Magnetic properties of self-organized systems [ Individual and - - PowerPoint PPT Presentation

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Magnetic properties of self-organized systems

[ Individual and collective aspects ]

O.Fruchart

Laboratoire Louis Néel (CNRS-UJF-INPG) Grenoble

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.2

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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TABLE OF CONTENTS

1 . I ntroduction to m agnetism 2 . I ndividual properties 3 . Collective properties 4 . Tow ards m aterials?

Magnetic properties of self-organized systems : TOC

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.3

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INTRODUCTION TO MAGNETISM – Currents, magnetic fields and magnetization

r I B π µ 2 =

⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − × = µ r µ.r) ( 3 4

2 3

r r B π µ µ

Magnetization: A.m-1 Magnetic moment: A.m2

Oersted field Magnetic moment Magnetic material

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.4

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INTRODUCTION TO MAGNETISM – Hysteresis loops

Hext M

Manipulation of magnetic materials: Application of a magnetic field

H.M

Z

µ − = E

Zeeman energy: Spontaneous magnetization Ms Remanent magnetization Mr

Hext M

Losses

M H E d

ext

∫

= µ

Coercive field Hc

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.5

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INTRODUCTION TO MAGNETISM – Magnetically soft and hard magnetic materials

Soft materials

Transformers Flux guides Magnetic shielding

Hard materials

Permanent magnets, motors Magnetic recording

Hext M

Hext M

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.6

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INTRODUCTION TO MAGNETISM – Sources of magnetic energy

2 2 1 2 , 1 Ech

) ( . θ ∇ = − = A J E S S ) ( sin 2

mc

θ K E =

H M .

S Z

µ − = E

1 2

d S d

. 2 1 H M µ − = E

Zeeman energy (enthalpy) Magnetocrystalline anisotropy energy Dipolar energy Echange energy

Hext M

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.7

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INTRODUCTION TO MAGNETISM – Magnetic characteristic length scales

Typical length scale: Bloch wall width λB

( )

θ θ

2 2

sin / K dx d A e + =

Exchange Anisotropy

J/m

3

J/m

Numerical values

K A/

B

π λ =

nm 3 2

B

− = λ nm 100

B ≥

λ

Hard Soft

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.8

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INTRODUCTION TO MAGNETISM – What is nanomagnetism?

Bulk material

One possible definition: mesoscopic magnetism

Co(1000) crystal – SEMPA

• A. Hubert, Magnetic domains

Mesoscopic scale

Numerous and complex magnetic domains Small number of domains, simple shape

Microfabricated dots Kerr magnetic imaging

• A. Hubert, Magnetic domains

Nanometric scale

Magnetic single-domain

R.P. Cowburn, J.Phys.D:Appl.Phys.33, R1 (2000)

Magnetic recording essentially makes use of single-domain particles

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.9

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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TABLE OF CONTENTS

1 . I ntroduction to m agnetism 2 . I ndividual properties 3 . Collective properties 4 . Tow ards m aterials?

Magnetic properties of self-organized systems : TOC

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.10

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INDIVIDUAL PROPERTIES – Ferromagnetic order

Naïve m odel

B c

k M Nw J J T . 3 1 . m + = µ

Molecular field N neighbors

N b N s

t N N N N ) ( 2

s b b

− − =

• 1

c

~ ) ( t t T ∆

Less naïve…

λ

• c

~ ) ( t t T ∆ 1 = λ

G.A.T. Allan, PRB1 , 3 5 2 ( 1 9 7 0 )

Thickness-dependant molecular field

Experim ents

• U. Gradm ann,

Handbook of Magn. Mater. Vol.7 , ch.1 ( 1 9 9 3)

Conclusion: Naïve views are roughly correct

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.11

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INDIVIDUAL PROPERTIES – Ferromagnetic order

J.Hauschild et al., Phys.Rev.B57, R677(1998)

250 x 250 nm

Fe( 0 .5 ML) / W ( 1 1 0 )

Stripes on vicinal surfaces Magnetic order in 2D: finite-size scaling

Curie tem perature as a function of stripe w idth

• H. J. Elmers et al., PRL73, 898 (1994)

03 . 1 = λ

in agreement with Ising model (=1)

3 . 2 . 1 ± = λ

for Fe stripes / Pd(110)

• D. Li et al., PRB64, 144410 (2001)

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.12

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INDIVIDUAL PROPERTIES – Magnetic anisotropy

(Derived from slide of A. Thiaville – CNRS/Orsay)

Electronic cloud Atom nucleus (crystal structure) Spin-orbit coupling the energy of both spin and orbital m om ent depends on orientation Series development on an angular basis:

...

