University of Twente Magnetic Force Microscopy Leon Abelmann and Martin Siekman Systems and Materials for Information storage MESA + Research Institute University of Twente Systems and Materials for Information storage 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 Control Electronics Computer Detector Magnetic element Cantilever M z Specimen 3 dimensional Scanner x y 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 Amplitude [nm] amplitude 0 frequency Phase difference [deg] 0 phase -90 -180 , Drive frequency [Hz] f res f res Leon Abelmann Constanta Summerschool, Sep 2005 Slide 5
Principle of MFM 18,18(3) Image formation Transform stray field to Fourier space: � ∞ � ∞ � H ( x, y, z ) e − i ( xk x + yk y ) dxdy H ( k x , k y , z ) = −∞ −∞ H ( k x , k y , z ) = exp( −| k | z ) · � � H ( k x , k y , 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 b Wavelength l [nm] 1000 200 100 20 10 non-magnetic bar h 125 10 Force derivative [mN/m] Frequency shift [Hz] magnetic 1 12.5 coating s 0.1 1.25 x 0 z 0 0.01 0.13 t 0.001 l 5 6 7 8 10 10 10 10 -1 Spatial frequency [m ] F z ( k , z ) = − µ 0 M t · b sinc ( k x b 2 ) · S sinc ( k y S � 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 Cantilever Tip plane MFM tip Cross-section determined by layer thicknesses Leon 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 � � 4 kTc ∆ B ∆ = ω n Q � z 2 osc � ∂z th rms Leon Abelmann Constanta Summerschool, Sep 2005 Slide 20
Instrumentation 10,10(3) Drift Noise (nm or Hz) D B’ D B 1/(N D t) 1/ D t frequency (Hz) Leon Abelmann Constanta Summerschool, Sep 2005 Slide 21
Instrumentation 13,13(3) Vacuum • Reduce damping, improves Q-factor by 10 5 • 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 400 300 Dots reversed 200 100 0 -300 -200 -100 0 100 200 300 Field (kA/m) 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 Storage Leon 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
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