Sepelák V, Myndyk M, Witte R, Röder J, Menzel D, Schuster R H, Hahn H, Heitjans P, Becker K-D
Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany.
Faraday Discuss. 2014;170:121-35. doi: 10.1039/c3fd00137g.
The response of the structure of the M-type barium hexaferrite (BaFe12O19) to mechanical action through high-energy milling and its impact on the magnetic behaviour of the ferrite are investigated. Due to the ability of the (57)Fe Mössbauer spectroscopic technique to probe the environment of the Fe nuclei, a valuable insight on a local atomic scale into the mechanically induced changes in the hexagonal structure of the material is obtained. It is revealed that the milling of BaFe12O19 results in the deformation of its constituent polyhedra (FeO6 octahedra, FeO4 tetrahedra and FeO5 triangular bi-pyramids) as well as in the mechanically triggered transition of the Fe(3+) cations from the regular 12k octahedral sites into the interstitial positions provided by the magnetoplumbite structure. The response of the hexaferrite to the mechanical treatment is found to be accompanied by the formation of a non-uniform nanostructure consisting of an ordered crystallite surrounded/separated by a structurally disordered surface shell/interface region. The distorted polyhedra and the non-equilibrium cation distribution are found to be confined to the amorphous near-surface layers of the ferrite nanoparticles with the thickness extending up to about 2 nm. The information on the mechanically induced short-range structural disorder in BaFe12O19 is complemented by an investigation of its magnetic behaviour on a macroscopic scale. It is demonstrated that the milled ferrite nanoparticles exhibit a pure superparamagnetism at room temperature. As a consequence of the far-from-equilibrium structural disorder in the surface shell of the nanoparticles, the mechanically treated BaFe12O19 exhibits a reduced magnetization and an enhanced coercivity.
研究了M型钡铁氧体（BaFe12O19）结构对高能球磨机械作用的响应及其对铁氧体磁性能的影响。由于（57）Fe穆斯堡尔光谱技术能够探测铁原子核的环境，从而在局部原子尺度上对材料六方结构中机械诱导的变化有了宝贵的认识。结果表明，BaFe12O19的球磨导致其组成多面体（FeO6八面体、FeO4四面体和FeO5三角双锥）变形，以及Fe(3+)阳离子从规则的12k八面体位置机械触发转变为磁铅石结构提供的间隙位置。发现六方铁氧体对机械处理的响应伴随着形成由有序微晶组成的非均匀纳米结构，该微晶被结构无序的表面壳层/界面区域包围/分隔。发现扭曲的多面体和非平衡阳离子分布局限于铁氧体纳米颗粒的非晶近表面层，其厚度延伸至约2nm。通过对BaFe12O19宏观磁性能的研究，补充了关于机械诱导的短程结构无序的信息。结果表明，球磨后的铁氧体纳米颗粒在室温下表现出纯超顺磁性。由于纳米颗粒表面壳层中远离平衡的结构无序，经机械处理的BaFe12O19表现出降低的磁化强度和增强的矫顽力。
