University of Twente

Systems and Materials for Information storage

Magnetic Force Microscopy

Leon Abelmann and Martin SiekmanSystems and Materials for Information storage

MESA+ Research Institute

University of Twente

Constanta Summerschool, Sep 2005 (September 14, 2005)

Introduction 2,2(2)

Contents

• Before break: MFM Operation� Principle of MFM� MFM tips

• After break:� Instrumentation� Artefacts

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 1

Principle of MFM 5,5(3)

Principle of MFM

3 dimensionalScanner

Cantilever

Specimen

DetectorMagnetic element

z

xy

ComputerControlElectronics

M

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 2

Principle of MFM 7,7(2)

Magnetic Force Microscopy

500 nm

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 3

Principle of MFM 12,12(5)

Change in resonance

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 4

Principle of MFM 15,15(3)

Amplitude, Phase, Frequency

0

0

-180

-90

Phas

e di

ffer

ence

[deg

]A

mpl

itud

e [n

m]

Drive frequency [Hz]fres fres

,

phase

frequency

amplitude

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 5

Principle of MFM 18,18(3)

Image formation

Transform stray field to Fourier space:

H(kx, ky, z) =∫ ∞

−∞

∫ ∞

−∞H(x, y, z)e−i(xkx+yky)dxdy

H(kx, ky, z) = exp(−|k|z) · H(kx, ky, 0)

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 6

Principle of MFM 28,0(10)

MFM Demonstrator

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 7

Principle of MFM 31,3(3)

Tip transfer function

l

non-magnetic bar

magnetic coating

s

h

b

z0x0

t

10

12.5

1.25

0.13

125

1

0.1

10201002001000

0.01

0.001

810

710

610

510

Forc

e d

eriv

ativ

e [m

N/m

] Freq

uenc

y shift [Hz]

-1Spatial frequency [m ]

Wavelength l [nm]

Fz(k, z) = −µ0Mt · b sinc(kxb

2) · S sinc(

kyS

2) · H(k, z)

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 8

Principle of MFM 41,0(10)

MFM Demonstrator 2

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 9

Principle of MFM 51,10(10)

Resolution versus distance

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 10

MFM Probes 53,12(2)

Probes

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 11

MFM Probes 55,14(2)

AFM sputtered

�

• Sputtered CoCr(X) hard disk materials• Low/high moment: layer thickness• Fe, NiFe for low coercivity tips

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 12

MFM Probes 57,16(2)

AFM side coated

�

• Co, NiFe evaporated• Shape anisotropy• Stable domain structure

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 13

MFM Probes 59,18(2)

CantiClever

Tip plane

Cantilever

MFM tip

Cross-section determined by layer thicknessesLeon Abelmann Constanta Summerschool, Sep 2005 Slide 14

MFM Probes 61,20(2)

SEM Images cantilever

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 15

MFM Probes 63,22(2)

SEM Images tip

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 16

Break 0,0(0)

Break

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 17

Instrumentation 2,2(2)

Beam Deflection

�����

�������

• Laser• LED

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 18

Instrumentation 4,4(2)

Interferometer���������� ��������

• factor 10 better sensitivity• difficult to align• better reflection coatings

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 19

Instrumentation 7,7(3)

Thermal Noise

(∆

(∂F

∂z

)th

)rms

=

√4kTc∆B

ωnQ〈z2osc〉

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 20

Instrumentation 10,10(3)

Drift

1/Dt1/(NDt)

frequency (Hz)

Noi

se (n

m o

r H

z)

DB

DB’

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 21

Instrumentation 13,13(3)

Vacuum

• Reduce damping, improves Q-factor by 105

• Sound isolation

• Remove most of water film (meniscus)

Be careful with break-down (Paschen curve)

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 22

Instrumentation 16,16(3)

Magnetic Field

Application of magnetic fields

• On sample� Simple� Only in-plane� Low field� Heating

• On microscope� Requires very small microscope

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 23

Instrumentation 18,18(2)

Instrumentation

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 24

Instrumentation 20,20(2)

Switching Field Distribution

-300 -200 -100 0 100 200 300Field (kA/m)

0

100

200

300

400

Dot

s re

vers

ed

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 25

MFM Artefacts 22,22(2)

Correct domain image

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 26

MFM Artefacts 24,24(2)

Correct bit pattern

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 27

MFM Artefacts 26,26(2)

Interference stripes

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 28

MFM Artefacts 28,28(2)

Topographic contrast

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 29

MFM Artefacts 30,30(2)

Topographic contrast 2

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 30

MFM Artefacts 31,31(1)

Interaction

• Sample disturbs tip

• Tip disturbs sample

• Reversible/Irreversible

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 31

MFM Artefacts 33,33(2)

Tip reversal on strong sample

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 32

MFM Artefacts 35,35(2)

Tip reversal in external field

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 33

MFM Artefacts 37,37(2)

Domain in tip

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 34

MFM Artefacts 39,39(2)

Sample disturbance

• Reversible (susceptibility contrast)

• Irreversible

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 35

MFM Artefacts 41,41(2)

Susceptibility contrast

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 36

MFM Artefacts 43,43(2)

Move domain walls

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 37

MFM Artefacts 45,45(2)

Disturb sample

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 38

MFM Artefacts 47,47(2)

Dot switch

Data StorageLeon Abelmann Constanta Summerschool, Sep 2005 Slide 39

Conclusions 49,49(2)

Conclusions

• Imaging principle (deflection, phase, frequency)

• Fourier transform for image formation

• Side coated tips

• Noise, bandwidth

• Artefacts (interference, topography)

• Tip/sample interaction

Leon Abelmann Constanta Summerschool, Sep 2005 Slide 40