We have determined the phase diagram of magnetic colloidal ellipsoids as a function of both packing fraction φ and external magnetic field B. We use core-shell particles with a magnetic core where the magnetic moment of the core is sufficiently small to avoid additional dipole-dipole interactions, but high enough to induce preferential particle alignment with an external magnetic field. By using a combination of small-angle x-ray scattering (SAXS) and magnetometry we have examined positional correlations of the charged ellipsoids (aspect ratio p = 2.3) and orientational order of their magnetic moments. By combining SAXS and computer simulations we have established structural criteria for the different phase and arrest transitions and map distinct isotropic, polarised fluid and nematic phases over an extended range of φ − B coordinates. We demonstrate that upon the isotropic-nematic transition of the ellipsoids, the bulk magnetic behavior of the suspensions switches from superparamagnetic to ferromagnetic. We extend the often-used atom-colloid analogy to spin systems and present a relationship between the structural topology of suspensions of magnetic colloids and their macroscopic magnetic response.
peter.schurtenberger@fkem1.lu.se
