univ ersity of t wen te magnetic force...
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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)
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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
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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
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Principle of MFM 7,7(2)
Magnetic Force Microscopy
500 nm
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 3
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Principle of MFM 12,12(5)
Change in resonance
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 4
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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
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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
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Principle of MFM 28,0(10)
MFM Demonstrator
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 7
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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
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Principle of MFM 41,0(10)
MFM Demonstrator 2
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 9
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Principle of MFM 51,10(10)
Resolution versus distance
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 10
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MFM Probes 53,12(2)
Probes
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 11
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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
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MFM Probes 57,16(2)
AFM side coated
�
• Co, NiFe evaporated• Shape anisotropy• Stable domain structure
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 13
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MFM Probes 59,18(2)
CantiClever
Tip plane
Cantilever
MFM tip
Cross-section determined by layer thicknessesLeon Abelmann Constanta Summerschool, Sep 2005 Slide 14
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MFM Probes 61,20(2)
SEM Images cantilever
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 15
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MFM Probes 63,22(2)
SEM Images tip
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 16
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Break 0,0(0)
Break
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 17
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Instrumentation 2,2(2)
Beam Deflection
�����
�������
• Laser• LED
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 18
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Instrumentation 4,4(2)
Interferometer���������� ��������
• factor 10 better sensitivity• difficult to align• better reflection coatings
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 19
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Instrumentation 7,7(3)
Thermal Noise
(∆
(∂F
∂z
)th
)rms
=
√4kTc∆B
ωnQ〈z2osc〉
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 20
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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
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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
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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
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Instrumentation 18,18(2)
Instrumentation
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 24
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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
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MFM Artefacts 22,22(2)
Correct domain image
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 26
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MFM Artefacts 24,24(2)
Correct bit pattern
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 27
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MFM Artefacts 26,26(2)
Interference stripes
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 28
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MFM Artefacts 28,28(2)
Topographic contrast
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 29
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MFM Artefacts 30,30(2)
Topographic contrast 2
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 30
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MFM Artefacts 31,31(1)
Interaction
• Sample disturbs tip
• Tip disturbs sample
• Reversible/Irreversible
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 31
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MFM Artefacts 33,33(2)
Tip reversal on strong sample
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 32
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MFM Artefacts 35,35(2)
Tip reversal in external field
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 33
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MFM Artefacts 37,37(2)
Domain in tip
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 34
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MFM Artefacts 39,39(2)
Sample disturbance
• Reversible (susceptibility contrast)
• Irreversible
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 35
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MFM Artefacts 41,41(2)
Susceptibility contrast
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 36
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MFM Artefacts 43,43(2)
Move domain walls
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 37
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MFM Artefacts 45,45(2)
Disturb sample
Leon Abelmann Constanta Summerschool, Sep 2005 Slide 38
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MFM Artefacts 47,47(2)
Dot switch
Data StorageLeon Abelmann Constanta Summerschool, Sep 2005 Slide 39
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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