4 2 2 1 mc

+ + =

z z

m K m K E

Uniaxial

... ) (

2 2 2 2 2 2 4 mc

+ + + =

x z z y y x

m m m m m m K E

Cubic

…

Anisotropy energy Alignem ent of m agnetization is favored along given axes of the crystal Norm alized m agnetization com ponents

Magnetocrystalline anisotropy energy

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.13

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INDIVIDUAL PROPERTIES – Magnetic anisotropy

+ + = >

... ) ( cos2

1 mel, mel

+ = θ K E ε

i i

K B ~

mel, Result Origin Deformation of orbitals Correction to the magneto-crystalline energy

Magneto-elastic anisotropy

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.14

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INDIVIDUAL PROPERTIES – Magnetic anisotropy

• L. Néel,
• J. Phys. Radium 15,

15 (1954)

« Cette énergie de surface, de l’ordre de 0.1 à 1 erg/ cm2, est susceptible de jouer un rôle important dans les propriétés des substances ferromagnétiques dispersées en éléments de dimensions inférieures à 100Å » « This surface energy, of the order of 0.1 to 1 erg/ cm2, is liable to play a significant role in the properties of ferromagnetic materials spread in elements of dimensions smaller than 100Å » « Anisotropie magnétique superficielle et surstructures d'orientation »

« Superficial magnetic anisotropy and orientational superstructures »

Overview Breaking of symmetry for surface/ interface atoms Correction to the magneto-crystalline energy Pair m odel of Néel:

• Ks estimated from magneto-elastic constants
• Does not depend on interface material
• Yields order of magnitude only: correct value from experiments or calculations

... ) ( cos ) ( cos

4 2 S, 2 1 S, s

+ + = θ θ K K E

Surface anisotropy

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.15

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INDIVIDUAL PROPERTIES – Magnetic anisotropy

History of surface anisotropy : STEP 1 (1/ t plot)

S V tot

2 ) ( k t k t E + = t 2 ) (

S V

k k t e + =

1/ t e(t) Bulk Slope --> Surfaces

First exam ple of perpendicular anisotropy

• U. Gradmann and J. Müller,
• Phys. Status Solidi 27, 313 (1968)

Bulk T= 2AL

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.16

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INDIVIDUAL PROPERTIES – Magnetic anisotropy

Structural relaxation

t 2 ) (

S V

k k t e + =

• W. A. Jesser et al., Phys. Stat. Sol. 19, 95 (1967)

tc

Pseudomorphic range Relaxation range (introduction of dislocation)

t t a a t

c bulk substrate

) ( ~ ) ( − ε

Effect on anisotropy

• C. Chappert and P. Bruno., JAP64, 5736 (1988)

ε

mel mel ~ B

k

Conclusion: Mixing of surface and magneto-elastic contributions

t / ) (

mel bulk

B k t k α + =

Magneto-elastic anisotropy: Strain relaxation regime:

• U. Gradmann, Appl. Phys.3, 161 (1974)

Co/ Cu(111)

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.17

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INDIVIDUAL PROPERTIES – Magnetic anisotropy

Magnetic Anisotropy Energy (MAE): Link with anisotropy of orbital moment

L B

µ µ ξ α ∆ = 4 MAE Theory

• P. Bruno,

PRB39, 865 (1989)

Perturbation theory for 3d metals:

Experim ents atom / 10 4

B L

µ µ

−

≈ ∆

Bulk (Fe, Ni, … )

eV 1 MAE µ ≤ Ab initio calculations

High precision needed:

eV 10 eV 1 << µ

• O. Hjortstam et al., PRB55, 15026 (1997)

Conclusions

• Origin of MAE = anisotropy of orbital moment
• No strict linearity
• α may also depend on thickness in thin films (band structure)

Direct measurement of MAE preferable

L

µ

does not rotate in 3d metals

• > MAE reflects cost in ξ

Covers magnetocrystalline, magnetoelastic and surface anisotropy

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.18

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INDIVIDUAL PROPERTIES – Magnetic anisotropy

• 2. Magnetic anisotropy

[ 3.5 From surfaces to atoms]

Co atoms P t 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 L 2 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)

Magnetic circular dichroïsm

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.19

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INDIVIDUAL PROPERTIES – Magnetic anisotropy

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 + 4 3 °

• 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

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.20

Laboratoire Louis Néel, Grenoble, France. Laboratoire Laboratoire Louis N Louis Né éel, Grenoble el, Grenoble, France , France. .

