[PPT] - Magnetic Force Microscopy Olivier Fruchart Institut Nel PowerPoint Presentation

SLIDE 1

http://perso.neel.cnrs.fr/olivier.fruchart/slides/ http://perso.neel.cnrs.fr/olivier.fruchart/slides/

Institut Néel, Grenoble, France

Magnetic Force Microscopy

Olivier Fruchart

Institut Néel (CNRS-UJF-INPG) Grenoble - France

http://neel.cnrs.fr

SLIDE 2

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.2

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

WHY DO WE NEED MAGNETIC MICROSCOPY ? – Origins 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

SLIDE 3

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.3

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

WHY DO WE NEED MAGNETIC MICROSCOPY ? – – Magnetic characteristic length scales

Typical length scale: Bloch wall width B

( )

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

SLIDE 4

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.4

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

WHY DO WE NEED MAGNETIC MICROSCOPY ? – Magnetic domains

Bulk material

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)

Nanomagnetism ~ mesoscopic magnetism

SLIDE 5

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.5

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Why do we need microcopy ?

What information may be sought

Distribution of magnetization in

sample

Direction & magnitude Depth resolution Elemental resolution Lateral resolution Time resolution

Conditions and environment

Temperature Magnetic field Electrical current, light etc. Additional microscopies

SLIDE 6

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.6

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Key elements of an Atomic Force Microscope (AFM)

Atomic Force Microscopy – Working principle

Measures forces (vertical

and lateral) between sample and tip

SLIDE 7

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.7

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Atomic Force Microscopy – Cantilevers and tips

Images : Olympus catalog (http://www.olympus.co.jp/probe) Millimeters 100μm Chip – Batch fabrication Cantilever Full tip + apex

Price 10-200€/tip Radius of curvature ≈ 5nm

SLIDE 8

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.8

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

AFM – Many uses

Static (deflection)

When contact is needed : electric,

friction etc.

Dynamic (cantilever oscillation)

Less damage to sample and tip
More sensitive

Measures

Topography Mechanical properties Electric properties Piezoelectric properties Long-range forces

(electrostatic, magnetic)

Micromanipulation &

fabrication

Modes of operation

SLIDE 9

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.9

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Mechanical oscillator – Equations

Mechanical excitation of cantilevers

m ¨ z+ Γ ˙ z+ k z=F (z ,t) m Γ k Inertia Damping Spring F (z ,t) External force

Notations

ω0=√ k m Q=√k m Γ z(t)=z0e

j ωt

Seek solutions for Reference angular velocity Quality factor H= z F = 1 k 1 −( ω ω0)

2

+ j Q( ω ω0)+ 1

Transfert function

F=0

SLIDE 10

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.10

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Mechanical oscillator – Solutions

G=∣H∣= 1 k 1

√[1−(

ω ω0)

2

]

2

+ 1 Q

2(

ω ω0)

2

cosφ= 1−( ω ω0)

2

√[1−(

ω ω0)

2

]

2

+ 1 Q

2(

ω ω0)

2

7 6 5 4 3 2 1 3 2 1

Gain

ω/ω0 kG kG(∞)=0 kG(0)=1 kG(ωr)=Q ωr=ω0√ 1− 1 2Q

2

Peak at :

Dephasing

ω/ω0 φ

Q=1/√2 Q=10/√2 Q=1/√2 Q=10/√2

φ(0)=0 φ(∞)=−π

Q>>1

ωr≈ω0 φ(ωr)≈−π/2 Δ ωr ω0 ≈√3 Q

3.0
2.5
2.0
1.5
1.0
0.5

0.0 3 2 1

SLIDE 11

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.11

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Phase shift

Mechanical oscillator

m ¨ z+ Γ ˙ z+ k z=F (z) F (z)=F (z0)+ (z−z0)∂z F ω0,eff=ω0(1− 1 2k ∂z F) Attractive force δφ=−Q k ∂z F

Tip-sample interaction treated as perturbation

with

Mere renormalization : Red shift

Repulsive force

Blue shift

ωexc=ω0

Forces monitored through phase shift

SLIDE 12

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.12

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

AFM – Tapping mode (topography)

Full resonance spectrum

500 kHz

Resonance spectrum

288 kHz 280 kHz 0 kHz φ=0 φ= - π Amplitude Phase

Resonance in tapping mode

79 kHz 82 kHz Amplitude Amplitude ← Peak is cut

SLIDE 13

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.13

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

AFM – Close-loop versus open-loop operation

Close-loop operation

Feedback signal and setpoint: amplitude

Map at constant force Map The force along a predefined

trajectory (plane, lift-height etc.)

