Transcript Magnetic Imaging - SLAC National Accelerator Laboratory

High Resolution Magnetic
Imaging
Lisa Qian
SASS talk: 3/4/09
Motivation: hard drive technology
Motivation: Nanomagnets
Magnetic Logic Devices
Wolfgang Porod, et. al
Nanomaterials for Cancer Therapy
Challa S. S. R. Kumar
magnet
Tumor
3-terminal majority logic gate
artery
injected
nanomagnets
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
All this at room
temperature!
Susceptibility
M H  
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
Silicon substrate after
RCA clean – RMS
surface roughness
0.73nm
Red blood cells
– 100um scan
Images from veeco.com
7nm FePt nanoparticles
(800nm scan)
Magnetic Force Microscopy
Coat AFM tip with magnetic material, measure
the magnetostatic force between tip and sample.
F  E  
www.veeco.com
0
M
tip

H sample dVtip
Magnetic Force Microscopy
• Tapping Mode AFM with magnetized tip
• LiftMode to obtain both topography and
magnetic force gradient.
Trace at constant
Cantilever lifts to
scan height
Trace & retrace to
measure topography
separation to measure
magnetic forces
Magnetic force gradient causes
shift in cantilever resonance
f 0
F
frequency:

f0
2k
Magnetic Force Microscopy
25nm
31nm
36nm
Commercial MFM probes
give 30nm resolution in
ambient conditions.
51nm
85nm
200nm
MFM image of Maxtor test tracks
Topography (L) and MFM
(R) images of hard disk track
(25um scan)
What Limits Resolution?
Lift Height
Tip Geometry
• Aspect ratio
• Tip radius
• Sidewall coating
Magnetic Material
Conventional tip
F  E  
0
M
tip

H sample dVtip
tip dipole moment
Carbon Nanotube Probes
Zhifeng Deng; Yenilmez, E.;
Leu, J.; Hoffman, JE; Straver,
EWJ; Hongjie Dai; Moler, KA:
Applied Physics Letters 85,
6263-5 (2004)
• 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
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
J.R. Kirtley, Z. Deng, L. Luan, E.
methods
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 Properties
Stoner-Wohlfarth potential
E  KV sin 2 (   )  0 HM SV cos 
K = anisotropy constant
Ms = saturation moment
V = particle volume
H
er
θ φ
m
E  KV  0 HM SV
Superparamagnetism
Thermally activated switching time:
   0e
E
k BT
Small V and/or high T: thermal flipping dominates,
Hc decreases – Superparamagnetic Limit
~15nm – 50nm for most
magnetic materials
FePt Nanoparticles
• 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
Shouheng Sun, C.B. Murray, Dieter Weller, Liesl Folks, Andreas Moser.
Science 2000, 287, 1989-1992.
FePt Nanoparticles
7nm particles
5nm particles
As-synthesized
Post-anneal
FePt nanorods and nanowires
Hc = 9.5 kOe
•Length tunable from 20nm to 200nm;
•Diameters 2-3nm
•(001) plane parallel to growth direction
•Thermally unstable under anneal
Chao Wang, Yanglong Hou, Jaemin Kim, Shouheng Sun. Angew. Chem.
Int. Ed. 2007, 46, 6333-6335.
What kind of resolution can we get?
r0
z
l
Mt
Mt
r0
z0 = 10nm
Sphere: r0 = 3nm
Rod: l = 200nm
r0 = 1.5nm
Ms = 295 kA/m
Mt = 1422 kA/m
t = 70nm
z0
Ms
t
x
  2 kx
MFM Transfer Functions
Sphere:
k x  7.2 108
  8.7nm
Rod:
k x  1.2 109
  5.3nm
Tip Functionalization
Amine groups have a partial positive charge and
covalently bond to metallic nanoparticles
Functionalized tips (ugly)
Functionalized Tips
Manipulation with AFM
Use 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