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INDIVIDUAL PROPERTIES – Magnetic anisotropy

• 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) 5.5K 10K 10K

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.21

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INDIVIDUAL PROPERTIES – Magnetic anisotropy

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 )

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.22

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‘Astroïd’ curve

INDIVIDUAL PROPERTIES – Single domain magnetization reversal

Hr(θ) is a signature of reversal modes

• 90°

0° 90° 180° 270°

H

• 90°

0° 90° 180° 270°

H H = 0.2 Ha H = 0.7 Ha H = Ha H = 0 EASY ~ HARD

30 60 90 120 150 180 210 240 270 300 330

E a s y a x i s E a s y a x i s H a r d a x i s H a r d a x i s

) ( H

r θ

H

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.23

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MAGNETISM – Superparamagnetism

UP DOWN

~25kT

τ / ) ( t e t P − =

T k E e

B B /

τ τ =

Phenomenological model

Brown, Phys.Rev.130, 1677 (1963)

Probability for non-reversal Mean waiting time for reversal

s 10

9 −

≈ τ

) / ln( τ t T k E

B B =

Energy barrier required to prevent

magnetization reversal during duration t Laboratory : t =1s Recording : t >>109s

K T k V

B B

/ 25 ≈ K T k V

B B

/ 60 40 − ≈

Anisotropy barrier Formalism for thermal excitations

Anisotropy barrier EB~KV

Orders of magnitude Magnetic recording with

• ne grain per bit (V=VB,

discrete media) more favorable than V=NVB

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.24

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MAGNETISM – Superparamagnetism

2 4 6 8 10 0.0 0.2 0.4 0.6 0.8 1.0 Magnetization Field (a.u.)

Brillouin 1/2 Langevin

Which fitting function ?

Extremely weak anisotropy Langevin function

H h h m µ βµ0 ), tanh( = =

Strong uniaxial anisotropy Brillouin 1/2 function

h h m / 1 ) tanh( / 1 − =

Other cases: moderate or tilted anisotropy, distributions etc. Numerical fitting

Yields an estimation of the magnetic moment per particle µ Fitting with inadequate functions yields errors on µ

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.25

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MAGNETISM – Superparamagnetism

Example: Co/Au dots

H.Takeshita et al., JMMM1 6 5 , 38 (1997)

350 x 350 nm

• O. Fruchart et al.

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.26

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MAGNETISM – Superparamagnetism

Curie temperature versus blocking temperature?

• H. J. Elmers et al., PRL73, 898 (1994)

Remanence plotted as a function

• f temperature

Transition from superparamagnetism to a blocked state

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.27

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COLLECTIVE PROPERTIES – Interactions and distributions

Hext M

Expected hysteresis loop for macrospins

M Hext

Hysteresis for assemblies of dots

Possible effects

• Distribution of coercive fields
• (Dipolar) interactions

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.28

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Solving

COLLECTIVE PROPERTIES – Evaluation of interactions

Preisach model

• G. Biorci et al., Il Nuov. Cim. VII, 829 (1958)
• I. D. Mayergoyz, Mathematical models of

hysteresis, S pringer (1991)

β α

Distribution function No true link between real particles and µ

β α β α µ > with ) , ( ' ' 2 1 ) ' , ' (

' , ' 2

β α β α µ

β α

∂ ∂ ∂ = f

Hext M

β’ α’

Long experiments (1D set of hysteresis curves) Better suited to bulk materials with strong interactions

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.29

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COLLECTIVE PROPERTIES – Evaluation of interactions

Henkel plots

• O. Henkel,
• Phys. S
• tat. S
• l. 7, 919 (1964)

S . Thamm et al., JMMM184, 245 (1998)

[ ]

) ( 2 1 ) ( ) (

r d

x M x M x M H − − = ∆

Measure of dipolar interactions

Long experiments (ac demagnetization) Better physical meaning than Preisach

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.30

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COLLECTIVE PROPERTIES – Evaluation of interactions

• 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

Minor loops: negative interactions Minor loops: negligible interactions

• 0.1
• 0.05

0.05 0.1 B (T)

• 50 mT
• 0.1
• 0.05

0.05 0.1 B (T)

• 17 mT
• O. Fruchart et al., unpublished

Exam ple: dipolar interactions in arrays of Co/ Au( 1 1 1 ) pillars

Faster than Henkel Other applications: characterization of exchange bias

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.31

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COLLECTIVE PROPERTIES – Evaluation of interactions

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)

( )

kT NH µ µ m / B

eff. Co ½

= m M r H H

s eff.

