Propriétés structurales et magnétiques d'un cristal liquide minéral, les suspensions de goethite (alpha-FeOOH)

Bruno J. Lemaire 1, *
Abstract : We carried experiments on different mineral liquid crystals and especially on suspensions of colloidal goethite particles. After characterizing the suspensions, we showed that they form a nematic phase. We described the isotropic nematic phase transition with the models of literature. Three properties of the isotropic phase under magnetic field are : a very high paranematic order, a non vanishing dipolar component of the orientation order and the change in orientation with the value of the field (parallel to a low field, perpendicular to a high one). We determined the nematic order parameter by birefringence and small angle X-ray scattering measurements, and the dipolar one by magnetization measurements. We interpreted the change in orientation as the competition between a longitudinal field-independent magnetic moment (probably uncompensated spins at the surface of the particles) and the induced magnetic moment (negative magnetic anisotropy). The former dominates at low fields and the latter at higher ones. In this frame, the orientation must remain perpendicular to a high-frequency field, which we observed experimentally. The model fitted the experimental data well. Then we studied the properties of the nematic phase under field. The Frederiks transition threshold is outstandingly low, while the nematic order is very high. The particles reorient perpendicular to a higher field, through a second Frederiks transition, forming a nematic biaxial domain. At even higher fields, the suspension forms a two-dimension colloidal crystal in the c2mm space group. The same phase occurs in zero field at high concentrations. Finally, we include our published articles, concerning suspensions of goethite, stiff covalent sheets of H3Sb3P2O14, flexible polymers of NaNb2PS10 and laponite disc-like moieties.
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Submitted on : Wednesday, December 18, 2002 - 1:47:20 PM
Last modification on : Wednesday, November 14, 2018 - 10:58:02 AM
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  • HAL Id : tel-00002172, version 1



Bruno J. Lemaire. Propriétés structurales et magnétiques d'un cristal liquide minéral, les suspensions de goethite (alpha-FeOOH). Analyse de données, Statistiques et Probabilités []. Université Paris Sud - Paris XI, 2002. Français. 〈tel-00002172〉



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