Magnetic Force Microscopy Fmagntic = mtip • Hsample so one images stray fields! Comprehensive review: Grutter, Mamin and Rugar, in ‘Scanning Tunneling Microscopy II’ Springer, 1991
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!!!
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.
Subtle, reversible tip stray field effects: Bloch walls (black and white lines) in Fe whisker
Less subtle effect… Displacement of Bloch line in a Bloch wall in a Fe(001) whisker, Hc < 1Oe
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.
Optimized coating depending on sample Max field components and their decay lengths for z=20nm
Tapping/Lift mode Good separation topography – magnetic information (in most cases)
Tip influence! MFM Tip Stray Field Distortion Reduce Distortion: Three consecutive scans. NiFe: 500nm200nm10nm 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, 024423 (2002).
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, 024423 (2002)
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.
Permalloy Circular Rings Domain wall propagation At Remanence H=-25 Oe H=-60 Oe H X. Zhu, PhD. Thesis 2002, McGill University
MFM Imaging Experiment Simulation Stray field Transverse domain wall Onion State NiFe: 700 nm Flux domain wall Public code: OOMMF NiFe: 5 mm
MFM Imaging Co (4nm) Cu(3nm) NiFe (6nm) Mixtures of antiparallel states and parallel states. Coexistence of the two different antiparallel states.
MFM Imaging of weak stray fields: pseudo spin valve structures C & D are antiparallel, but the two layers are not completely magnetically equivalent.
Magnetostatic Coupling: can one build magnetic cellular automata?
Coherence length relevant! Coupled 700 nm rings
Hysteresis Loop of Ensemble
Switching Field Distribution
Hysteresis Loop of Ensemble Switching field distribution is broader for switching form ‘onion’ to vortex.
Individual Hysteresis Loop, part II
Individual Hysteresis Loop
Permalloy Square Rings H 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
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
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.
Control of Domain Patterns H=500 -500 Oe H H=180 -180Oe
110 direction H=300 Oe H=-147 Oe H=-162 Oe H H=-187 Oe
Melting of Nb Vortex lattice between 4.5-9 K M. Roseman, Ph.D. 2001, McGill