Self-organized growth on the Au(111) surface

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1 Self-organized growth on the Au(111) surface Olivier Fruchart 02/03/2002 Olivier Fruchart - Laboratoire Louis Néel, Grenoble, France. Slides on-line:

2 TABLE OF CONTENTS The Au(111) reconstruction, conventional self-organization: growth and magnetism Grazing Incidence Small Angle Xray Scattering : diffraction on dots arrays Vertical self-organization : growth and magnetism Acknowledgments Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.2 ] Slides on-line:

Au(111) > A template for self-organization FACTS: 350 x 350 nm Co, Ni, Fe, Cu, Rh growth on Au(111) gives rise to self-organized arrays of dots ORIGIN? (0.20AL Co@300K) nm 0.4 2Co 0.3 0.2 0.

3 Au(111) > A template for self-organization FACTS: 350 x 350 nm Co, Ni, Fe, Cu, Rh growth on Au(111) gives rise to self-organized arrays of dots ORIGIN? (0.20AL Co@300K) nm 0.4 2Co Fe, Ni : 1 AL-high dots nm D.D. Chambliss et al., PRL 66, 1721 (1991) B.Voigtlander et al., PRB 44, (1991) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.3 ] Slides on-line:

Au(111) > Basic s x 3 reconstruction Planar view of Au(111) FCC HCP FCC HCP ZOOM 50 x 28 nm fcc : ABC Cross-sectional view Corrugation hcp : ABA C C A A B B B B B B B B B A A A A A A A A A C C C C C

4 Au(111) > Basic s x 3 reconstruction Planar view of Au(111) FCC HCP FCC HCP ZOOM 50 x 28 nm fcc : ABC Cross-sectional view Corrugation hcp : ABA C C A A B B B B B B B B B A A A A A A A A A C C C C C C C C C B B B B B B B B B A A A A A A A A A Second layer atoms Site B Atoms of the topmost layer Site A Site C Bridge position (smaller diameter for clarity only) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.4 ] Slides on-line:

Au(111) > secondary chevron reconstruction 250 x 250nm Corrugation ~ 0.2 Å Unit cell size : ~ 7.5 x 25nm Isotropic surface relaxation J. V. Barth et al., Phys. Rev.

5 Au(111) > secondary chevron reconstruction 250 x 250nm Corrugation ~ 0.2 Å Unit cell size : ~ 7.5 x 25nm Isotropic surface relaxation J. V. Barth et al., Phys. Rev. B 42 (15), 9307 (1990) A.R. Sandy et al., Phys. Rev. B 43 (6), 4667 (1991) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.5 ] Slides on-line:

Au(111) deposits > nucleation stage Fe, Co, Ni (etc.

25ML Ni@300K : 1ML-high dots 0.005ML Co@300K J.A.Meyer et al., Surf.Sci.

, B96, 178 (2002) Leading parameter : deposit has a higher surface energy.

6 Au(111) deposits > nucleation stage Fe, Co, Ni (etc.) nucleation: atomic place exchange mechnism with Au atoms 0.002ML 0.25ML : 1ML-high dots 0.005ML J.A.Meyer et al., Surf.Sci.365, L647 (1996) V. Repain et al., Mater. Sci. Eng., B96, 178 (2002) Leading parameter : deposit has a higher surface energy. (and Au atoms stress near chevrons) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.6 ] Slides on-line:

7 Co/Au(111) magnetism > the superparamagnetic limit issue Spontaneous magnetization is perpendicular to the plane [similar to Co/Au(111) films] Co/Au(111) Anisotropy barrier ~KV ~25kT UP DOWN H.Takeshita et al., JMMM165, 38 (1997) see also: S. Padovani et al., Blocking temperature Tb ~ 20K H.Dürr et al., PRB59, R701 (1999) K. Koide et al., PRL87, (2001) Ph. Ohresser, F. Scheurer et al., private comm. Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.7 ] Slides on-line:

