated processes to the electric-field effects on coercivity in FePt. Coercive field distributions were measured under different gate voltages in solid-state field-effect structures. For low voltages, a shift in the coercive field distribution can be observed; however, it is not larger than the width of the distribution. Higher voltages are needed to obtain the splitting from the negative (zero) voltage distribution allowing for the unambiguous characterization of the electric-field effect. A virtual unipolarity in the electric-field effect has been identified as a feature introduced by the dielectric layer that disappears upon annealing.
    Full-text · Article · Jan 2013 · Applied Physics Letters
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    ABSTRACT: The coercive field statistics in FePt nanostructures reveals the existence of multiple switching probability sub-distributions that can be asymmetric with respect to the field orientation. Each sub-distribution is correlated with an individual magnetization reversal path whose selection cannot happen at the magnetization reversal in negative (positive) field but rather at the moment of applying the initial positive (negative) magnetic field. This serves to determine the reference magnetic state from which reversal in negative (positive) field will develop. The disappearance of the asymmetric sub-distributions upon increasing the initial magnetic field μ0Hmax supports this model. However, the sub-distributions remaining at high μ0Hmax are not necessarily those characterized by the highest coercive field. This is attributed to the fact that the initial magnetization state hierarchy and the coercive field hierarchy are essentially decorrelated.
    Full-text · Article · Nov 2012 · New Journal of Physics
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    ABSTRACT: The magnetic moment induced on Rh atoms, forming 1.6 nm average diameter clusters, embedded in an Al_{2}O_{3} matrix, has been determined using x-ray magnetic circular dichroism measurements. The magnetic moment varies linearly with the applied magnetic field. At 2.3 K and under 17 T, the spin magnetic moment amounts to 0.067(2)  μ_{B}/Rh atom. The orbital moment does not exceed 2% of the spin moment. The susceptibility is highly temperature dependent. This is in agreement with a prediction due to Moriya and Kawabata, that in itinerant electron systems, close to the onset of magnetism, the renormalization of the magnetic susceptibility by electron correlations, leads to a Curie-like behavior.
    No preview · Article · Nov 2012 · Physical Review Letters

Publication Stats

6k Citations
524.86 Total Impact Points

Institutions

  • 2012-2015
    • Federal University of Rio de Janeiro
      • Instituto de Física (IF)
      Rio de Janeiro, Rio de Janeiro, Brazil
  • 2009-2014
    • University of Grenoble
      Grenoble, Rhône-Alpes, France
  • 2009-2013
    • Institut Néel
      Grenoble, Rhône-Alpes, France
  • 1988-2013
    • University Joseph Fourier - Grenoble 1
      • Institut Néel
      Grenoble, Rhône-Alpes, France
    • Okayama University
      • Department of Physics
      Okayama, Okayama, Japan
  • 1975-2012
    • French National Centre for Scientific Research
      • Institut Néel
      Lutetia Parisorum, Île-de-France, France
  • 2007
    • Universitatea Tehnica Cluj-Napoca
      • Department of Materials Science and Technology
      Klausenburg, Cluj, Romania
  • 2006
    • Tver State University
      Kalinin, Tverskaya, Russia
  • 1988-2000
    • Trinity College Dublin
      • School of Physics
      Dublin, Leinster, Ireland
  • 1982-2000
    • University of Hull
      Kingston upon Hull, England, United Kingdom
  • 1999
    • Schneider Electric
      Rueil, Île-de-France, France
  • 1998
    • Instituto de Ciencia de Materiales de Madrid
      Madrid, Madrid, Spain
    • Hochschule für Technik und Wirtschaft Dresden
      Dresden, Saxony, Germany
  • 1997
    • University of South Wales
      Понтиприте, Wales, United Kingdom
  • 1985
    • University of Oviedo
      • Department of Physics
      Oviedo, Asturias, Spain
  • 1976-1985
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France