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

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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 THyd. A new method is demonstrated for making highly aligned SrFe12O19 permanent magnets based on the conversion of antiferromagnetic six-line ferrihydrite platelets. This journal is

Original languageEnglish
JournalMaterials Chemistry Frontiers
Pages (from-to)3699-3709
Number of pages11
Publication statusPublished - May 2021

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