超细ZnFe2O4粒子的磁不可逆性
Magnetic irreversibility in ultrafine ZnFe2O4 partices
摘要：
采用机械合成法制备了ZnFe_2O_4超细粉体。由X-射线衍射估计平均粒径为<d>= 36（6）nm。中子衍射（ND）数据的细化表明，所得的立方尖晶石结构是缺氧的，约7%的Fe 3+离子占据四面体A位。在4.2 K下的磁化曲线表明，磁场H = 9特斯拉时没有饱和，这与研磨过程产生的自旋倾斜有关。场冷（FC）和零场冷（ZFC）曲线显示出远高于室温的不可逆行为，这与自旋无序有关。在300 °C下退火样品产生平均晶粒尺寸<d>= 50（6）nm，并且在A位点处产生约16%的Fe 3+离子。部分氧回收率也推导出退火样品中的中子数据细化。同时，注意到磁不可逆性的降低，归因于共线自旋结构的部分恢复。在室温和80 K下观测到了复杂的穆斯堡尔谱，其超精细场分布范围从~10 T到~40 T。在T = 4.2 K时，超精细场分布表明Fe局域环境中存在高度无序。上述数据表明富铁团簇的存在，产生强烈的超交换相互作用的铁离子之间的尖晶石结构的A和B网站。
英文摘要：
Pure ultrafine ZnFe2O4 particles have been obtained from mechanosynthesis of the ZnO and Fe2O3 oxides. The average grain diameter was estimated from x-ray diffraction to be <d> = 36(6) nm. Refinement of neutron diffraction (ND) data showed that the resulting cubic spinel structure is oxygen-deficient, with ~7% of Fe3+ ions occupying the tetrahedral A sites. Magnetization curves taken at 4.2 K showed absence of saturation up to fields H = 9 Tesla, associated to a spin-canted produced by the milling process. Field-cooled (FC) and zero-field-cooled (ZFC) curves showed irreversible behavior extending well above room temperature, which is associated to spin disorder. Annealing samples at 300 °C yields an average grain size <d> = 50(6) nm, and ~16% of Fe3+ ions at A sites. Partial oxygen recovery is also deduced from neutron data refinement in annealed samples. Concurrently, decrease of magnetic irreversibility is noticed, assigned to partial recovery of the collinear spin structure. Complex Mössbauer spectra were observed at room temperature and 80 K, with broad hyperfine field distributions spanning from ~10 T to ~40 T. At T = 4.2 K, hyperfine field distributions indicate high disorder in Fe local environments. The above data suggest the existence of Fe-rich clusters, yielding strong superexchange interactions between Fe ions at A and B sites of the spinel structure.
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