D. Givord

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

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Publications (285)435.53 Total impact

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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.
    Applied Physics Letters 05/2014; 104(18):182408. · 3.52 Impact Factor
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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.
    Journal of Applied Physics 04/2014; 115(17). · 2.21 Impact Factor
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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.
    Journal of Applied Physics 01/2014; 115(17):-. · 2.21 Impact Factor
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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.
    Nature Communications 12/2013; 4:2892. · 10.74 Impact Factor
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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.
    Acta Materialia. 08/2013; 61(13):4920–4927.
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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.
    Physical Review Letters 11/2012; 109(19):197204. · 7.73 Impact Factor
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    ABSTRACT: Understanding the subtle link between coercivity and microstructure is essential for the development of higher performance magnets. In the case of R–Fe–B (R = rare earth) based materials this knowledge will be used to enable the development of high coercivity, Dy-free permanent magnets, which are relevant for clean energy technologies. A combination of high resolution characterization, molecular dynamics and micromagnetic simulations and model thick film systems has been used to gain valuable new insights into the coercivity mechanisms in R–Fe–B magnets.
    Scripta Materialia 09/2012; 67(6):536–541. · 2.82 Impact Factor
  • Physical review. B, Condensed matter 07/2012; 86(1). · 3.77 Impact Factor
  • Y. Zhang, D. Givord, N.M. Dempsey
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    ABSTRACT: The influence of buffer/capping-layer-mediated stress on the coercivity of NdFeB films is demonstrated. NdFeB films, 5 μm thick and rich in Nd, were deposited on Si/SiO2 and Al2O3 substrates, with or without buffer/capping layers of Ta. The coercivity of Ta-free samples (∼0.5 T) is significantly less than that achieved in samples with Ta present as a buffer and/or capping layer (1.7–1.8 T). The as-sputtered Ta layers are under strong compressive stress due to peening. During post-deposition annealing to crystallize the Nd2Fe14B phase, stresses are relieved in the Ta layer. This leads in turn to a compressive stress in the NdFeB layer, inducing extrusion of a Nd-rich phase up through the NdFeB layer. The high values of coercivity achieved in Ta-containing structures are attributed to good coverage of individual Nd2Fe14B grains with the redistributed Nd-rich phase.
    Acta Materialia. 05/2012; 60(9):3783–3788.
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    A N Dobrynin, D Givord
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    ABSTRACT: The exchange-bias properties of a Co/CoO/Co sandwich structure are examined. The Co/CoO bottom bilayer was obtained by oxidation of a Co layer and the top bilayer was formed by subsequent deposition of Co on top of CoO. The strength of the interfacial coupling is shown to be stronger at the bottom Co/CoO interface than at the top interface. A number of original properties characterize exchange bias of the top bilayer. This includes weak temperature dependence of the bias field, weak coercivity due to exchange bias, and lack of the training effect. These properties suggest that the antiferromagnetic moment configuration is frozen during magnetization reversal of the top Co layer.
    Physical review. B, Condensed matter 01/2012; 85(1). · 3.77 Impact Factor
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    ABSTRACT: The behavior of the arc formed during electrical circuit opening is examined, in the presence of a pulsed magnetic field. The field is applied in the inter-electrode gap where the arc develops. The opening sequence may be separated into two temporal phases. During the first phase, the arc remains stuck at the same position as under zero field. This is ascribed to the existence of a sticking force, which overcomes the Laplace force. The sticking force is related to the high pressure existing in the plasma. During the second phase, the Laplace force dominates. The arc is blown out of the contact region in a time duration of 1 to 1.5 ms. A model is developed which provides a semi-quantitative account of the experimental behavior.
    Electrical Contacts (ICEC 2012), 26th International Conference on; 01/2012
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    ABSTRACT: A significant reduction in the Néel temperature of CoO ultrathin films is revealed by highly sensitive specific heat measurement. It is found that the films consist of weakly coupled antiferromagnetic (AF) grains. The TN reduction from large to small grain samples scales with the grain size reduction, according to the Binder theory of critical phenomena in systems of reduced dimensions. In these finite AF nanosystems, the intergrain exchange interaction is reduced by the presence of surface Co spins, weakly coupled to Co moments inside the grains.
    Physical Review B 04/2011; 83(14):140407(R). · 3.66 Impact Factor
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    ABSTRACT: First principles study of the electric field effect on magnetization and magnetic anisotropy of FeCo/MgO(001) thin film Appl. Phys. Lett. 99, 072503 (2011) Numerical study of the effective magnetocrystalline anisotropy and magnetostriction in polycrystalline FeGa films J. Appl. Phys. 110, 043902 (2011) Contribution of the easy axis orientation, anisotropy distribution and dot size on the switching field distribution of bit patterned media Appl. Phys. Lett. 99, 062505 (2011) Temperature dependence dynamical permeability characterization of magnetic thin film using near-field microwave microscopy The various forces experienced by magnetic particles pumped through microfluidic channels placed above a chessboard array of micromagnets were calculated as a function of particle size and device dimensions. A device incorporating magnetically microstructured hard magnetic NdFeB films was fabricated. Good agreement was achieved between the calculated and observed distance over which magnetic particles travel before they are trapped. Using this simple and autonomous device, mixed solutions of magnetic and non-magnetic micro-particles were separated into two distinct solutions containing a concentration of up to 99.9% and 94.5% of non-magnetic and magnetic particles, respectively. V C 2011 American Institute of Physics. [doi:10.1063/1.3664092] The range of applications for magnetic micro-and nano-particles is constantly expanding, in particular in medicine and biology. A number of applications involve particle trap-ping and deviation, achieved by attracting them to specific locations under the effect of a magnetic field gradient. The required magnetic fields and associated field gradients have been produced using soft magnetic elements polarized by an external magnetic field, 1–3 electromagnets, 4–6 or bulk perma-nent magnets. 7,8 The field gradients produced by a perma-nent magnet can be greatly enhanced by reducing its size to the micro-scale, and gradients as high as 10 6 T/m have been recently produced by micro-scale arrays of rare-earth perma-nent magnets. 9 Such micro-magnet arrays are autonomous, having no requirements for a cumbersome external field source nor power supply, and have been used to attract lipo-somes containing magnetic nanoparticles 10 and to levitate cells. 11 In this work, we exploit micro-magnet arrays to sepa-rate magnetic and non-magnetic particles flowing through a microfluidic channel. Magnetic separation exploits the difference in the bal-ance of forces acting, on the one side, on non-magnetic objects submitted to gravity (including buoyancy) and vis-cous forces (neglecting Brownian motion) and on the other side, on magnetic objects (e.g., magnetic particles), addition-ally submitted to a magnetic force (Fig. 1(a)). Gravity forces are expressed as F g ¼ (4/3) p r part 3
    Applied Physics Letters 01/2011; 99(82):232504-223101. · 3.52 Impact Factor
