Magnetic properties of graphitically encapsulated nickel nanocrystals
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Magnetic properties of graphitically encapsulated nickel nanocrystals J.-H. Hwang and V. P. Dravid Department of Materials Science and Engineering, Materials Research Center, Northwestern University, Evanston, Illinois 60208
M. H. Teng Department of Geology, National Taiwan University, Taipei, Taiwan, Republic of China
J. J. Host, B. R. Elliott, D. L. Johnson, and T. O. Mason Department of Materials Science and Engineering, Materials Research Center, Northwestern University, Evanston, Illinois 60208 (Received 8 July 1996; accepted 23 December 1996)
Graphitically encapsulated ferromagnetic Ni nanocrystals have been synthesized via a modified tungsten arc-discharge method. By virtue of the protective graphitic coating, these nanocrystals are stable against environmental degradation, including extended exposure to strong acids. The magnetic properties of the encapsulated particles are characterized with regard to the nanoscale nature of the particles and the influence of the graphitic coating which is believed to be benign insofar as the intrinsic magnetic properties of the encapsulated nanocrystals are concerned. The Curie temperature of graphitically encapsulated Ni nanocrystals is the same as that of microcrystalline Ni. However, saturation magnetization, remanent magnetization, and coercivity of these particles are reduced, for a range of temperatures. The unique features are compared with those of unencapsulated nanocrystalline and coarse microcrystalline nickel particles.
I. INTRODUCTION
Nanocrystalline materials with particle or grain sizes in the range 10 –100 nm have gained considerable attention in recent years because of the potential for enhanced or even unique physical properties compared to their microcrystalline or bulk counterparts.1–5 A variety of metals and nonmetallic materials have been synthesized by gas phase condensation6–12 and aqueous chemical routes.13–15 Nanocrystalline metals, although readily available, have the disadvantage of being prone to rapid environmental degradation, owing to a very high surface area to volume ratio and high reactivity. This tends to limit potential industrial applications and even scientific evaluation of nanocrystalline properties. A recent breakthrough in this regard is encapsulation of nanocrystals with chemically stable species such as graphitic layers,16–29 which not only protects the nanocrystals from environmental degradation, but is also believed to be benign insofar as the intrinsic nanocrystalline magnetic properties are concerned. The recent studies of the graphitic encapsulation of metal carbides16–23 have promoted an extensive research on similar protection of ferromagnetic materials, e.g., Fe, Co, and Ni.24–29 Appropriately encapsulated ferromagnetic materials may find applications ranging from ferrofluids and recording media to novel biomedical applications such as drug delivery and immunoassays. 1076
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 4, Apr 1997
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