Optical- and x-ray-scattering studies of ionic ferrofluids of ${\mathrm{MnFe}}_2$${\mathrm{O}}_4$, γ-${\mathrm{Fe}}_2$${\mathrm{O}}_3$, and ${\mathrm{CoFe}}_2$${\mathrm{O}}_4$

Ionic ferrofluids of ${\mathrm{MnFe}}_2$ ${\mathrm{O}}_4$, γ-${\mathrm{Fe}}_2$ ${\mathrm{O}}_3$, and ${\mathrm{CoFe}}_2$ ${\mathrm{O}}_4$ has been investigated by means of optical- and x-ray-scattering techniques. The optical birefringence measured as a function of temperature, without any magnetic field, showed the existence of a nematic-type order in these materials. Scattering results indicate the existence of bundlelike aggregates in Mn samples. The origin of the birefringence is discussed in terms of the shape anisotropy of the magnetic particles.