High-performance hexaferrite magnets tailored through alignment of shape-controlled nanocomposites

Nanoparticles of strontium hexaferrite, SrFe₁₂O₁₉, were prepared by two different synthesis methods: hydrothermal (autoclave) and sol-gel autocombustion (solid-salt-matrix). The two synthesis pathways yield nanoparticles with different morphologies and correspondingly different magnetic characteristics. The autoclave synthesis results in large plate-like crystallites, which spontaneously align with a preferred crystallographic orientation when applying a uniaxial pressure, but exhibit a relatively poor coercivity. Meanwhile, the solid-salt-matrix synthesis method results in smaller less anisotropic crystallites with enhanced coercivity, but with a relatively limited ability to align under a uniaxial applied pressure. The obtained nanocrystalline powders were dry or wet mixed in different ratios followed by Spark Plasma Sintering (SPS) into dense pellets. A clear correlation between mixing ratio, the level of alignment and resulting coercivity was observed for the dry mixed samples, i.e. as more solid-salt-matrix powder is added, the texture of the pellets decreases and the coercivity increases. The best performing pellet in terms of maximum energy product (BH_max = 32.1(6) kJ m^-3) was obtained by dry-mixing of 75 wt% autoclave prepared powder and 25 wt% solid-salt-matrix powder. The results presented here illustrate the potential of mixing magnetic nanoparticle powders with different shape characteristics to gain improved magnetic performance.