D. Halley

French National Centre for Scientific Research, Lutetia Parisorum, Île-de-France, France

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Publications (12)30.7 Total impact

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    ABSTRACT: Chemically disordered FePd epitaxial layers are grown at room temperature by molecular beam epitaxy on a Pd(001) buffer layer and then annealed in order to induce the chemically ordered L10 (AuCu I) structure. Contrary to what is observed in the case of ordering during growth above room temperature, the ordered structure appears here with the three possible variants of the L10 phase. The ratio of the three different variant volumes is set by the residual epitaxial strain in the layer before annealing. It thus explains that for long annealing times, the long-range order parameter associated with the L10 variant with c along the (100) growth direction saturates at a value close to 0.65, and never reaches unity. Magnetic consequences of the ordering are studied.
    Physical review. B, Condensed matter 11/2004; 70(17). · 3.77 Impact Factor
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    ABSTRACT: We observed that the relaxation mechanism of the epitaxial strain is dramatically dependent on the chemical ordering within the L10 structure in FePd(Pt) thin films. In disordered or weakly ordered layers, the relaxation takes place though perfect 1/2[101] dislocations, whereas well-ordered films relax through the partial 1∕6[112] Shockley dislocations, piled-up within microtwins, with a huge impact on both the morphology and the magnetic properties of the film. We show that the antiphase boundary energy is the key factor preventing the propagation of perfect dislocations in ordered alloys.
    Physical Review B 01/2004; 70(17). · 3.66 Impact Factor
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    ABSTRACT: We show how, combining He ion irradiation and thermal mobility below 600 K, the transformation from chemical disorder to order in thin films of an intermetallic ferromagnet (FePd) may be triggered and controlled. Kinetic Monte Carlo simulations show that the initial directional short range order determines the transformation. Magnetic ordering perpendicular to the film plane was achieved, promoting the initially weak magnetic anisotropy to the highest values known for FePd films. Applications to ultrahigh density magnetic recording are suggested.
    Physical Review Letters 09/2003; 91(7):077203. · 7.73 Impact Factor
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    ABSTRACT: FePd layers deposited by molecular beam epitaxy at 620 K on a (001) Pd buffer are investigated. At this growth temperature, an L1<sub>0</sub> structure is expected; we compare the crystal ordering at different depths in these thin films. <sup>57</sup> Fe is used as a local probe at an atomic scale, located at different places within an equimolar FePd layer. The relationship between the alloy structure and the magnetic anisotropy of the films is investigated by Mössbauer spectroscopy. The possible decrease of the L1<sub>0</sub> ordering during growth is not observed. On the contrary, the quality of the L1<sub>0</sub> structure seems to improve, as confirmed by the low number of near-neighbors for iron atoms located on the top level of the alloy layer. Iron-rich clusters shaping into large vertical defects across the whole alloy layer are also observed; these clusters correspond to chemically disordered areas within the film. At the buffer interface, disordered, and dilute iron alloys (mean composition: Fe <sub>0.25</sub> Pd <sub>0.75</sub>) with in-plane momenta are identified and explained assuming iron atom diffusion through about 1 nm thick Pd buffer, due to Pd segregation. Some iron rich aggregates could be the consequence of iron homocoordination or Pd surface segregation. The L1<sub>0</sub> structure is assumed to occur after about 1.7 nm alloy deposit. © 2002 American Institute of Physics.
    Journal of Applied Physics 07/2002; · 2.21 Impact Factor
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    ABSTRACT: We demonstrate that, if a high degree of short range order is present in FePd disordered films, a high value of the long range order parameter S can be obtained by using postgrowth ion irradiation at very low processing temperatures. FePd films deposited monolayer by monolayer at room temperature on MgO(001) substrates exhibit a very low degree of long range order (S∼0.1) but a high degree of short range order as demonstrated by extended x-ray absorption fine structure measurements. Irradiation with 130 keV He+ ions at low fluences (2.0×1016 ions/cm2) leads to a large increase in the long range order parameter and to a large increase in perpendicular anisotropy for irradiation at substrate temperatures lower than 200 °C. This could have a great impact on the current race toward high magnetic recording density media. © 2002 American Institute of Physics.
    Journal of Applied Physics 05/2002; 91(10):8082-8084. · 2.21 Impact Factor
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    ABSTRACT: Strain relaxation processes have been investigated in chemically ordered FePd thin layers grown on Pd(001). Microtwins due to the pileup of a/6 〈211〉 dislocations release most of the relaxation. A statistical analysis of scanning tunneling microscopy images provides a detailed understanding of the involved processes and indicates a linear dependence of the relaxation process upon the FePd thickness. Such an unusual behavior is confirmed by reflection high-energy electron diffraction and x-ray diffraction measurements. Through a modeling of the repulsion between the cores of the partial dislocations forming the microtwin, we put forward the mechanism explaining this discrepancy with the Matthews law.
    Physical review. B, Condensed matter 01/2002; 65(20). · 3.77 Impact Factor
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    ABSTRACT: Summary form only given. Due to the huge magnetocrystalline anisotropy associated with chemical ordering within the tetragonal L10 phase, FePt(Pd), CoPt... alloys have attracted a growing interest. Indeed, they may be used in future ultra high density recording media as a way to push forward the thermal stability limit. We demonstrate that medium energy (30 keV) light ion (He+) irradiation allows efficient chemical ordering at low processing temperatures (around 500 K) and provides a full mastering of the magnetic properties i.e. in plane to out of plane easy axis reorientation (FePd), creation of 100 % perpendicular magnetic remanence (FePt). Our results suggest that the ability of ion irradiation to induce chemical order depends critically on the initial microstructure of the alloy. We also show that the microtwins induced by the strain relaxation processes (FePt/Pt[001)] lead to a spontaneous square nanostructuring of the thin layer. By acting as pinning sites for the domain walls, these defects are responsible for the major part of the magnetic coercivity.
