1. Magnetic Force Microscopy
Fmagntic = mtip • Hsample so one images stray fields!
Comprehensive review:
Grutter, Mamin and Rugar, in
‘Scanning Tunneling Microscopy II’
Springer, 1991

2. Force sensor: tip needs to be magnetic
Typical coatings:
Co, Co80Cr20, Co71Pt12Cr17 (hard)
Ni81Fe19, Fe, and Ni50Co50 (soft)
by sputtering or thermal evaporation,
Often 5nm Au protection.
Magnetized in 1T field
Utke et al. APL, 80, 4792 (2002)
Different coatings for different MFM applications!!!

3. Domain wall movement as a function of external field
The magnetic field was applied diagonally along the scanned area with the magnetic field of (a)-(h) -2 Oe, 5 Oe, 15 Oe, 45 Oe, 25 Oe, 20 Oe, 12 Oe, -2 Oe respectively.
Tip: 50 nm Co71Pt12Cr17, constant frequency shift mode.

4. Subtle, reversible tip stray field effects:
Bloch walls (black and white lines) in Fe whisker

5. Less subtle effect…
Displacement of Bloch line in a Bloch wall in a Fe(001) whisker, Hc < 1Oe

6. Tip Stray Field Conical shell model calculation of
tip stray field as a function
of lateral distance r and at
different z (100 nm, 50 nm,
20 nm);
tip: 30 nm Co71Pt12Cr17.
Tip stray field close to the tip end is substantial.
Tip stray field decays slowly, especially for radial component.

7. Optimized coating depending on sample
Max field components and their decay lengths for z=20nm

8. Tapping/Lift mode Good separation topography – magnetic information
(in most cases)

9. Tip influence! MFM Tip Stray Field Distortion Reduce Distortion:
Three consecutive scans.
NiFe: 500nm200nm10nm
tapping/lift mode
Lift height: 80 nm
X. Zhu, et al., JAP
91, 7340 (2002).
Reduce Distortion:
Operate in the
constant height
mode
X. Zhu, et al., PRB
66, (2002).

10. MFM can be used to control local magnetic structure
Manipulation
elliptical NiFe, 600nm x 150nm x 30nm
MFM can be used to control local magnetic structure
X. Zhu, et al., PRB 66, (2002)

11. MFM Imaging Permalloy disk: diameter: 700 nm; thickness: 25 nm.
Constant height image with 30 nm CoPtCr tip in vacuum
Vortex state with
vortex core singularity
Zoom in 140 nm
Micromagnetic simulation
(OOMMF)
Vortex core moves closer to the edge perpendicularly to
the field directions with the presence of external magnetic fields.

12. Permalloy Circular Rings
Domain wall propagation
At Remanence
H=-25 Oe
H=-60 Oe H
X. Zhu, PhD. Thesis 2002, McGill University

13. MFM Imaging Experiment Simulation Stray field Transverse domain wall
Onion State
NiFe: 700 nm
Flux domain
wall
Public code:
OOMMF
NiFe: 5 mm

14. MFM Imaging Co (4nm) Cu(3nm) NiFe (6nm)
Mixtures of antiparallel states
and parallel states.
Coexistence of the two different
antiparallel states.

15. MFM Imaging of weak stray fields: pseudo spin valve structures
C & D are antiparallel, but the two layers are not completely magnetically equivalent.

16. Magnetostatic Coupling: can one build magnetic cellular automata?

17. Coherence length relevant!
Coupled 700 nm rings

18. Hysteresis Loop of Ensemble

19. Switching Field Distribution

20. Hysteresis Loop of Ensemble
Switching field distribution
is broader for switching form
‘onion’ to vortex.

21. Individual Hysteresis Loop, part II

22. Individual Hysteresis Loop

23. Permalloy Square RingsH
As-grown flux closure state
of Ni square rings
Remanent states after applying
magnetic field diagonally.
Permalloy square rings (t=20 nm,
w=500 nm, L=2mm)
Tip: 30 nm Co71Pt12Cr17

24. Micromagnetic Simulation
Head-head Domain Wall
Ni square rings
(t=10 nm, w=200
nm, L=2mm)
Simulation: OOMMF
cell size: 5nm
With field
Remanence
Stray Field
Tip Closer to the sample

25. Magnetic Moment State versus Magnetic Field
H1=130 Oe
H1=147 Oe
H2=256 Oe
H2=270 Oe
Remanence after
applying -300 Oe
Ni square rings (t=10 nm, w=200 nm, L=2mm)
Four segments of the square rings can be treated as four ‘single
domain state’. The magnetic field parallel to the four segments are
Hcos(): A and C; Hsin(): B and D.

26. Control of Domain Patterns
H=500
-500 Oe H
H=180
-180Oe

27. 110 direction
H=300 Oe
H=-147 Oe
H=-162 Oe H
H=-187 Oe

28. Melting of Nb Vortex lattice between 4.5-9 K
M. Roseman, Ph.D. 2001, McGill