+ =

T N µ k r M µ χ dm H µ d

Co

S

1 ) ( + − = =

a + b . T

Brillouin 1/2 function Effective field First order expansion: susceptibility

1/χ

(T)

(Demagnetizing dipolar interactions)

• O. Fruchart et al., PRL 2 3 , 2 7 6 9 ( 1 9 9 9 )

Superparamagnetic regime: plot of inverse susceptibility No need of hysteresis Analogy with Curie-Weiss law

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.32

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COLLECTIVE PROPERTIES – Distribution of properties

Distribution of properties

le irreversib r)

( dH dm H = ρ

Hext M Reversible Irreversible

Effect of distributions and dipolar interactions are sometimes difficult to disentangle

Hc( T) for a given population of the distribution can be studied at a given stage of the reversal ( 1 0 % , 2 0 % etc.)

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.33

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COLLECTIVE PROPERTIES – Distribution of properties

Ferrom agnetic order stabilized by dipolar interactions

• J. Hauschild et al., Phys. Rev. B 5 7 , R6 7 7 ( 1 9 9 8 )

Simulation without dipolar interactions Simulation with dipolar interactions Although w eak, dipolar interactions stabilize ferrom agnetic order and sharpen the transition at Tc, ow ing to their long range

Tc=179K

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.34

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COLLECTIVE PROPERTIES – Distribution of properties

Vicinal Fe(110)[1.2ML]/W(110)

(stabilized by dipolar interactions)

• M. Pratzer et al., PRL8 7 , 1 2 7 2 0 1 ( 2 0 0 1 )

J.Hauschild et al., Phys.Rev.B57, R677(1998)

250 x 250 nm

Magnetic order in 1 D

Fe( 0 .5 ML) / W ( 1 1 0 )

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.35

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COLLECTIVE PROPERTIES – Exploit order for magnetic scattering

Magnetic scattering on nanofabricated arrays of lines

Co/ Pt(111) multilayers I nform ations about m agnetic correlations can be extracted from m agnetic satellites

• K. Chesnel et al., PRB6 6 , 0 2 4 4 3 5 ( 2 0 0 2 )

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.36

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COLLECTIVE PROPERTIES – Exploit order for magnetic scattering

Structural scattering on self-organized systems

q CCD

INTRA-ROW ORDER: SUPER-CRYSTAL

Thickness: 1 Å ; coverage: 25%

Λ 4π/Λ

• O. Fruchart et al., Europhys. Lett. 6 3 , 2 7 5 ( 2 0 0 3 )

Evidencing magnetic scattering is in principle possible, however:

• Low temperature and weak signal
• Distribution of properties may prevent ordering to occur

Co/ Au(111) nanodots

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.37

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TABLE OF CONTENTS

1 . I ntroduction to m agnetism 2 . I ndividual properties 3 . Collective properties 4 . Tow ards m aterials?

Magnetic properties of self-organized systems : TOC

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.38

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TOWARDS MATERIALS – Micromagnetic studies

• M. Bode et al, J. Electr. Spectr. Rel. Phenom . 1 1 4 – 1 1 6 , 1 0 5 5 ( 2 0 0 1 )

Theorical prediction: P. Bruno, Phys. Rev. Lett. 8 3 , 2 4 2 5 ( 1 9 9 9 )

Spin-Polarized Scanning Tunneling Spectroscopy

Fe(1.5ML)/W(110)

200 x 200 nm

Domain walls in geometrical constriction

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.39

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TOWARDS MATERIALS – Micromagnetic studies

• A. W achow iak, Science 2 9 8 , 5 7 7 ( 2 0 0 2 )

… also in-field study Structural image

In-plane component

• f magnetization

Out-of-plane component

• f magnetization

Magnetic image Spin-polarized Scanning Tunneling Microscopy

• n Fe/ W ( 1 1 0 ) self-assem bled dots

Magnetic vortex core

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.40

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TOWARDS MATERIALS – Micromagnetic studies

Experim ent ( top) Sim ulation ( top w ith facets)

The main features are

reproduced

Asym m etry around vortex Strong asym m etry of end dom ains Néel cap displaced from m edian line y z Pseudo-2 D 3 D

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.41

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TOWARDS MATERIALS – Micromagnetic studies

Top Mid-height Bottom

Classical picture

A new type of wall / cap combination

• 3D thinking required
• Role of walls with constrained angles?
• Role of tilted facets?
• Perp. component

(mid-height view) Magnetization component

Present observation

• R. Hertel et al., cond-m at/ 0 5 0 7 1 1 9

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.42

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TOWARDS MATERIALS – nanomaterials

Manipulation of edge anisotropy

• S. Rusponi et al., Nature Mater. (2003)

(core-shell dots)

• Self-organization : concepts
• Self-assem bly : tow ards m aterials?

