Magnetic field induced instabilities of a doped lyotropic hexagonal phase

A new anisotropic magnetic fluid is obtained using a colloidal suspension o f magnetic particles of nanometric size (ferrofluid) as a component of a sw ollen lyotropic hexagonal phase. This doped hexagonal system exhibits speci fic behaviors when submitted to a magnetic field of weak intensity. The fie ld-induced instabilities are described and interpreted; they result from a high anisotropy of the magnetic susceptibility of the medium, which is meas ured. It is finally shown that the magnetic properties of the doped hexagon al phase allows one a determination of the compression modulus of the syste m.