R型铁氧体BaFe4-xTi2+xO11的化学组态以及磁性行为的研究

本文通过传统的固相反应法制备了R型六角铁氧体BaFe_4-xTi_2+xO₁₁ (x= 0, 0.25, 0.5, 0.75, 1), 并且对它的原子价态以及磁性行为进行了研究. X 射线光电子能谱(XPS)结果显示了随着掺杂含量的增加, 体系中Fe³⁺离子逐渐减少而Fe²⁺离子逐渐增加. 由于具有非对称结构的阻挫晶格中存在各种关联作用的竞争, 使得BaFe_4-xTi_2+xO₁₁体系表现出了复杂的磁有序行为, 在T₁～250 K和T₂～83 K两处存在磁转变. 对这一系列掺杂样品, 在相变温度T₁之上表现顺磁行为, 而在相变温度T₂前后的磁化强度都表现出低场下随磁场的增加快速增加, 高场下则线性变化且在5×10⁴ Oe时还未达到饱和的行为, 表明这一系列掺杂样品是典型的倾斜反铁磁态(canted antiferromagnetic) 或者亚铁磁态.

Magnetic property and electronic structure of BaFe4-xTi2+xO11

In this paper, polycrystalline BaFe_4-xTi_2+xO₁₁ (x=0, 0.25, 0.5, 0.75, 1) samples have been synthesized by the conventional solid-state reaction method. X-ray diffraction (XRD) patterns of all the samples show that the diffraction peaks correspond to that of an R-type hexagonal ferrite structure, and no trace of second phase is detected. Measurement of X-ray photoelectron spectroscopy (XPS) reveals that most of the Fe ions in BaFe₄Ti₂O₁₁ are trivalent and the fitting of two peaks in Fe 2p spectrum corresponding to different Fe ion sites, while the amount of Fe²⁺ ions increases with the increase of Ti ions in BaFe_4-xTi_2+xO₁₁. The spectroscopy of Ti ions confirms that the valence of Ti in BaFe_4-xTi_2+xO₁₁ are tetravalent. Magnetic susceptibility of BaFe_4-xTi_2+xO₁₁ (x= 0, 0.25, 0.5, 0.75, 1) reveals two magnetic transitions at T₁～250 K and T₂～83 K, which indicate a complex magnetic order driven by competing interactions on a frustrated lattice with a noncentrosymmetric structure. For all the samples, the magnetic susceptibility obeys Curie-Weiss law above T₁, and M-H curves exhibit a linear variation with magnetic field in this temperature range, which is consistent with the paramagnetic behavior. A decrease of the effective magnetic moment is due to the increase of Fe²⁺ ions with the increase of Ti content in BaFe_4-xTi_2+xO₁₁. Below T₁, the magnetization curve as a function of temperature (M-T) and the magnetization versus magnetic field (M-H) at different temperatures imply its characteristic of a typical canted antiferromagnetic or ferrimagnetic state. Meanwhile, the transition temperature T₂ drops gradually with the increase in Ti content, which might be related to the change of occupying of Fe ions in the kagome layers.