Pt brim s around Co/ Pt( 1 1 1 )

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.43

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Structure

TOWARDS MATERIALS – nanomaterials

CoFe 2O4 in BaTiO3 m atrix

Deposition from a Ti-Ba-Co-Fe oxide target by pulsed laser deposition

• H. Zheng et al., Science 303, 661 (2004)

BaTiO3 ( piezzoelectric) CoFe 2O4 ( ferrim agnetic)

Magnetis m

• Rooom-temperature

functionality

• Perpendicular anisotropy
• wing to matrix-induced strain

in the columns

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.44

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TOWARDS MATERIALS – nanomaterials

• M. Albrecht et al., Europhys. Lett. 5 6 , 8 8 4 ( 2 0 0 1 )

CoPt3(0.8ML)/WSe2

Chemical ordering (L12 phase)

• ccurs from 150°C, against

500°C for continuous films (search for high-anisotropy L10 phases)

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.45

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TOWARDS MATERIALS – Overcoming superparamagnetism

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? Thining the spacer layer superparamagnetism overcome ? Enhanced magnetic signal

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.46

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TOWARDS MATERIALS – Overcoming superparamagnetism

Co atoms grow only on 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

Co/ Au( 1 1 1 ) pillars

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.47

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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

TOWARDS MATERIALS – Overcoming superparamagnetism

Blocking temperature > 300K (~ 30K for flat Co/Au dots)

Pillar volume

Co/ Au( 1 1 1 ) pillars

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.48

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TOWARDS MATERIALS – Overcoming superparamagnetism

~ 2.5AL ~ 3.25AL ~ 2AL

Fe films grown at 150°C STM, 600nm x 600nm STM, 750nm x 750nm

Al2O3 Mo Fe

Step 1 : layer-by-layer grow th at 1 5 0 ° C Step 2 : annealing at 5 0 0 ° C --> stripes along steps

Fe/ Mo( 1 1 0 ) stripes

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.49

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TOWARDS MATERIALS – Overcoming superparamagnetism

• 100

100

• 1.0
• 0.5

0.0 0.5 1.0

100 K 200 K 300 K

M/Ms Applied field (mT)

H//[001]

• 100

100

• 1.0
• 0.5

0.0 0.5 1.0

100 K 200 K 300 K

M/Ms Applied field (mT)

H//[1-10]

[001] [ 1

• 1

] 10 µm

Sapphire \ W \ Fe(3nm) \ Mo

• O. Fruchart et al., APL 8 4 , 1 3 3 5 ( 2 0 0 4 )

Coercivity and rem anence at 3 0 0 K Coercivity and high remanence at 300K Weak temperature dependence

> behaves like a conventional material

Stripe height = 5 .5 nm

Fe/ W ( 1 1 0 ) stripes

Olivier Fruchart – SON School (Cargèse) – July 17-23, 2005 – p.50

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[1] O. Fruchart, Epitaxial self-organization: from surfaces to magnetic materials,

• C. R. Physique 6 (1), 61-73 (2005).

[2] J.P. Bucher, Magnetism of free and supported metal clusters, in: S.N. Khanna, A.W. Castleman Jr. (Eds.), Quantum Phenomena in Clusters and Nanostructures, in: Springer Series in Cluster Physics, Springer, Berlin, pp. 83–137 (2003). [3] J.P. Bucher, F. Scheurer, Self-organized clusters and nanosize islands on metal surfaces, in: J.S. Miller, M. Drillon (Eds.), Magnetism: Molecules to Materials III, Wiley–VCH, Weinheim, Germany, pp. 211–251 (2002). [4] J.I. Martin et coll., Ordered magnetic nanostructures: fabrication and properties,

• J. Magn. Magn. Mater. 256, 449-501 (2003).

[5] R. Skomski, Nanomagnetics, J. Phys.: Cond. Mat. 15, R841–896 (2003). [6] J. Bansmann et al., Magnetic and structural properties of isolated and assembled clusters,

• Surf. Sci. Rep. 56, 189 (2005)

SOME LITERATURE