Open-loop operation

Ex : map = topography with

setpoint on the amplitude

Ex : map = magnetic stray field

above sample

SLIDE 14

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.14

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Images

AFM – Tapping mode (topography)

Sample : self-organized Anodized Alumina (synthesis L. Cagnon, Institut Néel)

Height image (topography) Phase image

To notice

Non-contact part (bottom of image) Phase does not reflect topography Noise and phase depend on set point

Increasing pressure

SLIDE 15

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.15

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Sample : M. Miron (Spintec) Lithography : S. Pizzini (Néel)

Schematics

AFM – Tip shadowing effects

Tip Sample

Examples

1.8 x 2 microns

Lateral (base) size is over-estimated with AFM Shading, not convolution (no true retrieval possible) Tips are less sharp for MFM due to magnetic coating Notice

S. Y. Suck et al.,

APL95, 162503 (2009)

SLIDE 16

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.16

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

AFM – Tip effects

ZIP disk, 400x400 nm Usual tip SWCNT tip

Tip : A. M. Bonnot (Institut Néel)

SLIDE 17

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.17

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

MFM – Two-pass technique

First-pass

Feedback ON Monitor topography

(height) and any other signal (phase etc.) Second pass

Monitor long-range

forces (magnetic, electrostatic)

SLIDE 18

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.18

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Calculate energy with magnetic dipoles Calculate energy with magnetic dipoles

Reminder about magnetostatics

Hd(r)= μ0 4πr

3[

3(μ1.r)r r

2

−μ1] Magnetic field arising from a magnetic dipole F2(r)=−∇ E1,2=μ0 ∇(Hd.μ2) μ1 μ2

        − − + − − − =

∫∫ ∫∫∫

sample 2 3 space 3 3 s d

' d ' 4 ) ' )].( ' ( ). ' ( [ ' d ' 4 ) ' )].( ' ( [ div ) ( r r M r r r r r n r m r r r r r m r H π π

Analogy with electrostatics thanks to

) ( div ) ( div

d

M H

− =

] ) ( div[

)

(

s

r m r M

= ρ

) ( . ) ( ) (

s

r n r m r M

= σ

Volume charges Surface charges F2(r)=−∇ E1,2=μ0 ∇ (Hd.μ2) F2(r)=−∇ E 1,2 E 1,2=−μ0μ2.Hd with E 1,2=μ0σ.ϕ Hd=−∇ ϕ with

SLIDE 19

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.19

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

MFM – tip/sample interaction

Tip is a dipole

E 1,2=−μ0μ2.Hd E 1,2=−μ0(μ x. Hd , x+ μy.H d ,y+ μz. H d , z) δφ=Q k μ0μi∂z

2 Hd ,i

E 1,2=μ0σ.ϕ Fz=−μ0σ Hd ,z δφ=Q k μ0σ∂z Hd , z

Tip is a monopole

In practice, a combination of both models is best suited (dipole is more important) MFM is sensitive to some derivative(s) of the stray field from the sample MFM may be sensitive to in-plane field, depending on the tip moment.

SLIDE 20

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.20

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

MFM – Single-domain nanostructures (planar)

S. Y. Suck et al., APL95, 162503 (2009)

Single-domain in-plane magnetized dots appear as dipoles

Topography MFM, saturated MFM, partly reversed

SLIDE 21

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.21

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

MFM – Single-domain nanostructures (perpendicular)

T. Wang et al., APL 92, 192504 (2008)

Single-domain out-of-plane magnetized dots appear as monopoles

Structure (SEM) MFM, partly reversed

SLIDE 22

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.22

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

MFM – Single-domain nanostructures (mutual interaction)

Lithography : S. Pizzini (Institut Néel) Imaging : Z. Ishaque (Institut Néel)

It is a DOMAIN contrast Interaction is ALWAYS attractive : red shift Contrast is proportionnal to the square

f the tip moment

Permalloy (15nm), 3x8 microns Principle :

1. Stray field magnetizes sample
2. Sample is non -uniform → stray field
3. Tip measures sample's stray field

Contrast : -0.1°, LM tip

SLIDE 23

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.23

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

MFM – Domains in films with perpendicular anisotropy

Contrast : ±0.4°, LM tip FePt (4nm), 5x5 microns

Sample : A. Marty (CEA-Grenoble) Imaging : M. Darques (Institut Néel)

It is a DOMAIN contrast The direction of magnetization is deduced

Quantitative analysis :

L. Belliard et al., J. Appl. Phys. 81,

3849 (1997)

SLIDE 24

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.24

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

MFM – Imaging domain walls (Bloch)

Fe dot (25nm), 2.5x1 microns

Contrast is MONOPOLAR Informs about the polarity of the wall core

Up core Down core

SLIDE 25

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.25

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

MFM – Imaging domain walls (Néel)

