Self -organized growt h on t he Au(111) surf ace Olivier Fruchart - - PowerPoint PPT Presentation

Olivier Fruchart - Laboratoire Louis Néel, Grenoble, France. Olivier Fruchart - Laboratoire Louis Néel, Grenoble, France.

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Self -organized growt h on t he Au(111) surf ace

Olivier Fruchart

02/03/2002

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TABLE OF CONTENTS The Au(111) reconst ruct ion, convent ional self -organizat ion: growt h and magnet ism Grazing I ncidence Small Angle Xray Scat t ering : ‘dif f ract ion’ on dot s ar r ays Ver t ical self -organizat ion : growt h and magnet ism Acknowledgment s

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Au(111) > A t emplat e f or self -or ganizat ion

(0.20AL Co@300K)

Fe, Ni : 1 AL-high dots

5 10 15 20 0.1 0.2 0.3 0.4

2Co nm nm

350 x 350 nm

D.D. Chambliss et al., PRL 66, 1721 (1991) B.Voigtlander et al., PRB 44, 10354 (1991)

FACTS: Co, Ni, Fe, Cu, Rh growth on Au(111) gives rise to self-organized arrays of dots ORIGIN ?

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Au(111) > Basic s x √3 r econst r uct ion

Site B Second layer atoms Site A Atoms of the topmost layer Site C Bridge position

(smaller diameter for clarity only)

Cross-sectional view Planar view of Au(111)

A A A A B B C C C C C C C C C C C B B B B B B B B B B B B B B B B A A A A A A A A A A A A A A A A

fcc : ABC hcp : ABA Corrugation 50 x 28 nm

FCC HCP Z O O M FCC HCP

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Au(111) > secondary ‘chevr on’ r econst r uct ion

250 x 250nm

• J. V. Barth et al., Phys. Rev. B 4 2 ( 1 5 ) , 9 3 0 7 ( 1 9 9 0 )

Corrugation ~ 0.2 Å Unit cell size : ~ 7.5 x 25nm

A.R. Sandy et al., Phys. Rev. B 4 3 ( 6 ) , 4 6 6 7 ( 1 9 9 1 )

Isotropic surface relaxation

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Au(111) deposit s > nucleat ion st age

J.A.Meyer et al., Surf.Sci.3 6 5 , L647 (1996)

Fe, Co, Ni (etc.) nucleation: atomic place exchange mechnism with Au atoms

Leading parameter : deposit has a higher surface energy. (and Au atoms stress near chevrons)

0.25ML Ni@ 300K : 1ML-high dots

0.005ML Co@300K 0.002ML Ni@300K

• V. Repain et al., Mater. Sci. Eng., B96, 178 (2002)

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Co/ Au(111) magnet ism > t he super par amagnet ic limit issue Anisotropy barrier ~KV Co/Au(111)

H.Takeshita et al., JMMM1 6 5 , 3 8 ( 1 9 9 7 ) see also: S. Padovani et al.,

Blocking temperature Tb ~ 20K

H.Dürr et al., PRB5 9 , R7 0 1 ( 1 9 9 9 )

• K. Koide et al., PRL8 7 , 2 5 7 2 0 1 ( 2 0 0 1 )
• Ph. Ohresser, F. Scheurer et al., private

comm.

UP DOWN

~25kT

Spontaneous magnetization is perpendicular to the plane [similar to Co/ Au(111) films]

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Co/ Au(111) magnet ism > spin and or bit al moment s

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 0.1µB/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)

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Co/ Au(111) magnet ism > spin and or bit al moment s

H.Dürr et al., PRB5 9 , R7 0 1 ( 1 9 9 9 )

Conclusion: extra orbital 2µB/edge atome Problems:

• dots coalescence above 1300 atoms:

non-valid fit…

• Estimation of dot size by Langevin

function (Brillouin ½ better suited) 1/ N1/ 2 fit Bulk

• K. Koide et al., PRL8 7 , 2 5 7 2 0 1 ( 2 0 0 1 )

Conclusion: no extra orbital moment for edge atoms (dot=‘small thin film’). Problems:

• dot size is still large.

