In the present study we probe the bulk modulus and the structure ofconcentrated magnetic fluids by Small Angle X-ray Scattering. Theelectrostatically stabilized nanoparticles experience a repulsive interparticlepotential modulated by dipolar magnetic interactions. On the interparticledistance lengthscale, we show that nanoparticles are trapped under-field inoblate cages formed by their first neighbours. We propose a theoretical modelof magnetostriction for the field-induced deformation of the cage. This modelcaptures the anisotropic features of the experimentally observed scatteringpattern on the local scale in these strongly interacting colloidal dispersions.
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