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    ABSTRACT: In this paper, we describe the use of Giant Unilamellar Vesicles (GUV) as cellular models to illustrate the possibility to trap magnetically-marked cells on a flat micro-patterned hard magnetic film by magnetophoresis. GUV can encapsulate nano-or micro-functional objects during the electroformation process and the amount of magnetic material incorporated can be controlled by regulating the concentration of magnetic nanoparticles in the electroformation buffer. The micromagnet arrays described in this paper are capable of generating high magnetic field gradients, which favors attraction of the magnetic GUV. These flat magnetic microstructures are obtained using the recently developed thermo-magnetic patterning technique (TMP).
    AIP Conference Proceedings 12/2010; 1311(1):192.
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    ABSTRACT: A method for extracting the magnetization dependent part of the demagnetizing field from minor hysteresis loops is described. It applies to hard magnetic materials with irreversible magnetization switching. The method’s validity is tested on the simulated magnetization curves of an assembly of hard magnetic grains, as well as on a thin NdFeB film with out of plane magnetization. Effective demagnetization factors are extracted from the analysis. These factors are smaller than the usually applied sample shape dependent demagnetizing factors.
    Applied Physics Letters 11/2010; 97(19):192506-192506-3. · 3.52 Impact Factor
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    ABSTRACT: The phenomenon of magnetic blowing allows drastic reduction in arc duration. It is of specific interest in DC applications, more specifically in automotive applications. Blowing exploits the action of the magnetic force on the transitory arc formed during circuit opening. A new contact was described in a previous paper in which the magnetic field required for blowing is generated by the contact material itself. In the present study, aimed at a more thorough analysis of magnetic blowing, an external adjustable magnetic field (up to 320 mT) was produced by placing magnets on each side of the contact. The experiments were performed for voltage values from 42 V<sub>DC</sub> to 360 V<sub>DC</sub> and currents up to 100 A, using a resistive circuit. For considered magnetic field values, no magnetic blowing was observed below a certain critical current. At high current, the arc duration was found to be approximately current independent, following a phenomenological 1√B variation. Under 42 V<sub>DC</sub> and the maximum current of 100 A, the arc duration under 320 mT was reduced by a factor of 100 as compared to the arc duration under zero field. Qualitative account of the present phenomena is obtained by considering that blowing is governed by the competition between the magnetic force acting on the arc and the force acting against arc motion, originating from the enthalpy expense needed to move the arc from a heated region at the contact surface to a colder one.
    Electrical Contacts (HOLM), 2010 Proceedings of the 56th IEEE Holm Conference on; 11/2010
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    ABSTRACT: Scanning Hall probe microscopy has been used for the quantitative measurement of the z-component out-of-plane of the stray magnetic fields produced by Nd–Fe–B hard magnetic films patterned at the micron scale using both topographic and thermomagnetic methods. Peak-to-peak field values in the range 20–120 mT have been measured at scan heights of 25– 30 m above the samples. Quantitative comparison between calculated and measured field profiles gives nondestructive access to the micromagnets' internal magnetic structure. In the case of topographically patterned films the average value of remanent magnetization is extracted; in the case of thermomagnetically patterned films the depth of magnetization reversal is estimated. The measured field profiles are used to derive the spatial variation in the field and field gradient values at distances in the range 0.1– 10 m above the micromagnet arrays. These length-scales are relevant to the application of the micromagnet arrays for lab-on-chip applications trapping and confinement of magnetic particles. Very large field and field gradient values as high as 1.
    Journal of Applied Physics 09/2010; 108:63914. · 2.21 Impact Factor
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    ABSTRACT: We performed magnetic force microscopy (MFM) measurements in external magnetic fields parallel to the sample plane to qualitatively study their effect on the magnetization of different kinds of MFM probes. As a test structure we used an array of rectangular ferromagnetic thin film elements aligned with the external magnetic field direction. MFM images were taken while the field was increased stepwise to monitor the onset of a tilt in the MFM probe magnetization. Three different probes were investigated: a conventional pyramidal probe coated with 40 nm of CoCrTa, a coated high aspect ratio MFM probe, and a MFM probe based on an iron filled carbon nanotube (FeCNT). The results show that the magnetization of the pyramidal probe is only stable in in-plane fields of up to ∼60 mT , in larger fields a considerable in-plane component of the tip magnetization is observed. This makes it difficult to distinguish the effect of the external field on the sample from those on the MFM probe. The coated high aspect ratio probe is at first stable up to ∼200 mT and then shows a history dependent behavior, in the second sweep the onset of the magnetization tilt is already visible at ∼100 mT . With the FeCNT probe, no tilt of the probe magnetization could be observed in in-plane fields up to the experimental limit of 230 mT. Due to the large shape anisotropy of the enclosed iron nanowire its direction of magnetization stays mainly oriented along the long axis even in moderate fields that deviate from this easy axis. Consequently, this probe is best suited for MFM measurements in external fields.
    Journal of Applied Physics 08/2010; · 2.21 Impact Factor
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    ABSTRACT: Magnetization reversal in systems made of high coercivity exchange-decoupled grains is modeled. Dipolar collective effects, negligible for a sample in the form of a foil with out-of-plane magnetization, become more and more significant as the sample shape evolves toward three-dimensional geometry, and reach their maximum in case of a foil with in-plane magnetization. These collective effects are manifested by coercivity variations, which allow differences in coercivity for samples of the same material but having different shapes to be understood. Taking collective effects into account requires the introduction of a correction term to the demagnetizing factor. The numerical results obtained are experimentally confirmed for NdFeB permanent magnets.
    Physical Review B 05/2010; 81(17). · 3.66 Impact Factor
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    ABSTRACT: We report the microfabrication and operation of a highly sensitive ac-calorimeter designed to characterize small mass magnetic systems operating at very low frequencies (from 0.1 to 5 Hz) in a temperature range from 20 to 300 K. The calorimetric cell is built in the center of a 500 nm thick polymeric membrane of parylene C held up by a Cu frame. On both sides of the membrane defining a three layer structure, electrical leads, heater, and thermometer are deposited as thin film layers of NbN(x), with different nitrogen contents, taking benefit of the poor thermal conductance of niobium nitride to thermally isolate the system. This suspended structure ensures very low heat capacity addenda with values in the microJ/K over the 1 mm(2) area of the measurement cell. The structuring of the membrane along with suspending of the sensing part only by the parylene bridges leads to a highly reduced thermal link. The calorimeter has been characterized as a function of frequency, temperature, and magnetic field. The thermal link measured is really small reaching values well below 10(-8) W/K at 50 K. With these characteristics the frequency of adiabaticity is typically around few hertz and energy exchanges as small as 1 pJ can be detected. Measurements have been performed on Co/Au thin films and on the GdAl(2) microcrystal where the ferromagnetic phase transition is clearly evidenced.
    The Review of scientific instruments 05/2010; 81(5):053901. · 1.52 Impact Factor

