  In the present study we probe the bulk modulus and the structure of
concentrated magnetic fluids by Small Angle X-ray Scattering. The
electrostatically stabilized nanoparticles experience a repulsive interparticle
potential modulated by dipolar magnetic interactions. On the interparticle
distance lengthscale, we show that nanoparticles are trapped under-field in
oblate cages formed by their first neighbours. We propose a theoretical model
of magnetostriction for the field-induced deformation of the cage. This model
captures the anisotropic features of the experimentally observed scattering
pattern on the local scale in these strongly interacting colloidal dispersions.

Soft Condensed Matter;Materials Science