Magnetic Force Microscopy

Transcription

1 Magnetic Force Microscopy June 1, 1998 Kim Byung-Il Dept. of Physics Seoul National Univ. 6/19/1 1 Superconductivity Lab.

2 Development of MFM Noncontact mode weak interaction magnetic, electric and attr. VdW force resonance frequency shift: ω ~ ω ( F / k ). amplitude change lock-in technique tip-sample spacing dependence Principle of noncontact mode One dimentional harmonic osicillator F ( z ) = F + F ( z ) ζ m t z 1 z + γ + k ( z u ) = F ( z ) t The positions of the bimorph and the lever 6/19/1 Superconductivity Lab.

3 u = u + a exp( i ω t ) z then = z + ζ respectively. ζ m k a i t F + γ ζ ζ ω ζ t + [ exp( )] = ' t The amplitude of vibration is given by A (, ak i F ' ) exp( θ ω = ) b k ' ω m + iωγ where k ' = k F ' The motion of the lever is now influenced by the force derivative(resonance frequency shift). ω ~ ω ( F / k ). Amp interaction Amp change without interaction Driving frequency Frequency 6/19/1 3 Superconductivity Lab.

4 At the frequency with maximum sensitivity, ω m ω 1 1 ( + ) Q The amplitude change is given by A Q A = k F 3 3 contrast mechanism of MFM point dipole model ρ ρ ρ ρ F = ( m H ) m : magnetic moment of the tip H : stray field from sample F = ρ ( F z ) z H H x y = mx + m y + z z F m form ρ = m z z, = z H z z m z H z z 6/19/1 4 Superconductivity Lab.

5 bimorph driven system feedback in MFM 6/19/1 5 Superconductivity Lab.

6 operating condition oscillation freq:ω ω resonance frequency shift tip-sample distance:1-1nm long range magnetic interaction bias between tip and sample: V-1V electrostatic force servo force attractive or repusive magnetic force tip crash due to polarity change need of servo force for feedback addition of overall attractive force control of the spacing between tip and sample additional electrostatic force AQ = 3 3k A ( F + F ) m c under feedback: A = z F = Fservo m / z z = z 6/19/1 6 Superconductivity Lab.

7 data : contours of constant force gradient Force(F) Repulsive magnetic force(f m ) Distance(z) Attractive magnetic force(f m ) Electrostatic force(f c ) 6/19/1 7 Superconductivity Lab.

8 Force gradient(f') Capacitive force(f c ) Attractive Magnetic Force(F m ) Distance(z) Repulsive Magnetic Force(F m ) Co-Pd multilayer film sample preparation total thickness is about 3A (4.Co+13.Pd)17 layer glass substrate image earth s magnetic field observation of natural magnetic domain 6/19/1 8 Superconductivity Lab.

9 (1.8µm 1.8µm) (1.8µm 1.8µm) Summary MFM using resonnce frequency shift was constructed. The contrast mechanism of MFM was discussed by using one dimensional harmonic oscillator model. The role of electrostatic force as servo force under feedback was discussed. 6/19/1 9 Superconductivity Lab.

10 MFM was applied for the observation of magnetic domain of the magnetic multilayer films MFM using Electrostatic Force Modulation drawback of bimorph driven system indirect modulation uncontrolled vertical deflection unstable feedback 6/19/1 1 Superconductivity Lab.

11 complex amplitude vs. frequency restriction in op. frequency range electrostatic force modulation direct modulation ac voltage between tip and sample : V = V sin ω t ac driving force: f V ( t ) c = ac 1 cos ω m z t 1 C 4 z z + γ + t k z u = block schematics ( ) F + F + F m c vdw 6/19/1 11 Superconductivity Lab.

12 rms amplitude-frequencycurve electrostatic modulation system well-defined amp.-freq. spectrum bimorph driven system unwanted peak around the main peak RMS Amplitude(Arb. unit) bimorph driven electrostatic modulation Frequency(kHz) 6/19/1 1 Superconductivity Lab.

13 ω component m z t z + γ + t k ( z u ) 1 = F m + F vdw + C Vac ( 1 cos ωt ) Near average position z, 1 z = z + ζ ( ) +... then ζ = ( 1) Φz ( )sin( ωt + φ ) ( 1) ω ( ω 4ω ) + 16( γω) where ω = k, γ = b/m, Φ( z ) m In-phase output In-quadrature output Y 1 Ctip( z ) = V 4 m z X ω ω = 4 z, φ ( ) ω 4ω 1 = arctan 4γω ac k, = m ω F ( z ) m (1) Φ z m ( ) 4 Φ( z ) γω = ( ) ( ) 4 ω ω + 16 γω Φ( z )( 4ω ω ) 4ω ω + 16 γω ( ) ( ) in-phase component for different distances d5=µm, d4 =1µm, d3=5µm, d=µm, d1=1µm. 6/19/1 13 Superconductivity Lab.

14 In-phase w component(arb. unit) Frequency(kHz) rms amplitude-frequency curve various tip-sample distances: subharmonic peaks m z z γ t t k z u + + ( ) = F + F + F + F m c vdw rep nonlinear characteristics of capacitance truncated peaked near 1nm electrostatic tapping interaction. RMS Amplitude(Arb. unit.) ω component Electrostatic Force Modulation 45nm 3nm 15nm 1nm 7nm nm w component d5 d4 d3 d d /19/1 14 Superconductivity Lab.

15 resonance frequency in noncontact regime 53 5 Freq max 51 Frequency Shift Tip sample distance ω Amplitude-Distance Curve cantilever :Co coating( 4nm), k=.1n/m, f res =65kHz V ac =14V, fop=33.43khz sample: CoCr thin film( t 3nm) 6/19/1 15 Superconductivity Lab.

16 w component(arb. unit) tapping nm noncontact Amplitude(Arb. unit) (a) (b) Deflection(Arb. unit) Relative tip position to sample(m m) good amplitude vs. frequency curve(cf.bimorph) long range electrostatic capacitive modulation stable imaging condition ω increases as tip approaches surface(cf. inset) high signal to noise ratio Results noncontact regime(vibration 8nm) labyrinthine domain(period 4.5 5µm) tapping regime(vibration 85nm, d 9nm) 6/19/1 16 Superconductivity Lab.

17 elongated grains( 3nm 6nm) (1µm 1µm) noncontact regime(vibration amp. 8nm) contrast increase for larger distance 6/19/1 17 Superconductivity Lab.

18 4 (µm µm) Distance(nm) 3 1 A B C position(m m) 6/19/1 18 Superconductivity Lab.

19 1 w Component(nm) nm T 91nm C 87nm B 8nm A On the Positive Domain On the Negative Domain tip position from sample(nm) long range magnetic interaction 1 1 On the Positive Domain On the Negative Domain slope = w Component(nm) slope = tip position from sample(nm) C ( z ) z a ln( D / z ) conical tip model electrostatic long range order interaction stable feedback condition 6/19/1 19 Superconductivity Lab.

20 Summary development of MFM using electroststic force modulation good amplitude vs. frequency curve(cf. bimorph) long range electrostatic capacitive modulation stable imaging condition increase of ω as tip approaches surface high signal to noise ratio conical tip model in the noncontact regime labyrinthine domain(period 4.5 5µm) on CoCr thin film elongated grains( 3nm 6nm) promising tool for studying magnetic sample 6/19/1 Superconductivity Lab.