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