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High Resolution Magnetic Imaging

High Resolution Magnetic Imaging. Lisa Qian SASS talk: 3/4/09. Motivation: hard drive technology. Motivation: Nanomagnets. Nanomaterials for Cancer Therapy Challa S. S. R. Kumar . Magnetic Logic Devices Wolfgang Porod, et. al . magnet. Tumor. 3-terminal majority logic gate. artery.

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High Resolution Magnetic Imaging

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  1. High Resolution Magnetic Imaging Lisa Qian SASS talk: 3/4/09

  2. Motivation: hard drive technology

  3. Motivation: Nanomagnets Nanomaterials for Cancer Therapy Challa S. S. R. Kumar Magnetic Logic Devices Wolfgang Porod, et. al magnet Tumor 3-terminal majority logic gate artery injected nanomagnets

  4. Motivation: Domain wall interactions

  5. 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 Physics5, 35 - 39 (2009)

  6. What we want to measure All this at room temperature! Susceptibility

  7. Atomic Force Microscopy www.agilent.com/nano

  8. Contact Mode vs. Tapping Mode Feedback maintains constant DEFLECTION Feedback maintains constant oscillation AMPLITUDE, PHASE or FREQUENCY

  9. AFM Images Silicon substrate after RCA clean – RMS surface roughness 0.73nm Red blood cells – 100um scan 7nm FePt nanoparticles (800nm scan) Images from veeco.com

  10. Magnetic Force Microscopy Coat AFM tip with magnetic material, measure the magnetostatic force between tip and sample. www.veeco.com

  11. Magnetic Force Microscopy • Tapping Mode AFM with magnetized tip • LiftMode to obtain both topography and magnetic force gradient. Trace at constant separation to measure magnetic forces Cantilever lifts to scan height Magnetic force gradient causes shift in cantilever resonance frequency: Trace & retrace to measure topography

  12. 25nm 31nm 36nm 51nm 85nm 200nm Magnetic Force Microscopy Commercial MFM probes give 30nm resolution in ambient conditions. MFM image of Maxtor test tracks Topography (L) and MFM (R) images of hard disk track (25um scan)

  13. What Limits Resolution? Lift Height Conventional tip • Tip Geometry • Aspect ratio • Tip radius • Sidewall coating Magnetic Material tip dipole moment

  14. Carbon Nanotube Probes Zhifeng Deng; Yenilmez, E.; Leu, J.; Hoffman, JE; Straver, EWJ; Hongjie Dai; Moler, KA: Applied Physics Letters85, 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

  15. 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)

  16. Nanoparticle Tips • Attach magnetic nanoparticle to cantilever tip • No superflous magnetic material around apex • Intrinsically single domain • No need to coat

  17. θ φ H Nanomagnet Properties Stoner-Wohlfarth potential K = anisotropy constant Ms = saturation moment V = particle volume

  18. Superparamagnetism Thermally activated switching time: Small V and/or high T: thermal flipping dominates, Hc decreases – Superparamagnetic Limit ~15nm – 50nm for most magnetic materials

  19. 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.

  20. FePt Nanoparticles 7nm particles As-synthesized 5nm particles Post-anneal

  21. FePt nanorods and nanowires Chao Wang, Yanglong Hou, Jaemin Kim, Shouheng Sun. Angew. Chem. Int. Ed. 2007, 46, 6333-6335. Hc = 9.5 kOe • Length tunable from 20nm to 200nm; • Diameters 2-3nm • (001) plane parallel to growth direction • Thermally unstable under anneal

  22. z0 = 10nm Sphere: r0 = 3nm Rod: l = 200nm r0 = 1.5nm Ms = 295 kA/m Mt = 1422 kA/m t = 70nm z l Mt Mt t Ms x What kind of resolution can we get?

  23. MFM Transfer Functions Sphere: Rod:

  24. Tip Functionalization Amine groups have a partial positive charge and covalently bond to metallic nanoparticles

  25. Functionalized tips (ugly)

  26. Functionalized Tips

  27. 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

  28. 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.

  29. Acknowledgements • Kam Moler + Molerites • Jaemin Kim • Prof. Shouheng Sun • Park AFM • Cynthia Coggins • Doru Florescu • Stanford Nanocharacterization Laboratory • Bob Jones • Chuck Hitzman • Ann Marshall

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