Influence of microstructure and preparation methods on the magneto-crystalline structure and magnetic properties of subm
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D. Lisjak and D. Makovec “Jozef Stefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia
R.E. Vandenberghe Department of Subatomic and Radiation Physics, University of Gent, 9000 Gent, Belgium
A. Gilewski International Laboratory of High Magnetic Fields and Low Temperatures, 53-529 Wroclaw, Poland (Received 23 November 2005; accepted 12 July 2006)
We report studies on the correlation between the microstructure, the magneto-crystalline structure, and the magnetic properties of barium hexaferrite powders. The samples consisted of typical hexagonal plate-like particles with approximate sizes of 80, 180, and 500 nm, obtained by microemulsion, coprecipitation, and solid-state reaction techniques, respectively, and were characterized by x-ray powder diffraction and scanning and transmission electron microscopy. The hyperfine parameters of the hexaferrite powders with different particle size were investigated by Mössbauer spectroscopy. We also measured the magnetization-versus-magnetic field dependence of the submicron powders at high magnetic fields up to 30 T at 4.2 K. Finally, we comment on the surface effects observed because of particle size reduction from micron to nanoscale dimensions.
I. INTRODUCTION
The M-type barium hexaferrite, BaHF (BaFe12O19), is the hexaferrite family’s best known compound. Its crystal structure is the so-called magnetoplumbite structure that can be described as a stacking sequence of the basic blocks S and R.1,2 The larger O and Ba ions form a close-packed framework, and the interstices are occupied by smaller Fe cations. The O-Ba framework can be described as a stacking sequence of O layers and layers in which one-quarter of the O anions are replaced by Ba cations. The S block contains only O layers and the R block contains one layer with Ba. The magnetic ion (Fe3+) occupies five different interstitial positions in a ferrimagnetic order resulting in a net magnetic moment. Two of the possible 16 tetrahedral positions (4f1) and four of the possible octahedral positions (2a) are occupied by Fe3+ in the S block. Fe3+ in the R block occupies octahedral sites in the octahedra shared by common faces (12k), in octahedra at the interface of adjacent blocks (4f2), and trigonal bipyramidal sites (2b). The presence of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0314 2606 J. Mater. Res., Vol. 21, No. 10, Oct 2006 http://journals.cambridge.org Downloaded: 16 Nov 2015
magnetic Fe3+ cations in these positions is responsible for the magnetic properties of BaHF and for its magnetocrystalline anisotropy (K1 ⳱ 3.3 × 105 Jm−3).3 The recent developments in nanotechnology, allowing one to obtain still finer particles with quasi-clusteral structure where surface interactions play a leading role, have prompted the recent scientific interest in highly anisotropic magnetic materials.4 In the work reported here, our attention was focused on investigating BaFe12O19 powders with approximate sizes ranging from 80 to 500 nm synthesized by three different technological
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