Detecting the Magnetic Response of Iron Oxide Capped Organosilane Nanostructures Using Magnetic Sample Modulation and Atomic Force Microscopy Jie-Ren Li, † Brian R. Lewandowski, † Song Xu, ‡ and Jayne C. Garno* ,† Department of Chemistry and the Center for BioModular Multi-Scale Systems, Louisiana State University, 232 Choppin Hall, Baton Rouge, Louisiana 70803, and Nanotechnology Measurements Division, Agilent Technologies, Inc., 4330 W. Chandler Road, Chandler, Arizona 85226 A new imaging strategy using atomic force microscopy (AFM) is demonstrated for mapping magnetic domains at size regimes below 100 nm. The AFM-based imaging mode is referred to as magnetic sample modulation (MSM), since the flux of an AC-generated electromagnetic field is used to induce physical movement of magnetic nanomaterials on surfaces during imaging. The AFM is operated in contact mode using a soft, nonmagnetic tip to detect the physical motion of the sample. By slowly scanning an AFM probe across a vibrating area of the sample, the frequency and amplitude of vibration induced by the magnetic field is tracked by changes in tip deflec- tion. Thus, the AFM tip serves as a force and motion sensor for mapping the vibrational response of magnetic nanomaterials. Essentially, MSM is a hybrid of contact mode AFM combined with selective modulation of mag- netic domains. The positional feedback loop for MSM imaging is the same as that used for force modulation and contact mode AFM; however, the vibration of the sample is analyzed using channels of a lock-in amplifier. The investigations are facilitated by nanofabrication methods combining particle lithography with organic vapor deposi- tion and electroless deposition of iron oxide, to prepare designed test platforms of magnetic materials at nanom- eter length scales. Custom test platforms furnished suit- able surfaces for MSM characterizations at the level of individual metal nanostructures. INTRODUCTION Studies of the size-scaling effects for magnetic nanomaterials are an important direction for surface investigations, if the difficulties for characterizing properties at very small dimensions can be overcome. Understanding magnetic size scaling is not only important for understanding the behavior of existing nanomaterials but is also valuable for efforts to develop new materials with engineered properties. Magnetic nanomaterials are used for magnetic fluids, 1-3 catalysis, 4-6 magnetic separations, 7-9 magnetic resonance imaging, 10-12 data storage, 13-15 and biosensing/ biomedical technologies. 16-18 In this report, a hybrid AFM mode will be described for highly sensitive and selective characteriza- tions of the vibrational motion of magnetic nanomaterials respond- ing to the flux an alternating electromagnetic field. For scaling effects of magnetic nanomaterials, precise knowl- edge of the relationships between particle shape and size, surface structure, and the resulting magnetic properties is incomplete. The fundamental magnetic properties of nanomaterials, such as blocking temperature, spin lifetime, coercivity, and susceptibility, are influenced by size scaling. 19,20 Changes of magnetic properties with nanoscale dimensions are not well-defined even for simple particles composed of pure materials such as Fe, Co, or Ni, * To whom correspondence should be addressed. Phone: 225-578-8942. Fax: 225-578-3458. E-mail: jgarno@lsu.edu. † Louisiana State University. ‡ Agilent Technologies, Inc. (1) Berkowitz, A. E.; Lahut, J. A.; Vanburen, C. E. IEEE Trans. Magn. 1980, 16, 184–190. (2) Chikazumi, S.; Taketomi, S.; Ukita, M.; Mizukami, M.; Miyajima, H.; Setogawa, M.; Kurihara, Y. J. Magn. Magn. Mater. 1987, 65, 245–251. (3) Taketomi, S.; Ukita, M.; Mizukami, M.; Miyajima, H.; Chikazumi, S. J. Phys. Soc. Jpn. 1987, 56, 3362–3374. (4) Guillou, N.; Gao, Q.; Forster, P. M.; Chang, J. S.; Nogues, M.; Park, S. E.; Ferey, G.; Cheetham, A. K. Angew. Chem., Int. Ed. 2001, 40, 2831–2834. (5) Yoon, T. J.; Lee, W.; Oh, Y. S.; Lee, J. K. New J. Chem. 2003, 27, 227–229. (6) Lu, A. H.; Schmidt, W.; Matoussevitch, N.; Bonnemann, H.; Spliethoff, B.; Tesche, B.; Bill, E.; Kiefer, W.; Schuth, F. Angew. Chem., Int. Ed. 2004, 43, 4303–4306. (7) Olsvik, O.; Popovic, T.; Skjerve, E.; Cudjoe, K. S.; Hornes, E.; Ugelstad, J.; Uhlen, M. Clin. Microbiol. Rev. 1994, 7, 43–54. (8) Uehara, M.; Mori, S.; Chen, C. H.; Cheong, S. W. Nature 1999, 399, 560– 563. (9) Dagotto, E.; Hotta, T.; Moreo, A. Phys. Rep. 2001, 344, 1–153. (10) Weissleder, R.; Elizondo, G.; Wittenberg, J.; Rabito, C. A.; Bengele, H. H.; Josephson, L. Radiology 1990, 175, 489–493. (11) Wang, Y. X. J.; Hussain, S. M.; Krestin, G. P. Eur. Radiol. 2001, 11, 2319– 2331. (12) Mornet, S.; Vasseur, S.; Grasset, F.; Duguet, E. J. Mater. Chem. 2004, 14, 2161–2175. (13) Ross, C. Annu. Rev. Mater. Res. 2001, 31, 203–235. (14) Moser, A.; Takano, K.; Margulies, D. T.; Albrecht, M.; Sonobe, Y.; Ikeda, Y.; Sun, S. H.; Fullerton, E. E. J. Phys. D: Appl. Phys. 2002, 35, R157– R167. (15) Terris, B. D.; Thomson, T. J. Phys. D: Appl. Phys. 2005, 38, R199–R222. (16) Tartaj, P.; Morales, M. D.; Veintemillas-Verdaguer, S.; Gonzalez-Carreno, T.; Serna, C. J. J. Phys. D: Appl. Phys. 2003, 36, R182–R197. (17) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167–R181. (18) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995–4021. (19) Roduner, E. Chem. Soc. Rev. 2006, 35, 583–592. (20) Jun, Y. W.; Seo, J. W.; Cheon, A. Acc. Chem. Res. 2008, 41, 179–189. Anal. Chem. 2009, 81, 4792–4802 10.1021/ac900369v CCC: $40.75  2009 American Chemical Society 4792 Analytical Chemistry, Vol. 81, No. 12, June 15, 2009 Published on Web 05/19/2009