8 Co/Au(111) magnetism > spin and orbital moments Spin and orbital moments at films interfaces: Spin moment : slightly increased because of band narrowing (up to 30%) Orbital moment : quenched in bulk 3d (~0). Symmetry breaking at the interface Non-zero orbital moment (up to some 0.1µ B /atome) Probe: XMCD (X-ray Magnetic Circular Dichroism) and sum rules. Expected in dots: further symmetry breaking at the dots edges: enhanced effects? (similarity with atomic steps) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.8 ] Slides on-line:

9 Co/Au(111) magnetism > spin and orbital moments H.Dürr et al., PRB59, R701 (1999) K. Koide et al., PRL87, (2001) 1/N 1/2 fit Bulk Conclusion: extra orbital 2µ B /edge atome Problems: dots coalescence above 1300 atoms: non-valid fit Estimation of dot size by Langevin function (Brillouin ½ better suited) Conclusion: no extra orbital moment for edge atoms (dot= small thin film ). Problems: dot size is still large. Need smaller systems! Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.9 ] Slides on-line:

10 Fe/Au(111) magnetism > spin and orbital moments P. Ohresser et al., PRB64, (2001) High-spin fcc Conclusions: Spin moment not modified at edges (spin more influenced by deformation) Edge orbital moment ~ 0.5µ B, similar to steps on vicinal Fe. Orbital moment anisotropy: not constant during growth. Strain-dependent interface anisotropy? Problem = not even solved in thin films Low-spin fcc Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.10 ] Slides on-line:

, Nature 416, 301 (2002) P. Gambardella et al.

11 Orbital moment > Co/Pt(997) Co/Pt(997) Co/Pt(111) A. Dallmeyer et al., Phys.Rev.B 61(8), R5153 (2000) Conclusions Bulk: m L =0.14µ B /at. Surface: m L =0.31µ B /at. Bi-atomic wire: m L =0.37µ B /at. Mono-atomic wire: m L =0.68µ B /at. bi-atom: m L =0.78µ B /at. atom: m L =1.13µ B /at. P. Gambardella et al., Nature 416, 301 (2002) P. Gambardella et al., Science 300, 1130 (2003) Conclusions From bulk to atoms: considerable increase of orbital moment 2 atoms closer to wire than 1 atom bi-atomic wire closer to surface than wire Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.11 ] Slides on-line:

faces to atoms] P. Gambardella et al., Natur

12 Magnetic anisotropy > From surfaces to atoms From surface to wires (1D) Self-organized Co/Pt(997) Co atoms Pt terrace x z 2. Magnetic anisotropy 4 [3.5 From surfaces to atoms] P. Gambardella et al., Nature 416, 301 (2002) y µ µ L C a.u. a.u L 3 a Monatomic chains L 2 + b 1 monolayer C Bulk Photon energy (ev) Photon energy (ev) Photon energy (ev) L3 + L2 eff s 4 2L3 + L2 + - Conclusion: Increase of orbital moment (necessary condition for anisotropy) Anisotropy of orbital moment? + - Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.12 ] Slides on-line:

13 Magnetic anisotropy > From surfaces to atoms From surface to wires (1D) z z Method XMCD > Orbital moment Fit magnetization curves > Anisotropy functional MAE Bulk Co: 40µeV/atom Co ML: 140µeV/atom Co bi-wire: 0.34meV/atom Co wire: 2meV/atom M (a.u.) y (deg) T = 45 K T = 10 K x (deg) 0 Conclusions: Easy axis of magnetization perpendicular to the wires, but not the the mean film surface, nor to Pt(111) See anisotropy of orbital moment on the saturation XMCD. M (a.u.) B (T) B (T) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.13 ] Slides on-line:

5K Cf question by Dominique GIVORD Qualitatively: Easy axis of magnetization perpendicular to Pt(111) See