    01/2002;
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    ABSTRACT: FePt and FePd films are presently attracting a wide interest because in the L10 tetragonal chemically ordered phase they exhibit a very high magnetic anisotropy. These properties make them attractive candidates as high-density magnetic recording media. However, the high growth temperature needed for ordering (∼ 400°C) is not compatible with manufacturing techniques. In this study, we show that if a high degree of short range order (SRO) is present in FePt and FePd disordered films, a high value of the long range order parameter S can be obtained by using post growth ion irradiation at very low processing temperatures (T1rr < 200°C). FePd and FePt [001] films were deposited monolayer by monolayer by MBE on MgO[001] at room temperature. In such films the degree of long range order (LRO) is very weak (S∼0.1). However as indicated by EXAFS measurements the degree of SRO is high due the directional order induced by the layer by layer growth. We used He+ ions at low fluence and temperatures of irradiation T1rr ranging from 20-300°C. In such conditions the ions move the atoms on a few interatomic distances and allow to modify in a controlled way the SRO. We demonstrate that high long range chemical ordering and high perpendicular anisotropy can be obtained at very low irradiation temperatures (< 200°C). This could have a great impact on current race toward high magnetic recording density media.
    01/2002;
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    ABSTRACT: In order to increase the storage capacities of the media of magnetic recording one seeks to decrease the size of the magnetic fields which carry information. The growth of thin layers of magnetic alloys in ordered phases makes it possible to plan structures with very strong perpendicular magnetic anisotropy, necessary to obtain stable fields in spite of their reduced size. We studied, thanks to an original device of in temperature X-rays diffraction, the ordering of FePd layers or FePt deposited by MBE in different states of initial order and constraint, then submitted to heat treatment. The ordering occured along the growth axis. The structure of the layers is very inhomogeneous: not ordered and ordered domains coexist, with or without defect in the stacking of the L10 phase, even domains of other phases. The small zones, which have been ordered during the growth, do not seem to take part in the development of the long range order during the treatment. The evolution of the ordered fraction is identical whatever the imposed state of epitaxial stress. Modifications of the experimental conditions make it possible from now to plan more systematic measurements with a temperature history more rigorous in order to confirm these observations. Afin d'augmenter les capacités de stockage des media d'enregistre ment magnétique, on cherche à diminuer la taille des domaines magnétiques qui portent l'information. L'élaboration de couches minces d'alliages magnétiques dans des phases ordonnées permet d'envisager des structures à très forte anisotropie magnétique perpendiculaire, nécessaire pour obtenir des domaines stables malgré leur taille réduite. Nous avons étudié, grâce à un dispositif original de diffraction X en température, la mise en ordre de couches FePd ou FePt déposées par MBE dans des états différents d'ordre initial et de contrainte. La mise en ordre se fait selon l'axe de croissance. La structure des couches est très inhomogène : coexistent des domaines non ordonnés et ordonnés, avec ou sans défanrt dans l'empilement de la phase Ll$_0$, voire d'autres phases. Les petites zones ordonnées lors de la croissance ne semblent pas participer au développement de l'ordre à grande distance lors du traitement en température. L'évolution de la fraction qui s'ordonne est identique quel que soft l'état de contrainte éphaxiale imposé. Des modifications des conditions expérimentales permettent désormais d'envisager des mesures plus systématiques avec un historique en tempérance plus rigoureux afin de confirmer ces observations.
    Journal de Physique IV (Proceedings) 01/2002; · 0.29 Impact Factor
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    ABSTRACT: Thin FePt (001) films, grown by molecular-beam epitaxy on Pt(001), exhibit a very large perpendicular magnetic anisotropy (Ku = 5×106 J m−3) and a 100% magnetic remanence in perpendicular field. The lattice misfit between FePt and Pt (1.5%) relaxes through the pileup of a/6 〈112〉 partial dislocations along {111} planes, leading to the formation of microtwins. Atomic force microscopy images demonstrate that this process induces a spontaneous rectangular nanostructuration of the sample, while magnetic force microscopy shows that the microtwins act as pinning sites for the magnetic walls. This leads to square magnetic domains and explains the large coercivity associated with the domain wall propagation. © 2001 American Institute of Physics.
    Applied Physics Letters 08/2001; 79(6):794-796. · 3.79 Impact Factor
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    ABSTRACT: We have developed an ion irradiation technique that allows us to tune the magnetic properties of several magnetic alloys which are considered as promising candidates for ultra-high density magnetic recording such as FePd, FePt and CoPt<sub>3</sub> films. Depending both on the initial microstructure of the films and temperature used during irradiation, the chemical order and the resulting magnetic anisotropy can be either increased or decreased in a very controlled way. We discuss the possible mechanisms involved in ordering processes under irradiation
    IEEE Transactions on Magnetics 08/2001; · 1.42 Impact Factor
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    ABSTRACT: The FePd alloy exhibits an L10 (CuAu-I type) chemically ordered phase which can be grown by molecular beam epitaxy at a low temperature (620 K) on Pd(0 0 1). The surface of the alloy has been observed by scanning tunnelling microscopy to determine the link between the surface morphology and the underlying chemical order. Steps are commonly observed with a bi-atomic height as a result of the surface processes leading to the chemical ordering. Mono-atomic steps are also observed and there is some evidence that they are linked to the antiphase boundaries between ordered domains within the growing layer.
    Surface Science 01/2001; · 1.84 Impact Factor