Small-angle polarized neutron scattering study of strontium hexaferrite nanoplatelet ferrofluid

Abstract

The influence of a magnetic field on the structural ordering in concentrated (∼30 g l⁻¹) aqueous colloids of strontium hexaferrite (SrFe₁₂O₁₉) nanoplates with an average diameter of 500 Å and a thickness of 50 Å was studied with polarized small-angle neutron scattering (SANS). Two-dimensional SANS intensity maps illustrate isotropic diffuse scattering in the absence of a magnetic field and a series of diffraction reflections arising along the field at Q ≈ 0.0185 Å⁻¹ and its multiples. The appearance of Bragg maxima with the magnetic field is attributed to the formation of columns (chains) of flat strontium hexaferrite nanoplatelets distanced at ∼340 Å. According to the dependence of the Bragg peak intensity on the magnetic field, three structural states of the ferrofluid were identified during the magnetization process. At H ≈ 0, individual platelets or segments of uncorrelated platelet chains are randomly oriented in the isotropic phase. At H > 20 Oe the platelets orient preferentially along the magnetic field and assemble into stacked plate columns. In the intermediate state, 20 < H < 250 Oe, these stacked columns exhibit 1D-nematic order. At H > 250 Oe the columns arrange themselves, presumably, into a hexagonal smectic-like lateral ordering with a period exceeding 750 Å. The amplitude of nuclear magnetic interference scattering also reveals a stepwise increase with amplification of the magnetic field at H ≈ 20 Oe and H ≈ 250 Oe, showing the correlation of the magnetic and the structural ordering and collinear magnetization in the neighboring columns within the magnetized ferrofluid.