14 Magnetic anisotropy > From surfaces to atoms From surface to atoms (0D) Co/Pt(111) 10K 8 atoms 4 atoms 10K P. Gambardella et al., Science 300, 1130 (2003) 1 atom STM, 8.5nm, 5.5K 5.5K Cf question by Dominique GIVORD Qualitatively: Easy axis of magnetization perpendicular to Pt(111) See anisotropy of orbital moment on the saturation XMCD. Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.14 ] Slides on-line:

15 Magnetic anisotropy > From surfaces to atoms From surface to atoms (0D) Co/Pt(111) MAE Bulk Co: 40µeV/atom Co ML: 140µeV/atom Co bi-wire: 0.34meV/atom Co wire: 2meV/atom Co bi-atom: 3.4meV/atom Co atom: 9.2meV/atom Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.15 ] Slides on-line:

16 Co/Au(111) magnetism > the superparamagnetic limit issue Spontaneous magnetization is perpendicular to the plane [similar to Co/Au(111) films] Co/Au(111) Anisotropy barrier ~KV ~25kT UP DOWN H.Takeshita et al., JMMM165, 38 (1997) see also: S. Padovani et al., Blocking temperature Tb ~ 20K H.Dürr et al., PRB59, R701 (1999) K. Koide et al., PRL87, (2001) Ph. Ohresser, F. Scheurer et al., private comm. Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.16 ] Slides on-line:

Problem: K does not increase as fast as V decreases Co/Au(111) Increase V.

17 Co/Au(111) magnetism > the superparamagnetic limit issue T b = KV / 25k B TWO ROUTES TO OVERCOME SUPERPARAMAGNETISM in SO Increase K. Problem: K does not increase as fast as V decreases Co/Au(111) Increase V. Problem : lateral coalescence occurs 0.25AL 1.75AL INCREASE THE HEIGHT OF NANOSTRUCTURES? Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.17 ] Slides on-line:

18 Answer (?) Piling up dots vertically Assembly of isolated dots Vertical 3D self-organization of In x Ga 1-x As/GaAs : Q.Xie et al., Phys.Rev.Lett.75(13), 2542 (1995) Thinning the spacer layer Strong interaction between dots? superparamagnetism overcome? Enhanced magnetic signal Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.18 ] Slides on-line:

Step 1 : +Co Co2 Step 1 : 0.2AL Co @300K 350 x 350 nm Typical cross-section : nm 0.4 2Co 0.3 0.2 0.

19 Step 1 : +Co Co2 Step 1 : 0.2AL 350 x 350 nm Typical cross-section : nm 0.4 2Co nm Co2 Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.19 ] Slides on-line:

Step 2 : +Au Co2Au4 400 x 225 nm Step 2 : 3.8AL Au @375 410K nm Typical cross-section : nm 1AL Au decoration See also : Wollschläger et al., Surf.Sci.277, 1 (1992) Co2Au4 1AL Co hcp 0.

20 Step 2 : +Au Co2Au4 400 x 225 nm Step 2 : 3.8AL 410K nm Typical cross-section : nm 1AL Au decoration See also : Wollschläger et al., Surf.Sci.277, 1 (1992) Co2Au4 1AL Co hcp 0.205nm 1AL Au fcc 0.235nm Array of hollows Surface smoothing Unaffected Co dots Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.20 ] Slides on-line:

Step 3 : +Co2 Co4Au4 Step 3 : 0.1AL Co @500K nm Typical cross-section : 200 x 200 nm 0.15 0.10 0.05 0 1Au 4 (C o -A u) -.05 -.10-0.15 0 12.5 25 nm?

