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College Park, Maryland      June 6 - 10 , 2004

T1-A3 (9:15 AM): Field and Shear Induced Orientational Order in Magnetic Nanoparticle Dispersions from SANS

Vemuru V. Krishnamurthy, B. He, M. Piao ( MINT Center, The University of Alabama, Tuscaloosa, Alabama 35487-0209), L. Porcar ( NIST Center for Neutron Research), D. E. Nikles, J. M. Wiest, G. J. Mankey ( MINT Center, The University of Alabama, Tuscaloosa, Alabama 35487-0209)

Acicular magnetic nanoparticles have technological applications. The suitability of nanoparticles for magnetic recording applications depends on the control of the orientational order in an external force field, such as a shear flow and a magnetic field. Using small angle neutron scattering, we have investigated the orientational order of polydisperse rod shaped iron nanoparticles in cyclohexane based magnetic dispersions in longitudinal shear flow and transverse magnetic field. The anisotropy of the scattering intensity observed in 3.2 vol. % and 3.9 vol. % iron nanoparticle dispersions in shear flow and/or magnetic field, shows that the particles start to orient either in a shear flow of 100 s-1 or in a magnetic field of 8 mT. In zero-applied-field, the orientational order is measured as a function of steady shear flow. In an applied field, both the degree of order and the tilt angle of the particles is measured as a function of steady shear flow. At zero shear rate, the anisotropy, i.e., the order parameter exhibits hysteresis as a function of applied magnetic field showing that the nanoparticles order in domains. The scattering from nanoparticles in the 3.9 vol. % dispersion exhibits second order anisotropy in an applied field greater than 5 mT.

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