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RESEARCH SUMMARY
Importance of nanotechnology and magnetic nanoparticles
The field of nanostructure science and technology has grown explosively in the past 15 years. When materials are reduced in size to the nanometer scale, they exhibit novel electrical, optical, magnetic, and chemical properties that are not found in the bulk materials. The magnetic nanoparticles with diameters less than 20 nm represent an important class of nanomaterials because of their potential applications in data storage, advanced permanent magnets, catalysis, biomedicine as well as several other applications. Their magnetic properties can be substantially controlled by changing their size, shape, composition and structure. My research has been focused on synthesis and characterization of monodisperse FePt nanoparticles as well as their use in potential applications.
Synthesis and characterization of FePt nanoparticles
FePt nanoparticles has a very high anisotropy constant and their excellent chemical stability makes them especially useful for practical applications over high moment Co and Fe, as well as the large coercive SmCo5 and Nd2Fe14B nanoparticles which are extremely air sensitive. My first goal was to prepare these FePt nanoparticles with different size and shape and observe their dependence on magnetic properties. I have used chemical solution route to synthesize FePt nanoparticles. Solution phase synthesis is a unique way for producing monodisperse nanoparticles and is also scalable. The particle size, shape, and composition were controlled by systematically adjusting the reaction parameters. However, as-synthesized FePt nanoparticles have a chemically disordered face centered cubic (fcc) structure and are of superparamagnetic nature. Thermal annealing was needed to transform the fcc structure into the chemically ordered face centered tetragonal (L10) structure. Later I did several modifications on FePt nanoparticles based on the requirements for the particular applications which have been discussed in details below.
1. “Size and shape control of monodisperse FePt nanoparticles” V. Nandwana, K. E. Elkins, N. Poudyal, G. S. Chaubey, K. Yano, and J. P. Liu, J. Phys. & Chem. C, 111
(2007) 4185-4189.
2. “Magnetic hardening in ultrafine FePt nanoparticle assembled films” V. Nandwana, K. E. Elkins, and J. P. Liu, Nanotechnology 16 (2005) 2823–2826.
Recording Media - Preparation of monodisperse ferromagnetic FePt nanoparticles
The current magnetic media are rapidly reaching the ‘superparamagnetic
limit’, which means that the magnetic bits of information are so small
that they are not able to retain the stored information for an extended
time. To obtain high density recording media, the material should have
a very high anisotropy constant so that it can retain the magnetization
at a very small size. Due to their high anisotropy and chemical
stability, ferromagnetic FePt nanoparticles can be stable at as low as
3.5 nm which makes them ideal candidate for future recording media.
However, the annealing to convert FePt nanoparticles from fcc to L10
phase results in particle aggregation leading to wide size
distributions and deterioration of the monodispersity of the particles.
To prevent this uncontrolled agglomeration, we developed a new approach
in our lab where FePt nanoparticles were mixed in high amounts of salt
before annealing. The salt was washed away with water after annealing
and monodisperse ferromagnetic FePt nanoparticles were obtained. As an
alternative approach to inhibit particle growth at higher annealing
temperature, the transition temperature of fcc to L10 FePt was reduced
by more than 200 oC by doping Au in FePt nanoparticles.
3. “Monodisperse face-centred tetragonal FePt nanoparticles
with giant coercivity” (an IoP Select article and reported in
PhysicsWorld, NanotechWeb and other
medias) K. E. Elkins, D. Li, N. Poudyal, V. Nandwana, Z.
Jin, K. Chen, and J. P. Liu, J. Phys D: Appl. Phys. 38 (2005)
2306-2309.
4. “Magnetic properties of
FexPtyAu100-x-y nanoparticles” V. Nandwana, G. S. Chaubey, Y. Zhang, and J. P. Liu, J Nanosc. Nanotech. 10 (2010) 2979-2983.
TEM image of as-synthesized FePt nanoparticles of size (a) 2 nm, (b) 3 nm, (c) 4 nm, (d) 5 nm, (e) 6 nm, (f) 7 nm, (g) 8 nm, and (h) 9 nm.
Permanent Magnets - Synthesis of bimagnetic FePt/Fe3O4 nanoparticles
The figure of merit by a permanent magnetic material is judged, is the maximum energy product, (BH)max. To obtain high (BH)max, the magnet should have high magnetization (Ms) as well as high coercivity (Hc). The exchange-coupled nanocomposite which is two phase system of hard (high Hc) and soft (high Ms) magnetic nanoparticles, is called the next generation of high energy magnets. FePt nanoparticles can be the best choice for hard magnetic material for this application because of their high coercivity and chemical stability. To obtain exchange-coupled nanocomposite, bimagnetic FePt/Fe3O4 nanoparticles with different morphologies; core/shell and heterodimer structured nanoparticles were synthesized. After annealing in reducing atmosphere, Fe3O4 reduced to Fe3Pt which is very good soft phase. The size of FePt as well as Fe3O4 was tuned in a very fine manner to obtain highest energy product. The 36% increase in energy product was observed over single phase FePt nanoparticles.
5. “Bimagnetic nanoparticles with enchanced exchange coupling and energy products” V. Nandwana, G. S. Chaubey, K. Yano, C. B. Rong and J. P. Liu, J. Appl. Phys. 105
(2009) 014303 (selected for the January 2009 issue of Virtual Journal of Nanoscale Science & Technology).
TEM images of the as-synthesized (a) 7 nm FePt nanoparticles, (b) 7 nm FePt nanoparticles coated with 1 nm, and (c) 7 nm FePt nanoparticles coated with 3 nm Fe3O4 shell.
TEM images of the as-synthesized FePt/Fe3O4 heterodimer nanoparticle with 8 nm FePt attached with (a) 5 nm, (b) 8 nm, and (c) 10 nm Fe3O4
Biomedical applications - Synthesis of FePt/Au nanoparticles
Magnetic nanoparticles are desired in biomedical application because their controllable sizes ranging from a few nanometres up to tens of nanometres are comparable to those of a cell, virus, protein or a gene. Also their movement can be controlled under an external magnetic field. Many The high moment nanoparticles are desirable since they can respond more readily to an applied magnetic field. Ferromagnetic nanoparticles like FeCo (2000 emu/cc), Fe(1700 emu/cc) or Co(1400 emu/cc) are ideal candidates but they have extremely high air sensitivity. Though the moment of FePt (1100 emu/cc) nanoparticles is not as high as FeCo or Fe, their excellent chemical stability make them a very unique candidate. Their magnetic moment is still much higher than superparamagnetic iron oxide (350 emu/cc) nanoparticles which are currently being used for these applications. Therefore, Au was coated on the surface of FePt nanoarticles since it not only provides a good surface for subsequent functionalization for biomedical applications due to its very well known chemistry and these gold coated FePt nanoparticles were attached successfully to DNA in a collaboration with a team in the University of Texas at Dallas.
6. “Synthesis and Characterization of Magnetic FePt/Au Core/Shell Nanoparticles” K. Yano, V. Nandwana, G. S. Chaubey, N. Poudyal, S. Kang, and J. P. Liu, J. Phys.
& Chem. C 113 (2009) 13088–13091.
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