Nowadays single domain hexaferrite nanoparticles are in the focus of intense scientific activity. They can be used in highly effective permanent magnets, magnetic fluids and in several biomedical applications. In this study we report on the synthesis of strontium hexaferrite SrFe12O19 nanoparticles with average particle sizes <30nm, their stable dispersions in water and a composite material on their base that exhibits high magnetic anisotropy. The colloids were obtained by dissolving in acetic acid the ceramics synthesized by glass crystallization method. Hysteresis parameters, losses and dependence of the initial susceptibility versus temperature were explored. Particles size investigations were done by SEM, TEM and AFM. The average particle size and crystal shape were also determined by Rietveld analysis. The diffusion parameters were studied by DLS. The particles exhibit superparamagnetic behavior that allows the colloids to avoid agglomeration and sedimentation. After sustained interaction with a magnetic field the solution splits. One of the liquids, enriched by hexaferrite, behaves like a magnetic fluid: retains its volume and can be drawn by a magnet. If the system is stirred it becomes homogeneous again. The particles have the shape of thin platelets with only two unit cells along the c axis. The composite was obtained by high pressure filtration of the colloids through anodized alumina films and further thermal treatment. The particles embedded in the pores are orientated along the c axis. After sintering they form nanowires of strontium hexaferrite inside the pores. The magnetic properties of the material differ greatly along and normal to the pores. The coercitive power along the pores reaches 6500 Oe and doesn't exceed 4500 Oe in the other direction. The saturation magnetisation is equal in both cases (approx. 1 emu/g) while the remanent magnetisation is two times higher along the pores (approx. 0.7 emu/g).

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