High Resolution Magnetic Imaging
Lisa Qian
SASS talk: 3/4/09
Motivation: hard drive technology
Motivation: Nanomagnets
Tumor
injectednanomagnets
Nanomaterials for Cancer TherapyChalla S. S. R. Kumar
artery
magnet
Magnetic Logic DevicesWolfgang Porod, et. al
3-terminal majority logic gate
Motivation: Domain wall interactions
Motivation: Vortices in superconductors
Ophir M. Auslaender, Lan Luan, Eric W. J. Straver, Jennifer E. Hoffman, Nicholas C. Koshnick, Eli Zeldov, Douglas A. Bonn4, Ruixing Liang, Walter N. Hardy & Kathryn A. Moler: Nature Physics 5, 35 - 39 (2009)
What we want to measure
Susceptibility
M H
All this at room temperature!
Atomic Force Microscopy
www.agilent.com/nano
Contact Mode vs. Tapping Mode
Feedback maintains constant DEFLECTION
Feedback maintains constant oscillation AMPLITUDE, PHASE or FREQUENCY
AFM Images
Images from veeco.com
Silicon substrate after RCA clean – RMS surface roughness 0.73nm
Red blood cells – 100um scan
7nm FePt nanoparticles(800nm scan)
Magnetic Force Microscopy
Coat AFM tip with magnetic material, measure the magnetostatic force between tip and sample.
0 tip sample tipF E M H dV
www.veeco.com
Magnetic Force Microscopy• Tapping Mode AFM with magnetized tip
• LiftMode to obtain both topography and magnetic force gradient.
Trace & retrace to measure topography
Cantilever lifts to scan height
Trace at constant separation to measure magnetic forces
Magnetic force gradient causes shift in cantilever resonance frequency:
k
F
f
f
20
0
Magnetic Force Microscopy
Topography (L) and MFM (R) images of hard disk track (25um scan)
25nm
31nm
36nm
51nm
200nm
85nm
MFM image of Maxtor test tracks
Commercial MFM probes give 30nm resolution in ambient conditions.
What Limits Resolution?
tip dipole moment
Lift Height
Tip Geometry• Aspect ratio• Tip radius• Sidewall coating
Magnetic Material
Conventional tip
0 tip sample tipF E M H dV
Carbon Nanotube Probes
• Grow CNTs on commerical AFM cantilevers
• Shorten to a few hundred nm
• E-beam deposit 3nm Ti/7nm Co/3 nmTi
commercial tip CNT tip
Zhifeng Deng; Yenilmez, E.; Leu, J.; Hoffman, JE; Straver, EWJ; Hongjie Dai; Moler, KA: Applied Physics Letters 85, 6263-5 (2004)
Problems with CNT tips
• Frequency doubling– Coating on CNT
divides into domains– Tip magnetic moment
flips at low tip-sample spacing and low bit density
• Need to increase anisotropy and improve fabrication methods J.R. Kirtley, Z. Deng, L. Luan, E.
Yenilmez, H. Dai, and K.A. Moler: Nanotechnology 18 , 465506 (2007)
Nanoparticle Tips
• Attach magnetic nanoparticle to cantilever tip
• No superflous magnetic material around apex
• Intrinsically single domain
• No need to coat
Nanomagnet PropertiesStoner-Wohlfarth potential
K = anisotropy constant
Ms = saturation moment
V = particle volume
20sin ( ) cosSE KV HM V
0 SE KV HM V
Hθ
re
mφ
Superparamagnetism
Thermally activated switching time: 0B
Ek Te
Small V and/or high T: thermal flipping dominates, Hc decreases – Superparamagnetic Limit
~15nm – 50nm for most magnetic materials
FePt Nanoparticles
Shouheng Sun, C.B. Murray, Dieter Weller, Liesl Folks, Andreas Moser. Science 2000, 287, 1989-1992.
• As synthesized: disordered FCC phase, superparamagnetic at room temp
•After anneal, ordered FCT phase, ferromagnetic, with uniaxial anisotropy along [001]
• Ku ~ 107 J/m3, Hc ~ 0.9T at RT
• Monodisperse, tunable sizes and composition
FePt Nanoparticles
As-synthesized
Post-anneal
7nm particles
5nm particles
FePt nanorods and nanowires
Chao Wang, Yanglong Hou, Jaemin Kim, Shouheng Sun. Angew. Chem. Int. Ed. 2007, 46, 6333-6335.
•Length tunable from 20nm to 200nm;•Diameters 2-3nm•(001) plane parallel to growth direction•Thermally unstable under anneal
Hc = 9.5 kOe
What kind of resolution can we get?
Ms
2 xk
t
0r
0r
0z
l
x
z
MtMt
z0 = 10nmSphere: r0 = 3nmRod: l = 200nm r0 = 1.5nmMs = 295 kA/mMt = 1422 kA/mt = 70nm
MFM Transfer Functions
91.2 10
5.3xk
nm
87.2 10
8.7xk
nm
Sphere:
Rod:
Tip Functionalization
Amine groups have a partial positive charge and covalently bond to metallic nanoparticles
Functionalized tips (ugly)
Functionalized Tips
Manipulation with AFMUse functionalized AFM tip to pick up nanomagnets:
• Disperse nanoparticles onto substrate, anneal• Scan to locate particles• Push down on desired particle• Rescan area to confirm that particle is attached
Summary
• High resolution magnetic imaging useful in technology and science
• Magnetic force microscopy is a great technique for room temperature imaging.
• Current MFM resolution is limited to 30nm. We want to pus this to under 10nm.
• We want to push resolution to under 10nm by attaching FePt nanomagnets to functionalized cantilevers.
Acknowledgements• Kam Moler + Molerites
• Jaemin Kim• Prof. Shouheng Sun
• Park AFM• Cynthia Coggins• Doru Florescu
• Stanford Nanocharacterization Laboratory• Bob Jones• Chuck Hitzman• Ann Marshall