J. M. Garcia et al., APL 79, 656 (2001)

Permalloy dot (16nm) 2x2 microns Permalloy film (20nm) 10x10 microns

Néel wall give rise to DIPOLAR contrast Informs about the chirality of the wall core

SLIDE 26

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.26

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

MFM – Imaging domain walls (head-to-head)

Permalloy stripes (15nm), 250nm wide Contrast : ±0.3°, tip 5nm CoCr

R. McMichael and M. Donahue, IEEE Trans. Magn. 33, 4167 (1997)

Walls in in-plane magnetized stripes MONOPOLAR → Contrast informs about head-to-head ot tail-to-tail

Known as vortex domain-wall

SLIDE 27

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.27

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Tips influencing samples

Moderate

Permalloy (15nm) 500nm wide FePt (4nm) Image 5 microns

Sample : A. Marty Imaging : M. Darques Sample : S. Pizzini Imaging : Z. Ishaque

Repulsion Attraction

Moderate

Permalloy (15nm) 500nm stripes

Sample : S. Pizzini Imaging : Z. Ishaque

Permalloy (15nm) 500nm wide

Sample : S. Pizzini Imaging : Z. Ishaque

Scanning Attraction Repulsion 1. 2. Co (0.6nm)/ graphene (20 mic)

C. Dieudonné

Repeat measurement and/or change scanning direction Low-coercive samples require low-moment tips Commercial 'low-moment' may not be low enough

SLIDE 28

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.28

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Sample influencing tip

Forward scan Backward scan

Moment of tip reverses during scan Permanent magnet Choose high-coercivity tips (eg Asylum) →

G. Ciuta (Institut néel)

FePt thick film

SLIDE 29

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.29

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Sample influencing cantilever

Stiff cantilever (300kHz) Soft cantilever (70kHz) NdFeB thick film Contrast : ±20° Contrast : ±0.5°

Cantilever repeled for tip and sample opposite 'Attractive' domains are

ver-dominant in the image

'Repulsive' domains lack

f resolution

Use stiff cantilever for permanent magnets

Sample : G. Ciuta (Institut néel)

SLIDE 30

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.30

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

MFM under magnetic field

In-plane field (30mT)

Sample : A. Masseboeuf (CEA-Grenoble)

Co(5)\Cu(5)\Py(15nm) 5 microns image Agglomerated Fe3O4 superparamagnetic particles Out-of-plane field (400mT)

Sample : K. Aissou (LCPO , Bordeaux)

Coercive field of usual tips of the order of 50mT

SLIDE 31

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.31

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Moment

Tips and cantilevers – Summary of choices

Low moment, or even : Normal moment for most samples (30-50nm CoCr) Superparamagnetic or ultra-low moment

Low-coercivity samples Increased resolution At the expense of sensitivity

Cantilever

Large stifness Normal stifness ('force modulation') → resonance at 70kHz

High stray-field samples

(typically permanent magnets)

At the expense of sensitivity

Coercivity

High coercivity (eg Asylum) Normal (30-70mT)

High stray-field samples

(typically permanent magnets)

At the expense of cost

Tip sharpness

Sharp tips (resolution 25nm), expense : Normal resolution (50nm)

Commercial and/or spacial

fabrication (FIB etc.)

Loss of sensitivity

SLIDE 32

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.32

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Calibration of tips and cantilevers

Sample : MnAs(001)

R. Belkhou (Soleil)

25nm 40nm 55nm 80nm 120nm

Compare tips ! Use identical sample and imaging parameters

SLIDE 33

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.33

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Comparing MFM with other microscopies – Game of the seven differences...

XMCD-PEEM MFM

SLIDE 34

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.34

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Comparing MFM with other microscopies

XMCD-PEEM MFM Lorentz

SLIDE 35

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.35

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

Some references

References

Why we DO need 'SOLEIL'

Y. Zhu Ed., Modern techniques for charact-

erizing magnetic materials, Springer (2005)

A. Schwartz et al., Scanning probe

techniques: MFM and SP-STM, in : series Handbook of magnetism and advanced magnetic materials, Novel techniques for characterizing and preparing samples (vol.3), H. Kronmüller, S. Parkin Ed. (2007)

A. Hubert, R. Schäfer, Magnetic domains,

Springer (1999) Repository of the European School on Magnetism : http://esm.neel.cnrs.fr See 2005 School : « New experimental approaches to Magnetism »

SLIDE 36

Olivier Fruchart – Solemio School – Synchrotron Soleil – 2011-05-05 – p.36

Institut Néel, Grenoble, France

http://perso.neel.cnrs.fr/olivier.fruchart/slides

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