Modeling Analysis of Exchange-Coupled Nanocomposite Magnets
- Novel Rare-Earth-free Magnets based on Nanowire Technology Single domain ferromagnetic nanowires (NWs) are promising materials for next generation high-performance permanent magnets. Our approach is to develop high coercivity in cobalt NWs by using the magnetic shape anisotropy, which strongly depends on the aspect-ratio and uniformity of the NWs. Single crystalline hcp Co NWs with the aspect ratio from 10 to 66 are synthesized via a solvothermal method by controlling the growth process. The increased aspect ratio leads to enhanced coercivity of aligned Co nanowire assemblies up to an optimum value of 12.5 kOe at 300 K, which is closed to the theoretical limit of the coercive field (14.3 kOe). As a result, the energy product of the wires reaches 44 MGOe. These cobalt nanomagnets are excellent building blocks for future bonded, consolidated and thin film nanostructured permanent magnets. The compaction of the prealigned NWs assemblies at various pressure results in packing density in the range from 5.3 to 7.3 g/cm3 and energy product as high as 20 MGOe. The obtained nanowire based permanent magnets with well controlled aspect ratios exhibited energy products in the intermediate range between Alnico and rare-earth containing magnets.
- Shape-Controlled Magnetic Oxide Nanoparticles Iron oxide (Fe3O4) nanoparticles are the most studied magnetic nanoparticles (NPs) for their uses in vast areas of technology. Researchers have been doing excellent work to control their magnetic properties, which are key to all of their applications. Incorporation of different transition metal ions and control over their sizes from nanometre to submicron scale serve the purpose effectively. A smarter alternate approach to optimize the magnetic properties is to tailor the shape of the NPs since anisotropy plays a crucial role in deciding magnetic characteristics. For the synthesis of monodisperse NPs we have modified the conventional thermal decomposition to a ‘solvent-less’ synthesis approach where a long-chain amine/acid acts as reducing and surface functionalizing agent. Various shapes like spheres, rods, octahedrons and cubes are achieved through simple alterations in reaction conditions. Octahedral and cube shaped Fe3O4 NPs exhibits bulk magnetization (~ 92 emu/g) value, due to lesser surface spin disorder. More surprisingly, a characteristic Verwey transition near 120 K is observed in octahedral and cubic NPs of the same size with spherical NPs, which indicates better stoichiometry in the cubes and octahedral shape. Other than peculiar physical properties, these “smarter” NPs are more effective for their applications.
See , IEEE Trans. On Magnetics, 38 (2002), 2907.
See Appl. Phys. Lett., 81 (2002), 2029.
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).
Synthesis of Monodisperse Ferromagnetic Nanoparticles
By varying synthetic parameters, the composition from 15-90% Fe [1] and particle size from 2 to 9 nm can be tuned with 1 nm accuracy for the monodisperse fcc FePt nanoparticles [2]. 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 [6].
[1] C.B. Rong, Y. Li and J.P. Liu, J. Appl. Phys., 101, 09K505 (2007); [2] V. Nandwana, K. E. Elkins, N. Poudyal, G. S. Chaubey, K. Yano, and J. P. Liu, J. Phys. Chem. C, 111, 4185 (2007); [3] K.E. Elkins, D. Li, N. Poudyal, N. Nandwana, Z. Jin, K. Chen, J.P. Liu, J. Phys. D: Appl. Phys. 38, 2306 (2005); [4] D. Li, N. Poudyal, N.; Nandwana, Z. Jin, K.E. Elkins, J.P. Liu. J. Appl. Phys., 99, 08E911 (2006); [5] J.P. Liu, K.E. Elkins, D. Li, V. Nandwana, N. Poudyal, IEEE Trans. Magn., 42, 3036(2006); [6] 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.
Bulk Nanocomposite Magnets
Based on our experimental demonstration of enhanced energy products in nanoparticle-made exchange-coupled nanocomposite[1], 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 [2] and spark plasma sintering [3] 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.[2]
Others (magnetocaloric, etc.)
The Fe-rich FexPt100-x (x>78) alloys undergo a first-order phase transition from the disordered g phase with fcc structure to ordered a phase with bcc structure on cooling, and a reverse process on heating, accompanied by a magnetization jump [1]. A magnetic-field-induced phase transition g phase to the a phase is also observed in these alloys. The coupled temperature- and magnetic-field-induced phase transitions give rise to a huge negative thermal expansion and thus a giant magnetic entropy change (up to 39.8 J/kgK for the alloy with x=79) [2].
