Modeling Analysis of Exchange-Coupled Nanocomposite Magnets
The equations describe the correlations between the exchange strength and the hard- and soft-phase parameters. The figure to the right is a typical 3-D energy surface for ferromagnetically coupled bilayers.
For details, see V.M. Chakka, Z. S. Shan, and
J. P. Liu, J. Appl. Phys., 94 (2003), 6673, and Z. J. Guo, J. S. Jiang, J. E. Pearson, S. D. Bader, and J. P. Liu, Appl. Phys. Lett., 81, 2029 (2002).
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Synthesis of Monodisperse Ferromagnetic Nanoparticles
By varying synthetic parameters, the composition from 15-90% Fe  and particle size from 2 to 9 nm can be tuned with 1 nm accuracy for the monodisperse fcc FePt nanoparticles . The salt-matrix annealing method then can be used to develop the high anisotropy L10 structure from the disordered fcc structure without sintering
[3-5]. Quantitative correlations between particle size and the structural and magnetic properties of the L10 nanoparticles show that the long-range chemical ordering parameter S, Curie temperature and saturated magnetization drop significantly with decreasing particle size d .
 C.B. Rong, Y. Li and J.P. Liu, J. Appl. Phys., 101, 09K505 (2007);  V. Nandwana, K. E. Elkins, N. Poudyal, G. S. Chaubey, K. Yano, and J. P. Liu, J. Phys. Chem. C, 111, 4185 (2007);  K.E. Elkins, D. Li, N. Poudyal, N. Nandwana, Z. Jin, K. Chen, J.P. Liu, J.
Phys. D: Appl. Phys. 38, 2306 (2005);  D. Li, N. Poudyal, N.; Nandwana, Z. Jin, K.E. Elkins, J.P. Liu. J. Appl. Phys., 99, 08E911 (2006);  J.P. Liu, K.E. Elkins, D. Li, V. Nandwana, N. Poudyal, IEEE Trans. Magn., 42, 3036(2006);  C.B. Rong, D. Li, V. Nandwana, N. Poudyal, K.E. Elkins, J.P. Liu, Adv. Mater., 18, 2984 (2006).
FeCo nanoparticles are ideal building blocks for nanostructured magnetic materials and biomedical applications due to the high saturation magnetization. A simple route has been developed to synthesize air-stable FeCo nanoparticles. It is based on reductive decomposition of Fe(III) acetylacetonate and Co(II) acetylacetonate in a mixture of surfactants and
1,2-hexadecanediol (HDD) under a gas mixture of Ar 93% + H2 7% at 300oC.
For details, see G.S. Chaubey, C. Barcena, N. Poudyal, C.B. Rong, J.M. Gao, S.H. Sun and J. P. Liu, J.Am.Chem.Soc., (June 2007)
Nanoparticles of Fe, Co, FeCo, SmCo and NdFeB systems with sizes smaller than 30 nm and narrow size distribution have been successfully prepared by surfactant-assisted ball milling, which opens a new approach to obtain monodisperse magnetic nanoparticles.
For details, see V. M. Chakka, B. Altuncevahir, Z. Q. Jin, Y. Li, J. P. Liu,
J. Applied Physics, 99 (2006), 08E912.
Magnetic-field milling shows promise for
producing nanostructured anisotropic hard magnetic particles. These nanostructured anisotropic submicrometre particles can be used for fabricating anisotropic bulk nanocomposite magnets.
For details, see N. Poudyal, B. Altuncevahir, V. Chakka, K. Chen, T.D. Black, J. P. Liu, Y. Ding and Z.L. Wang, J. Phys. D: Appl. Phys., 37, L45 (2004).
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Bulk Nanocomposite Magnets
Based on our experimental demonstration of enhanced energy products in nanoparticle-made exchange-coupled nanocomposite, we continue to work in fabrication of bulk nanocomposite magnets made from compaction of nanoparticles. High density (95% of theoretical value) bulk FePt/Fe3Pt nanocomposite magnets with a homogenous microstructure have been prepared by high-pressure warm compaction  and spark plasma sintering  of chemically synthesized FePt and Fe3O4 nanoparticles. Energy products up to 16.3 MGOe of the isotropic bulk nanocomposite magnets have been achieved, which is significantly higher than the theoretical limit for fully dense isotropic single-phase FePt magnets. A pressure-induced phase transition was observed.
Hao Zeng, Jing Li, J. P. Liu, Zhong L. Wang and Shouheng Sun, Nature, 420, 395-398 (2002). C.B. Rong, V. Nandwana, N. Poudyal, J.P. Liu, M.E Kozlov, R.H. Baughman, Y.Ding, and Z.L. Wang, J. Appl. Phys., July (2007).  C.B. Rong, V. Nandwana, N. Poudyal, J.P. Liu, T. Saito, Y.Q. Wu, and M.J. Kramer, J. Appl. Phys., 101, 09K515 (2007).
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