LaFeO₃纳米磁性材料电四极超精细相互作用PAC研究

【作者】 杜恩鹏；

【导师】 朱升云；

【作者基本信息】 中国原子能科学研究院 ， 粒子物理与原子核物理， 2004， 硕士

【摘要】 钙钛矿结构氧化物LaFeO₃纳米磁性材料具有特异的电、磁以及结构特性，是一种有重要意义的新型磁性材料。理想的LaFeO₃具有立方结构，位于立方体体心的La与十二个氧配位，位于立方体顶点的Fe与六个氧配位。由于较大的La原子和较小的Fe原子的大小匹配失衡，立方结构发生形变，LaFeO₃呈现正交晶或菱方晶结构。晶格结构微小变化都会导致作用在La处电场梯度的变化，因此通过La受到的电四极相互作用测量，就能得到晶体结构微小变化的信息。纳米状态时的结构变化及其与纳米尺度的关系是当前一个非常感兴趣的热点问题。本论文工作采用扰动角关联方法研究了LaFeO₃纳米磁性材料电四极相互作用及其微观晶格结构，比较了纳米和晶体、不同尺度纳米材料微观结构的差异。主要研究结果如下： 1，采用溶胶—凝胶法制备了实验所需的各种LaFeO₃磁性材料样品。 2，在室温下对20nm、40nm LaFeO₃纳米磁性材料和晶体LaFeO₃磁性材料进行了扰动角关联测量。实验只观察到一个La晶位的四极相互作用，在这三种材料中测量的四极相互作用频率ω₀分别为：687.4 Mrad／s、698.3 Mrad／s和742.9 Mrad／s，频率分布宽度系数σ分别为：0.014、0.009和0.001，电场梯度不对称系数η=0。实验结果表明：电场梯度主轴与晶轴方向一致；所有三种样品都具有斜交菱形结构，晶体到纳米和纳米尺度变小发生斜交到正交的结构演化的趋向；四极相互作用频率分布宽度系数σ随纳米颗粒增大而变小，晶体时最小。 3，移植和采用了Uppsala大学DEPAC数据处理软件，使数据处理更可靠精确。更多还原

【Abstract】 Perovskite LaFeO3 nano-magnetic material that shows a variety of unusual and interesting electronic, magnetic and structural properties is a new important magnetic material. Ideal LaFeO3 possesses a cubic structure, and La and Fe occupy a body center site with twelve-fold oxygen coordination and an octahedral site with six-fold oxygen coordination, respectively. This ideal cubic structure is distorted by size mismatch of large La and small Fe atoms. The distorted structure is mostly orthorhombic or rhombohedral. Any tiny alteration of crystal structure causes a change of the electric field gradient (EFG) exerted on La. Therefore, the measurement of the quadropole interaction of La with EFG can yield information on the change of crystal structure. The crystal structure of nano-materials and its variation with grain size is a currently interesting topic. In the present work the perturbed angular correlation is used to investigate the quadrupole interaction and micro-structure for the perovskite LaFeO3 nano-magnetic materials, and the comparison is made of the micro-structure between the LaFeO3 nano- and crystalline magnetic materials and between the LaFeO3 nano- magnetic materials with different grain size. The main research results are as follows:1, a sol-gel method is employed to prepare all the LaFeO3 samples used in the experiment.2, the perturbed angular correlation measurements are performed at room temperature for the 20 nm and 40 nm nano-LaFeO3 and crystalline LaFeO3. Only one electric quadrupole interaction is detected for each material, which is assigned to the La site. The quadrupole interaction frequencies are 687.4Mrad7s, 698.3Mrad/s and 742.9Mrad/s with a distribution coefficient of 0.014, 0.009 and 0.001, respectively. The fitting yields an EFG asymmetry parameter = 0. The experimental results show that a) the principal axis of the EFG is aligned with the crystallographic axis, b) the structure of the nano- and crystalline LaFeO3 is rhombohedral, and there is a tendency that the structures from the crystalline LaFeO3 to the nano- LaFeO3 and with the decreasing of the nano-grain size evolve towards the orthorhombic structure, and c) the frequency distribution coefficient increases with the decreasing of the nano-grain size and the crystalline LaFeO3 arrives at its minimum.3, a DEPAC dada processing program is imported from Uppsala University, and installed and used in the experimental data analyses, which makes the obtained data more reliable and accurate.更多还原