Publication Stats

2k Citations
435.53 Total Impact Points

Institutions

  • 2012–2014
    • Federal University of Rio de Janeiro
      • Instituto de Física (IF)
      Rio de Janeiro, Rio de Janeiro, Brazil
  • 2010–2012
    • University Joseph Fourier - Grenoble 1
      • Institut Néel
      Grenoble, Rhône-Alpes, France
  • 2009–2012
    • Institut Néel
      Grenoble, Rhône-Alpes, France
  • 1974–2009
    • French National Centre for Scientific Research
      • Institut Néel
      Paris, Ile-de-France, France
  • 1998–2000
    • Trinity College Dublin
      • School of Physics
      Dublin, Leinster, Ireland
  • 1997
    • University of South Wales
      Понтиприте, Wales, United Kingdom
  • 1995
    • University of São Paulo
      • Institute of Physics (IF) (São Paulo)
      São Paulo, Estado de Sao Paulo, Brazil
    • Trinity College
      Hartford, Connecticut, United States
  • 1993
    • Complutense University of Madrid
      • Departamento de Física de Materiales
      Madrid, Madrid, Spain
    • Keio University
      • Department of Physics
      Edo, Tōkyō, Japan
  • 1978–1985
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France
  • 1976
    • Oak Ridge National Laboratory
      • Solid State Division
      Oak Ridge, Florida, United States
    • Vietnam National University, Hanoi
      Hà Nội, Ha Nội, Vietnam