Structure and Dynamics of the Ferrosmectic Phase
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STRUCTURE AND DYNAMICS OF THE FERROSMECT]IC PHASE PASCALE FABRE Collge de France, Laboratoire de Physique de la Mati~re Condensde 11, Place Marcelin Berthelot 75231 PARIS Cedex 05
FRANCE
ABSTRACT We have elaborated a system -that we call a ferrosmectic- which contains magnetic particles included in a lamellar phase. Because of the presence of the confined particles, this phase exhibits specific features that are revealed by submitting it to a magnetic field as well as by neutrons or quasi-elastic light scattering experiments. We present here a review of the different properties of this ferrosmectic phase.
THE FERROSMECTIC
A ferrosmectic phase is a swollen lamellar phase in the layers of which small magnetic particles have been incorporated. The existence of such stable phases has recently been established and they have been characterized by their optical and magnetic properties[1][2]. To realize these new systems, we proceed as follows : starting with the chemical composition of a quaternary swollen lamellar phase -cyclohexane, water, sodium dodecylsulfate, pentanol- we simply replace the "oil" (cyclohexane) by a ferrofluidic dispersion, stabilized in cyclohexane[31 ; in the new phase, the particles are thus embedded in the oil layers of the lamellar phase ; the large flexibility of the system allows to fit the thickness of the oil layers to the size of the colloidal magnetic particles. The mean diameter of the particles being of the order of 10 nm, the oil layers thickness eo can be varied from 0 to 50 nm, keeping the water layer ("membrane") thickness ew constant and equal to 4 nm. The range of stability of the ferrosmectic phase has been studied versus two parameters : e, the periodicity of the smectic (e = eo + ew) and 0 the volumic fraction in particles; they are stable from 0) = 0,3% to 0 = 3% and for e varying from 16 nm to 38 nm. In the case of the thinnest layers, two particles can not be superposed in the same lamella, thus leading to a layered structure of a bidimensional ferrofluid.
Mat. Res. Soc. Symp. Proc. Vol. 248. 01992 Materials Research Society
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H (a)
fir.
(b)
: Schematic representation of the ferrosmectic phase. A magnetic field is applied either (a) parallel to the layers ; no effect is observed, (b) perpendicular to the layers ; existence of a magnetic instability.
MAGNETIC FIELD EFFECTS
These phases exhibit a striking and unusual magnetic behavior : unlike the comparable thermotropic or lyotropic smectics which poorly respond[4] to magnetic forces, the ferrosmectics undergo a magnetic instability at very low field values[l] (2.10-3 T). When submitted to a magnetic field perpendicular to the oriented layers and larger than a threshold value Hc, the ferrosmectic reorients as a whole in order that the plane of the layers be parallel to the field. This behavior displays the magnetic anisotropy of the system, which is the driving force of the instability, while the restoring force is of the elastic type and involves the elastic constants of the phase.
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Fir
: Threshold field HC
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