• Pillai, Harikrishnan Vijayan
• Mørch, Mathias I.
• Christensen, Mogens
• Povlsen, Amalie
• Thomas-Hunt, Jack

Abstract

Sintered cold compacted hexaferrite magnets with appreciable magnetic properties and crystallite align- ment were made from non-magnetic precursors without applying an external magnetic field. This work presents a novel approach employing non-ferromagnetic interacting precursors comprising of platelet shaped six-line ferrihydrite and needle shaped goethite nanoparticles. A hydrothermal synthesis route was employed to produce platelet shaped six-line ferrihydrite of ~5 nm thickness. Needle shaped goethite na- noparticles were likewise prepared by hydrothermal synthesis with apparent dimensions of ~10 × 27 × 10 nm3 extracted from X-ray powder diffraction data. The powder diffraction Rietveld modelling also revealed the presence of an amorphous phase in the six-line ferrihydrite and a SrCO3 impurity. The presence of needle shaped goethite nanoparticles improves the alignment of magnets, while retaining the coercivity (Hc), in contrast to hexaferrite magnets prepared from six-line ferrihydrite by spark plasma sintering (SPS). The non-ferromagnetically interacting precursors were directly converted to the SrFe12O19 magnets by pressing them with conventional compaction technique followed by subsequent sintering of the pellets. Decoupling the pressing and sintering step is interesting for industrial production of magnets. The hexaferrite magnets prepared displayed good combination of saturation magnetization Ms = 70 Am2/kg and coercivity Hc = 297 kA/m with some degree of alignment of the crystallites Mr/Ms = 0.71. This procedure exploits the anisotropic shape of the crystallites and compaction using uniaxial pressure followed by sin- tering into aligned bulk magnets. Two sets of hexaferrite bulk magnets were prepared by sintering at 900 °C and held for 2 h and 1050 °C with a holding time 0 min. The hexaferrite magnets sintered at 1050 °C were subjected to transmission pole figure analysis. The texture index for each pellet were extracted from the pole figure analysis. Employing needle shaped goethite nanoparticles actually enhanced the alignment of the hexaferrite magnets. The magnet obtained from only six-line ferrihydrite displayed only a slightly improved texture index when compared with mixture of six-line ferrihydrite and goethite nanoparticles.

Topics

• nanoparticle
• impedance spectroscopy
• amorphous
• phase
• anisotropic
• mass spectrometry
• texture
• magnetization
• sintering
• saturation magnetization
• coercivity
• aligned