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Aligned fibers of barium and strontium M hexaferrite (BaFe12O19 and SrFe12O19) were manufactured from an aqueous inorganic sol-gel-based spinning process, but halides have been found to be retained up to 1000 °C, inhibiting the formation of the hexagonal ferrite phases. Therefore an investigation was carried out into the removal of the halides at lower temperatures through steaming between 500 and 900 °C/3 h, and the subsequent effects upon microstructure and magnetic properties. The fibers were prefired to 400 °C to remove all organic components, and in all cases the steaming process resulted in loss of alignment of the fibers. It was found that the M phase began to form at only 600 °C, becoming single-phase SrM or virtually pure phase BaM at 700 °C, confirming that halides had indeed delayed M phase formation. Both materials had a grain size below 100 nm, but other unusual surface features not seen before on ferrite fibers were observed. The fibers steamed at 700 °C had Ms and Hc values comparable to random M ferrite fibers fired to 1000 °C in air, while steaming over a temperature range from 400 to 800 °C/3 h gave products with improved magnetic properties, with SrM fibers having an Ms of 81.4 emu g−1 and Hc of 457 kA m−1.

I. INTRODUCTION

The M ferrites, BaFe12O19 and SrFe12O19, are the most commercially important hard magnetic ceramics globally; they are used in a huge range of applications from loudspeakers and electrical motors to high-technology niche applications. They arc magnetically hard, with high coercivities and magnetic permeabilities, and contain a high magnetocrystalline anisotropy along the c axis of the hexagonal structure.1 BaM has a high maximum saturation magnetization (Ms) of 72 emu g−1, and the uniaxial character gives BaM a large theoretical maximum coercivity (Hc) of 593 kA m−1, although the reported Hc values for polycrystalline BaM are much lower.2 The magnetic properties of SrM are similar to those of BaM, with the Ms of single-crystal SrM being variously reported at values between 92.6 emu g−1 3 and 74.3 emu g−1 4

a)

Present address: Centre for Physical Electronics and Materials, School of Electrical, Electronic and Information Engineering, South Bank University, 103 Borough Road, London SE1 0AA, United Kingdom e-mail: [email protected] b) Address all correspondence to this author. e-mail: [email protected] 3162

http://journals.cambridge.org

J. Mater. Res., Vol. 16, No. 11, Nov 2001 Downloaded: 16 Mar 2015

and the maximum coercivity at around 533 kA m−1,5 but again polycrystalline samples rarely approach these high values. It has been shown that a ferrite in a fibrous form should exhibit an enhancement of magnetic permeability along the fiber axis,6 even in a magnetically isotropic material, which will increase as the aspect ratio of length to diameter increases.7 Aligned fiber blankets of such materials should show this effect on mass when the applied magnetic field is parallel to the axis of alignment and could be incorporated into composite matrices. Continuous BaM8 and Sr