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Bulk magnetization and nuclear magnetic resonance of magnetically purified layered silicates and their polymer-based nanocomposites

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5 Author(s)
Levin, E.M. ; Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011-3160 ; Rawal, A. ; Budko, S.L. ; Kracher, A.
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The bulk magnetization and the 1H and 29Si nuclear-magnetic-resonance (NMR) spectra of two layered silicates, montmorillonite (MMT) and hectorite (HCT), purified by high-gradient magnetic separation, and of HCT-polymer nanocomposites have been measured. At 300 K, the magnetization of MMT as received shows a behavior typical of paramagnets and does not change significantly even after ∼100 h of magnetic separation. The magnetization of HCT as received is typical of a weak ferromagnet but it changes drastically after magnetic separation. The extracted particles have sizes varying from a few to about 150 μm and show ferromagneticlike properties at 300 K. While the magnetization/magnetic-field ratio, M/H, of HCT is reduced 50-fold after ∼15 min of magnetic separation and shows predominantly diamagnetic properties at 300 K after ∼30 h, we have also found that HCT contains ∼0.2 mass % of paramagnetic Fe ions, with an effective magnetic moment of ∼5.2μB per Fe ion, as a regular element of its lattice. This is much smaller than the ∼3.8 mass % in MMT. Spinning sidebands in 1H NMR spectra of HCT as received are reduced by magnetic separation. This indicates that dipolar interactions between nuclei and paramagnetic ions even in separated phases can affect the NMR spectra. Magnetically purified hectorite was used for preparing polymer-- HCT nanocomposites where the NMR spectra show increased signal-to-noise ratios.

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Journal of Applied Physics  (Volume:98 ,  Issue: 11 )