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

2. Motivation: hard drive technology

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

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. Bonn 4, Ruixing Liang, Walter N. Hardy & Kathryn A. Moler : Nature Physics 5, 35 - 39 (2009) 4

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

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

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 & 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:

12. 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.

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

14. 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)

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

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

19. 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] K u ~ 10 7 J/m 3, H c ~ 0.9T at RT Monodisperse, tunable sizes and composition

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

21. 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 H c = 9.5 kOe

22. What kind of resolution can we get? MsMs t l x z MtMt MtMt z 0 = 10nm Sphere: r 0 = 3nm Rod: l = 200nm r 0 = 1.5nm M s = 295 kA/m M t = 1422 kA/m t = 70nm

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