Need smaller systems !

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Fe/ Au(111) magnet ism > spin and or bit al moment s

• P. Ohresser et al., PRB6 4 , 1 0 4 4 2 9 ( 2 0 0 1 )

Conclusions:

• Spin moment not modified at edges

(spin more influenced by deformation)

• Edge orbital moment ~ 0.5µB, similar to steps
• n vicinal Fe.
• Orbital moment anisotropy: not constant

during growth. Strain-dependent interface anisotropy ? Problem = not even solved in thin films… Low-spin fcc High-spin fcc

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Orbit al moment > Co/ Pt (997)

Conclusions

• From bulk to atoms:

considerable increase of orbital m om ent

• 2 atom s closer to w ire than 1 atom
• bi- atom ic w ire closer to surface than w ire

Conclusions

• Bulk: m L= 0.14µB/ at.
• Surface: m L= 0.31µB/ at.
• Bi-atomic wire: m L= 0.37µB/ at.
• Mono-atomic wire: m L= 0.68µB/ at.
• bi-atom: m L= 0.78µB/ at.
• atom: m L= 1.13µB/ at.
• A. Dallmeyer et al., Phys.Rev.B 61(8), R5153 (2000)

Co/ Pt( 9 9 7 )

• P. Gambardella et al., Science 300, 1130 (2003)
• P. Gambardella et al., Nature 416, 301 (2002)

Co/ Pt( 1 1 1 )

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Magnet ic anisot ropy > From sur f aces t o at oms

• 2. Magnetic anisotropy [ 3.5 From surfaces to atoms]

Co atoms Pt terrace x z y

770 780 790 800 810

−10 −8 −6 −4 −2

770 780 790 800 810 770 780 790 800 810

a Monatomic

chains

b 1 monolayer

C Bulk

L 3 L2 Photon energy (eV) Photon energy (eV) Photon energy (eV) C a.u. a.u.

• +

−

+

−

+

• +

−

+

• +

∫ ∫

+ ≈

2 3 L

L L µ

∫ ∫

+ − ≈

2 3 eff s

4 2 L L µ

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

Self-organized Co/ Pt(997)

From surface to w ires ( 1 D)

Conclusion:

• Increase of orbital moment

(necessary condition for anisotropy)

• Anisotropy of orbital moment?

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Magnet ic anisot ropy > From sur f aces t o at oms

From surface to w ires ( 1 D)

y M (a.u.) (deg)

• 9
• 9
• 6
• 6
• 30
• 30

30 60 90 0.0 0.1 0.2 0.3 0.4 (deg) 30 60 90 0.0 0.1 0.2 z x z

−57°

+ 43° T = 45 K B (T) B (T) T = 10 K M (a.u.)

• 6
• 4

4

• 2

2 6

• 6
• 4

4

• 2

2 6

+ 43°

• 5 7 °

0 ° 9 0 ° 0 ° 9 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.

• XMCD > Orbital moment
• Fit magnetization curves

> Anisotropy functional Method

• Bulk Co: 40µeV/ atom
• Co ML: 140µeV/ atom
• Co bi-wire: 0.34meV/ atom
• Co wire: 2meV/ atom

MAE

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Magnet ic anisot ropy > From sur f aces t o at oms

• P. Gambardella et al., Science 300, 1130 (2003)

Co/ Pt( 1 1 1 )

From surface to atom s ( 0 D)

8 atom s 4 atom s 1 atom STM, 8 .5 nm , 5 .5 K Qualitatively:

• Easy axis of magnetization

perpendicular to Pt(111)

• See anisotropy of orbital

moment on the saturation XMCD. 5.5K 10K 10K Cf quest ion by Dominique GI VORD

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Magnet ic anisot ropy > From sur f aces t o at oms

