Ultrathin nanoplatelets of six-line ferrihydrite enhances the magnetic properties of hexaferrite

Abstract

High-performance bulk hexaferrite permanent magnets have been fabricated through dry-processing of ferrites without applying a magnetic field in the alignment process. This novel method takes advantage of the anisotropically platelet shaped antiferromagnetic six-line ferrihydrite in the presence of Sr²⁺ ions. The precursor has been prepared using a simple hydrothermal synthesis and the nanocrystallites have been subjected to subsequent compaction by Spark Plasma Sintering (SPS). The synthesis produced ∼7 nm thick six-line ferrihydrite platelets. A low SPS temperature of ∼750 °C was sufficient to obtain highly aligned strontium hexaferrites (SrFe₁₂O₁₉) with the magnetic easy axis parallel to the pressing direction. The best performing magnet produced a square hysteresis loop, with a high (BH)_max of 33 kJ m⁻³. In short, the alignment of the crystallites was controlled merely by the anisotropic crystallite shape obtained from the hydrothermal synthesis, rather than an applied external magnetic field. Three different hydrothermal synthesis temperatures were employed 100, 150, and 200 °C to synthesize thin six-line ferrihydrite nanoplatelets. Powder diffraction Rietveld modelling revealed the precursor to contain goethite (α-FeOOH) in addition to the six-line ferrihydrite, at higher hydrothermal processing temperatures. Rietveld refinements, texture investigations, and measured magnetic properties of the SPS pellets revealed an increase in alignment along the c-axis with an increase in hydrothermal processing temperature T_Hyd. A new method is demonstrated for making highly aligned SrFe₁₂O₁₉ permanent magnets based on the conversion of antiferromagnetic six-line ferrihydrite platelets.