Magnetic Force Microscopy Fmagntic mtip Hsample so one

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, Co 80 Cr 20, Co 71 Pt 12 Cr 17 (hard) Ni 81 Fe 19, Fe, and Ni 50 Co 50 (soft) by sputtering or thermal evaporation, Often 5 nm Au protection. Magnetized in 1 T 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, 20 Oe, 12 Oe, -2 Oe respectively. Tip: 50 nm Co 71 Pt 12 Cr 17, 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 < 1 Oe

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 Co 71 Pt 12 Cr 17. 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=20 nm

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

Tip influence! MFM Tip Stray Field Distortion Three consecutive scans. Ni. Fe: 500 nm 200 nm 10 nm 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).

Manipulation elliptical Ni. Fe, 600 nm x 150 nm x 30 nm 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 Co. Pt. Cr 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 H At Remanence H=-25 Oe X. Zhu, Ph. D. Thesis 2002, Mc. Gill University H H=-60 Oe

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

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! H 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

Melting of Nb Vortex lattice between 4. 5 -9 K M. Roseman, Ph. D. 2001, Mc. Gill