From surface to atom s ( 0 D)

Co/ Pt( 1 1 1 )

• 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

MAE

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Co/ Au(111) magnet ism > t he super par amagnet ic limit issue Anisotropy barrier ~KV Co/Au(111)

H.Takeshita et al., JMMM1 6 5 , 3 8 ( 1 9 9 7 ) see also: S. Padovani et al.,

Blocking temperature Tb ~ 20K

H.Dürr et al., PRB5 9 , R7 0 1 ( 1 9 9 9 )

• K. Koide et al., PRL8 7 , 2 5 7 2 0 1 ( 2 0 0 1 )
• Ph. Ohresser, F. Scheurer et al., private

comm.

UP DOWN

~25kT

Spontaneous magnetization is perpendicular to the plane [similar to Co/ Au(111) films]

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Co/ Au(111) magnet ism > t he super par amagnet ic limit issue

B b

25 / k KV T =

TW O ROUTES TO OVERCOME SUPERPARAMAGNETI SM in SO

• Increase K. Problem: K does not increase as fast

as V decreases

• Increase V. Problem : lateral coalescence occurs

Co/ Au( 1 1 1 ) 0 .2 5 AL 1 .7 5 AL I NCREASE THE HEI GHT OF NANOSTRUCTURES ?

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Answer ( ?) – Piling up dot s ver t ically

Vertical 3D self-organization of

InxGa1-xAs/ GaAs :

Q.Xie et al., Phys.Rev.Lett.75( 13) , 2542 ( 1995)

Assembly of isolated dots Strong interaction between dots? Thinning the spacer layer

superparamagnetism overcome ? Enhanced magnetic signal

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St ep 1 : +Co Co2

Step 1 : 0.2AL Co @ 300K Typical cross-section : Co2

5 10 15 20 0.1 0.2 0.3 0.4

2Co nm nm

350 x 350 nm 350 x 350 nm

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St ep 2 : +Au Co2Au4

400 x 225 nm 400 x 225 nm Step 2 : 3.8AL Au @ 375À410K

See also : W ollschläger et al. , Surf.Sci.2 7 7 , 1 ( 1 9 9 2 )

1AL Co hcp ≈ 0.205nm 1AL Au fcc ≈ 0.235nm Array of hollows

nm nm Typical cross-section :

1AL Au decoration Surface smoothing Unaffected Co dots Co2Au4

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St ep 3 : +Co2 Co4Au4

Step 3 : 0.1AL Co @ 500K Co/Au exchange mechanism The dots are now 4ML high Typical cross-section :

12.5 25

• 0.15
• .10
• .05

0.05 0.10 0.15

1Au 4 (C o -Au) nm nm

Co4Au4

?

200 x 200 nm 200 x 200 nm

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Ver t ical self -organizat ion – Scanning Tunneling Spect r oscopy

Co atoms grow onlyon existing dots

VERTICAL SELF-ORGANIZATION

100 x 100 nm TOPOGRAPHY SPECTROSCOPY SAME AREA

• 0.8

1 2

• 0.4

Sample voltage (V) dI/dV (nA/V)

0.4 0.8

Co Au

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Furt her sequences : +Co(0.1MC)Au(0.9MC) CoxAux

One step = (0.1ML Co) + (0.9ML Au) @ 500K

Co6Au6 Co10Au10 80 x 80 nm 80 x 80 nm 300 x 300 nm 300 x 300 nm

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Co pillars of height 20ML

300 x 300 nm 300 x 300 nm Self-organization nearly undisturbed Pillars with 2:1 vertical aspect ratio Unclear to this point :

• Exchange mechanism
• Limitating factors ?
• Composition, microstructure ?