21 Step 3 : +Co2 Co4Au4 Step 3 : 0.1AL nm Typical cross-section : 200 x 200 nm Au 4 (C o -A u) nm? Co4Au4 Co/Au exchange mechanism The dots are now 4ML high Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.21 ] Slides on-line:

Vertical self-organization Scanning Tunneling Spectroscopy

8 Sample voltage (V) Co atoms grow only on existing dots

22 Vertical self-organization Scanning Tunneling Spectroscopy 100 x 100 nm TOPOGRAPHY SPECTROSCOPY SAME AREA di/dv (na/v) 2 1 Au Co Sample voltage (V) Co atoms grow only on existing dots VERTICAL SELF-ORGANIZATION Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.22 ] Slides on-line:

23 Further sequences : +Co(0.1MC)Au(0.9MC) CoxAux Co6Au6 One step = (0.1ML Co) + (0.9ML 80 x 80 nm 300 x 300 nm Co10Au10 Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.23 ] Slides on-line:

24 Co pillars of height 20ML 300 x 300 nm 7.5nm 3nm Co Au 6nm Au Au Self-organization nearly undisturbed Pillars with 2:1 vertical aspect ratio Unclear to this point : Exchange mechanism Limitating factors? Composition, microstructure? Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.24 ] Slides on-line:

25 Growth overview Sequential deposition process : from flat dots to vertical pillars. vertical replication of the sub-al flat pattern Partial control of the pillars geometry: lateral size (% of AL per cycle), height (number of cycles). Requirements on materials : lattice parameter mismatch, surface energies? May be OK for other elements as well. Open questions : composition, microstructure, destabilizing factors. O. Fruchart et al., Phys. Rev. Lett. 23 (14), 2769 (1999) O. Fruchart et al., Appl. Surf. Science , 529 (2000) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.25 ] Slides on-line:

26 Exchange versus nucleation mechanisms Facts 80 x 80 nm First Co layer : nucleation on reconstructions Further layers: reconstructions disturbed Questions What drives nucleation over buried dots? Why no nucleation on reconstructions? Understanding Driving parameters: lattice parameter mismatch, immiscibility, surface energy. Nucleation on reconstructions still possible, but energetically less favorable. Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.26 ] Slides on-line:

by incoming Co atoms Role of lattice parameter mismatch O. Fruchart et al., J. Cryst. Growth 237-239, p.

27 Role of lattice parameter mismatch Au(2.5AL)/Co(0.65AL)/Au(111) Au atoms layer Co atoms layer Au dislocations on Co Au easily expelled by incoming Co atoms Role of lattice parameter mismatch O. Fruchart et al., J. Cryst. Growth , p (2002) ; Proceedings of ICCG13/ICVGE11. Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.27 ] Slides on-line:

28 Magnetism superparamagnetic regime 7.5nm 3nm Co Magnetization essentially perpendicular 6nm Au 2 states: up and down (Ising macrospin) Superparamagnetism fitted using Brillouin 1/2 function Normalized magnetization 300 K 185 K 90 K H dip ~25kT 67 K µ 0 H (T) UP DOWN Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.28 ] Slides on-line:

29 Brillouin versus Langevin Classical spin Uniaxial anisotropy H // anisotropy axis βe = dm 2 d = βk hm K K v = V Anisotropy h = βµ 0 µh Zeeman Exact solution Partition function Z exp( dm + hm)dm = Obstacle (?) 2 t exp( x )dx =? 0 Imaginary Error function, Erfi(t) Magnetization m = h / 2d + (2 / π d ) Erfi( 2 exp( d + h / 4d )sinh( h) d + h / 2 d ) + Erfi( d h / 2 d ) Zero field susceptibility χ = 1/ 2d + exp( d) /( π d Erfi d ) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.29 ] Slides on-line:

30 Brillouin versus Langevin Asymptotic behavior High temperature χ = 1/3+ 4d /45 Langevin-like Low temperature χ =1 1/ d Brillouin ½ -like Blocking temperature T B = K / 25k B τ = τ 0 exp ( β B E ) 1s s Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.30 ] Slides on-line:

31 Brillouin versus Langevin T<5T B Zero field susceptibility Our data Initial susceptibility T>5T B Exact solution High temperature expansion: 1/3+4d/45 Low temperature expansion: 1-1/d d=β K /(β K)=T/T B O. Fruchart et al., J. Magn. Magn. Mater. 239, 224 (2002) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.31 ] Slides on-line:

