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SUMMARY:The Outstanding Magnetic Behaviour of Nematic Suspensions of Goeth
 ite Nanorods - Patrick Davidson\, Laboratoire de Physique des Solides\, Or
 say\, France
DTSTART:20050513T123000Z
DTEND:20050513T133000Z
UID:TALK4308@talks.cam.ac.uk
CONTACT:Duncan Simpson
DESCRIPTION:Aqueous suspensions of goethite (-FeOOH) nanorods form a lyotr
 opic nematic phase in which the nanorods show orientational long-range ord
 er but only short-range positional order. It is actually one of the few ex
 amples of mineral liquid-crystals. Despite the fact that bulk goethite is 
 a typical antiferromagnetic material\, colloidal suspensions of goethite n
 anorods display unique magnetic properties [1-3]. Small angle X-ray scatte
 ring (SAXS) and birefringence measurements show that the nematic phase ali
 gns in magnetic fields much lower than usual liquid crystals. The nanorods
  orient along the field direction at field intensities smaller than 350 mT
 \, but they reorient perpendicularly to the field beyond 350 mT. This surp
 rising behaviour is also observed in the isotropic phase which has a very 
 strong magnetic-field-induced birefringence. These effects most probably r
 esult from a competition between a nanorod remanent magnetic moment and a 
 negative anisotropy of its magnetic susceptibility. At still larger magnet
 ic fields (around B l 1 T)\, the nematic suspensions form 2-dimensional co
 lumnar phases. This magnetic-field-induced transition\, at constant concen
 tration\, is reversible. We could record the SAXS patterns of single cryst
 als\, which allowed us to determine the organisation of the nanorods in th
 e columnar phase.\nMost of the experimental techniques and formalism used 
 in these studies may be applied to (or even stem from) the field of suspen
 sions of rod-like biomolecules (TMV\, DNA\, microtubules\, F-actin\, chiti
 n\, cellulose ....)\n\n[1] Q.Majorana\, Comptes Rendus Acad. Sci.\, 135\, 
 159 (1902)\n[2] A.Cotton\, H.Mouton\, Comptes Rendus Acad. Sci.\, 141\, 31
 7 (1905) \n[3] B.J.Lemaire et al\, Phys. Rev. Lett.\, 88\, 125507 (2002). 
 idem\, 93\, 267801 (2004). \n
LOCATION:Mott Seminar Room\, Cavendish Laboratory
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