6nm 7.5nm 3nm Co Au Au Au

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Growt h – over view 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. 2 3 ( 1 4 ) , 2 7 6 9 ( 1 9 9 9 )
• O. Fruchart et al. , Appl. Surf. Science 1 6 2- 1 6 3 , 5 2 9 ( 2 0 0 0 )

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Exchange versus nucleat ion mechanisms

First Co layer : nucleation on

reconstructions

Further layers: reconstructions disturbed Facts What drives nucleation over buried dots ? Why no nucleation on reconstructions ? Questions Driving parameters: lattice parameter

mismatch, immiscibility, surface energy.

Nucleation on reconstructions still possible,

but energetically less favorable.

Understanding 80 x 80 nm 80 x 80 nm

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Role of lat t ice paramet er mismat ch Au dislocations on Co

Au easily expelled by

incoming Co atoms

Role of lattice parameter

mismatch

+1 +2 +3 +3 +1 +2 Au atoms layer Co atoms layer

Au(2.5AL)/ Co(0.65AL)/ Au(111)

• O. Fruchart et al. , J. Cryst. Grow th 2 3 7- 2 3 9 , p. 2 0 3 5 ( 2 0 0 2 ) ;

Proceedings of I CCG1 3 / I CVGE1 1 .

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300 K 67 K

• 0.8 -0.6 -0.4 -0.2

0.2 0.4 0.6 0.8 185 K 90 K Hdip µ0H (T) Normalized magnetization

Magnet ism – super par amagnet ic r egime

6nm 7.5nm 3nm Co Au Magnetization essentially

perpendicular

2 states: up and down (Ising

macrospin)

Superparamagnetism fitted using

Brillouin 1/2 function UP DOWN

~25kT

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Br illouin ver sus Langevin hm dm E − − =

2

β

Classical spin

K d β = v K K × =

V

H h µ βµ0 =

Uniaxial anisotropy H / / anisotropy axis Anisotropy Zeeman

∫−

+ =

1 1 2

d ) exp( m hm dm Z

Exact solution

Partition function Obstacle (?)

? d ) exp(

2

=

∫

t

x x

Imaginary Error function, Erfi(t)

) 2 / Erfi( ) 2 / Erfi( ) sinh( ) 4 / exp( ) / 2 ( 2 /

2

d h d d h d h d h d d d h m − + + + × + − = π ) Erfi /( ) exp( 2 / 1 d d d d π χ + − =

Magnetization Zero field susceptibility

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Br illouin ver sus Langevin 45 / 4 3 / 1 d + = χ d / 1 1− = χ

B B

25 / k K T =

( )

E

B 0 exp β

τ τ =

High temperature Low temperature 1s 10- 9 – 10- 12 s Brillouin ½ -like Langevin-like

Asym ptotic behavior Blocking tem perature

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10 20 30 40 50 0.4 0.5 0.6 0.7 0.8 0.9 1.0

25/(β K)=T/TB

Zero field susceptibility

Br illouin ver sus Langevin

5 10 15 20 0.4 0.5 0.6 0.7 0.8 0.9 1.0

d=β K

Exact solution High temperature expansion: 1/3+4d/45 Low temperature expansion: 1-1/d

T< 5TB T> 5TB

Initial susceptibility

Our data

• O. Fruchart et al. , J. Magn. Magn. Mater. 2 3 9 , 2 2 4 ( 2 0 0 2 )

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0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 50 100 150 200 250 300 y = 0.042584 + 0.00030788x R= 0.96311 T(K)

Magnet ism – super par amagnet ic r egime

( )

kT NH µ µ m / B

eff. Co ½

= m M r H H

s eff.

+ =

T N µ k r M µ ? dm H µ d

Co

S

1 ) (

+ − = =

a + b . T

Brillouin 1/ 2 function Effective field First order expansion:

susceptibility

1/ ?