32 Magnetism superparamagnetic regime Brillouin 1/2 function ( µ µ NH kt) m = B / ½ 0 Effective field Co Heff. = H + r M s m eff. (T) y = x R= /? (Demagnetizing dipolar interactions) First order expansion: susceptibility d( µ 0H) = 1= µ dm? 0 M T µ k Sr+ N a + Co b. T Deduced from STM N=3300 atoms Hdip= -32 mt T(K)... from magnetism N=2800 atoms Hdip= -42 mt Good quantitative agreement 1 pillar = 1 magnetic entity O. Fruchart et al., Phys. Rev. Lett. 23 (14), 2769 (1999) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.32 ] Slides on-line:

33 Magnetization process Textbook cases Loop + M M Distributed assembly Minor loops Easy-axis single domain Symmetrical Easy axis domains Nonsymmetrical Hard axis Symmetrical Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.33 ] Slides on-line:

34 Magnetization processes Polar MOKE T = 67K (b5.5; 3) T = 69K (b8; 4.2) -0.8T +0.8T B (T) B (T) B (T) Similar to «nucleation-annihilation» : domain structure, with zero wall energy. Larger H sat. and more model hard-axis loop for (b8; 4.2) : stronger dipolar interactions. ~ agreement with calculated r z (32 mt and 77 mt, respectively). Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.34 ] Slides on-line:

35 Magnetization processes Polar MOKE (b8; 4.2) 0.25 M + M r z (calculated) Minor loops mt B (T) B (T) Signature of demagnetizing dipolar interactions: agrees with model Antiferromagnetic ground state not reached: Frustration, metastability mt B (T) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.35 ] Slides on-line:

36 Magnetization processes Longitudinal MOKE (b8; 4.2) Loop 0.35 M + M % of M irr B (T) Less than 10% of awaited signal (large reversible part substracted) Very weak hysteresis : perpendicular magnetization. Very weak in-plane interdots dipolar fields : 0-17 mt B (T) Minor loops -50 mt B (T) B (T) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.36 ] Slides on-line:

37 Blocking temperature effect of sample volume T (K) B Pillar volume A B C v (nm ) D Blocking temperature > 300K (~ 30K for flat Co/Au dots) 300 K 185 K 90 K 67 K 70K 285K 61K 290K Sample A Sample C Sample B Expected: KV~25kTb K decreases for the largest pillars 60K 293K Sample D Applied Field (T) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.37 ] Slides on-line:

38 Rise of blocking temperature 18nm Sample with bigger pillars 7.5nm Au 4nm Co Normalized Polar MOKE Mr Field (T) 60K 150K 214K 293K 0.30 Moderate temperature effect Blocking temperature > 300K (~ 20K for flat Co/Au dots) Normalized remanence No saturation at 0.8T below 150K T >300K T (K) O. Fruchart et al., Proceedings of ISPMM/ISAMT 2001, to appear in J. Magn. Magn. Mater. B Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.38 ] Slides on-line:

39 Co/Au(111) pillars conclusion New growth process: vertical replication of sub-ml flat pattern self-organized pillars with vertical aspect ratio Driving forces: lattice parameter mismatch, immiscibility, surface energy. validity for other elements than Co/Au? Motivation for magnetism: dramatic rise of Tb rise of magnetic signal (stray fields, etc.) Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.39 ] Slides on-line:

J 12 J 23 J 34. Driving forces in the nano-magnetism world. Intra-atomic exchange, electron correlation effects: Inter-atomic exchange: MAGNETIC ORDER

J 12 J 23 J 34. Driving forces in the nano-magnetism world. Intra-atomic exchange, electron correlation effects: Inter-atomic exchange: MAGNETIC ORDER Driving forces in the nano-magnetism world Intra-atomic exchange, electron correlation effects: LOCAL (ATOMIC) MAGNETIC MOMENTS m d or f electrons Inter-atomic exchange: MAGNETIC ORDER H exc J S S i j