Etudes in situ et ex situ de multicouches C/FePt

Transcription

1 Etudes in situ et ex situ de multicouches C/FePt : influence de la température sur la structure et les propriétés s magnétiques D. Babonneau, G. Abadias, F. Pailloux Laboratoire de Physique des Matériaux (PHYMAT) UMR 6630 CNRS Université de Poitiers N. Jaouen, E. Fonda Synchrotron SOLEIL, Gif-sur-Yvette F. Petroff Unité Mixte de Physique CNRS/THALES, Palaiseau J.-S. Micha UMR SPrAM 5819 CNRS, CEA DRMFC, Grenoble

2 Ultrahigh density magnetic recording and superparamagnetism Applications expected in magnetic data storage Ordered L1 0 FePt phase (tetragonal structure) Fe Pt Pt K u = erg.cm -3 Higher areal densities of magnetic recording media require smaller grains Nanometric grains have a higher probability of flipping Superparamegnetic limit: K k V T u B > 40 Grains with high magnetic anisotropy are required to preserve thermal stability

3 Effects of thermal annealing in cosputtered Fe Fe 50 Pt films As-deposited (RT) 350 C 450 C 550 C 650 C Grain size nm Disordered fcc structure (a = 3.83 Å) Grain size increases to nm Phase transformation to the ordered fct structure (c/a = 0.96) D. Babonneau et al., Rev. Adv. Mater. Sci. 15 (2007) Encapsulation of FePt nanoparticles in carbon: protection against oxidation, reduction of magnetic coupling, limitation of grain growth during deposition and annealing

4 Growth of C/FePt granular multilayers by ion-beam sputtering Target holder Sputtering gun Ar ev Alternate deposition from pure C and FePt targets Si 4 nm Carbon (T = 20 C) Si 1 nm Fe 50 Pt 50 N Si 4 nm Carbon Substrate holder Structural properties: TEM, GISAXS, XDR, XAS Magnetic properties: SQUID, XMCD

5 Structure of the C/FePt granular multilayers High-resolution TEM (cross-section section view) Layered structure with dark grains separated by a-c Lateral grain size D 3 nm and height H 2 nm Sharp size distribution 1,8 As-deposited at RT Intensity (Arb. Units.) 1,2 0,6 C K-edge As deposited Annealed XANES 0, Photon Energy (ev) Annealed 1 h at 600 C Increase of the grain size (lateral and vertical) Carbon matrix becomes more graphitic: protection of the FePt nanoparticles against oxidation

6 Structure of the C/FePt granular multilayers High-resolution TEM (cross-section section view) Grazing incidence X-ray X diffraction (GIXRD) Disordered fcc structure a = 3.79 Å, c/a = 1, S = 0 As-deposited at RT Partially ordered fct structure a = 3.89 Å, c/a = 0.92, S = 0.55 Annealed 1 h at 600 C

7 Energy filtered transmission electron microscopy After annealing at 800 C CTEM Si map C map Bimodal size distribution tends to develop Self-organization is destroyed Formation of precipitates in the Si substrate oriented at 54 with respect to the Si/C interface Fe map Chemical interaction of the Fe atoms with the Si substrate

8 In-situ GISAXS and GIXRD of the C/FePt granular multilayers Annealing from RT to 820 C C (ESRF BM kev) GISAXS map contains information on the size and organization (lateral and vertical) of the carbonencapsulated FePt nanoparticles Disordered fcc phase with a = 3.79 Å In situ GISAXS In situ GIXRD D. Babonneau et al., J. Phys.: Condens. Matter 20 (2008)

9 In-situ GISAXS analysis of the C/FePt granular multilayers I( q) = k1 P( q, D1 ) S( q, D1, ηhs ) + k2 P( q, D2 ) Correlated nanoparticles Uncorrelated Increase of the nanoparticle size Increase of the bilayer thickness! Increase of the interparticle distance Self-organization is destroyed

10 Magnetic properties of the C/FePt granular multilayers As-deposited at RT Annealed 1h at 600 C 30 K A1 30 K L1 0 H c = 120 Oe M r /M s = 0.71 µ at = 1.24 µ B H c = 1600 Oe M r /M s = 0.57 µ at = 1.65 µ B Strong interactions between as-deposited FePt nanoparticles M s is 20% smaller than the bulk value for the disordered A1 phase FePt nanoparticles annealed at 600 C are weakly coupled M s increases due to FePt precipitation and chemical ordering H 0 = 2.2 koe T B = 250 K For superparamagnetic systems: H 1 c T ( T ) = H 0 T B K = erg.cm -3 K 25kBT = V B Disordered A 1 FePt phase: K = erg.cm -3 Ordered L1 0 FePt phase: K = erg.cm -3

11 Magnetic properties of the C/FePt granular multilayers XMCD at RT (Daresbury( 5U.1 beamline) Intensity (arb. units.) Intensity (arb. units.) a) b) Fe L 2,3 Fe L 2, Photon Energy (ev) XAS XMCD As-deposited at RT XAS XMCD Annealed at 600 C No multiplet: FePt particles are protected against oxidation XMCD at RT (ESRF ID12 beamline) Intensity (Arb. Units.) Intensity (Arb. Units.) 1,5 1,0 0,5 0,0-0,5 1,5 1,0 0,5 0,0 a) Pt L 2,3 b) XAS As-deposited at RT XAS XMDC x2-0, Photon Energy (ev) XMCD x2 Annealed at 600 C Spin and orbital magnetic moments are increased with annealing at both the Fe and Pt sites N. Jaouen et al., Phys. Rev. B 76 (2007)

12 Conclusion Carbon encapsulation can be used to restrain the growth of the FePt nanoclusters both during the deposition process and during annealing Partial chemical ordering of the FePt nanoclusters can be achieved after annealing 600 C Annealing at 800 C leads to destruction of the self-organization and to chemical interaction of iron atoms with the silicon substrate As-deposited at RT C/FePt multilayer As-deposited at RT BN/FePt multilayer Annealed at 600 C Annealed at 600 C

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14 Magnetic properties of the C/FePt granular multilayers As-deposited at RT Annealed 1h at 600 C

15 Coll. J.M. Tonnerre, N. Jaouen, D. Carbone 2- Magnetic properties by x -ray resonant scattering - FePt/C a granular multilayer: depth informations from XRMS Intensity 10 0 E= 707.5eV I(Pol+, H+) I(Pol+, H-) x10-4 1x Ratio [a.u.] 0.8 E= 707.5eV (I + -I - )/(I + +I - ) ϑ [ ] ϑ (a) uniform magnetisation (b) magnetic core (non-magnetic shell) (c) magnetic shell (non-magnetic core) Asymmetry Ratio ϑ[ ] Model (a) Model (b) Model (c)