(T) Deduced from STM ... from magnetism

N=3300 atoms Hdip= -32 mT N=2800 atoms Hdip= -42 mT

Good quantitative agreement 1 pillar = 1 magnetic entity

(Demagnetizingdipolar interactions)

• O. Fruchart et al. , Phys. Rev. Lett. 2 3 ( 1 4 ) , 2 7 6 9

( 1999)

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Magnet izat ion process – Text book cases

Easy-axis ‘ single domain ’ ‘ Easy ’ axis ‘ domains ’ Hard axis

Distributed assembly

− + − M

M

Loop Minor loops Symmetrical Symmetrical Non- symmetrical

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• 0.4
• 0.2

0.2 0.4

B (T)

• 0.4
• 0.3
• 0.2
• 0.1

0.1 0.2 0.3 0.4

B (T)

• 0.8
• 0.6
• 0.4
• 0.2

B (T)

T = 69K (b8;∅4.2) Magnet izat ion processes – Polar MOKE Similar to « nucleation-annihilation » : ‘ domain ’ structure, with zero wall energy. Larger Hsat. and more ‘ model ’ hard-axis loop for (b8;∅4.2) : stronger dipolar interactions. ~ agreement with calculated rz (32 mT and 77 mT, respectively). T = 67K (b5.5;∅3)

• 0.8T

+0.8T

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• 0.4
• 0.3
• 0.2
• 0.1

0.1 0.2 0.3 0.4 B (T)

• 185 mT
• 0.4
• 0.3
• 0.2
• 0.1

0.1 0.2 0.3 0.4 B (T)

• 15 mT

0.05 0.1 0.15 0.2 0.25

• 0.8
• 0.6
• 0.4
• 0.2

0.2 0.4 0.6 0.8 B (T)

Magnet izat ion processes – P

• lar MOKE (b8;∅4.2)

Minor loops

rz (calculated)

− + − M

M

Signature of demagnetizing dipolar interactions:

agrees with model

‘ Antiferromagnetic ’ ground state not reached:

Frustration, metastability.

Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.36 ] Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.36 ]

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• 0.1
• 0.05

0.05 0.1 B (T)

• 50 mT
• 0.1
• 0.05

0.05 0.1 B (T)

• 17 mT

0.05 0.1 0.15 0.2 0.25 0.3 0.35

• 0.15
• 0.1
• 0.05

0.05 0.1 0.15 B (T)

• 0.1
• 0.05

0.05 0.1 B (T)

Magnet izat ion processes – Longit udinal MOKE (b8;∅4.2)

Loop

Less than 10% of awaited signal (large reversible part substracted) Minor loops

• Very weak hysteresis : perpendicular

magnetization.

• Very weak in-plane inter-

dots dipolar fields :

% of Mirr.

− + − M

M

Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.37 ] Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.37 ]

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100 200 100 200 300

v (nm )

3

T (K)

B

A B C D

• 0.8
• 0.4

0.0 0.4 0.8

Applied Field (T)

300 K 67 K 185 K 90 K

Sample A

61K 290K

Sample B

285K 70K

Sample C

60K 293K

Sample D Blocking t emperat ure – ef f ect of sample volume

Blocking temperature > 300K

(~ 30K for flat Co/Au dots) Expected: KV~25kTb

K decreases for the largest pillars

Pillar volume

Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.38 ] Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.38 ]

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Rise of blocking t emperat ure

18nm 7.5nm 4nm Co Au Sample with bigger

pillars

Moderate temperature effect Blocking temperature > 300K

(~ 20K for flat Co/Au dots)

• 0.6
• 0.4
• 0.2

0.0 0.2 0.4 0.6

• 1.0
• 0.5

0.0 0.5 1.0

60K 150K 214K 293K

Normalized Polar MOK E Field (T)

100 200 300 0.00 0.10 0.20 0.30 T (K) Normalized remanence

No saturation at 0.8T below 150K

T >300K

B

Mr

• O. Fruchart et al. , Proceedings of I SPMM/ I SAMT 2 0 0 1 , to appear in J. Magn. Magn. Mater.

Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.39 ] Olivier Fruchart - LLN-CNRS. [ 02/03/2002 / p.39 ]

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Co/ Au(111) pillar s – 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.)