Synchrotron Small-Angle Scattering Study of Concentrated Ferrofluids Based on Strontium Hexaferrite Nanoparticles in a Magnetic Field

The structural composition of a concentrated aqueous solution of 50 × 5-nm plate-shaped ferrimagnetic nanoparticles of strontium hexaferrite (SrFe 12 O 19 ) in a magnetic field has been studied using synchrotron small-angle scattering. A characteristic feature of these magnetic fluids is the formation of randomly oriented periodic structures even in the absence of a magnetic field. Two-dimensional maps of the synchrotron small-angle scattering intensity show isotropic scattering in zero field and growth of a series of diffraction reflections along the direction of the applied magnetic field at Q = nq b , where q b ≈ 0.226 nm –1 (period ~27.8 nm), which is due to the formation and orientation of nanoplate chains along the field. The analysis of the broadening of diffraction reflections has shown that the coherence of the chains even in extremely high magnetic fields, is 8–10 periods. However, this does not limit the length of the chain, but is due to disorientation and the dispersion of particle sizes. The degree of orientation of the segments is determined by the azimuthal width and intensity of the Bragg peaks. The chains of nanoplates are rotated toward the field direction with the field increase in the range H < 70 Oe (isotropic fluid–one-dimensional nematic transition); they form an intermediate state in the range 70 Oe < H < 300 Oe and a smectic (three-dimensional) phase at H > 300 Oe. The intermediate state is characterized by a logarithmic dependence of the intensity and width of the Bragg peak on the magnetic field, which presumably indicates a self-organized criticality state of the magnetic colloid.