Charge and Spin Ordering in Layered Perovskite-type Oxide Studied by Synchrotron X-ray Diffraction

近年来，人们认识到电荷、自旋和轨道自由度对过渡金属氧化物的电、磁性质起着重要的作用。特别是在钙钛矿型锰铁中，各种各样的磁场诱导现象的发现，如巨大的磁电阻和磁结构转变，刺激了该领域的许多活动。利用中子衍射和电子衍射等技术可以直接观察到自旋和电荷的顺序。然而，观察轨道有序的实验技术是有限的。在这里，我们开发了一种共振x射线散射技术来直接检测轨道有序。利用共振x射线散射，我们成功地观察了钙钛矿氧化物和f电子系统中多种类型的电荷、自旋和轨道有序态。最近取得的重大成就是在人工晶格中观察到电荷和轨道的短程有序以及轨道-超晶格。通过对近程序的观察，阐明了电荷和轨道有序的机理。同时，我们期望通过轨道超晶格的观测，找到一种用人工晶格制造电子器件的方法。

Recently, it has been recognized that the charge, spin, and orbital degrees of freedom play important roles in the electric and magnetic properties of the transition-metal oxides. In perovskite-type manganited, especially, the discover of a wide variety of magnetic-field-induced phenomena, such as a colossal magnetoresistance and a magnetostructural transition, has stimulated much activity this field. The spin and charge ordering can be directly observed by neutron and electron diffraction, and some other techniques. The experimental technique to observe the orbital ordering, however, has been limited. Here, we have developed a resonant x-ray scattering technique to detect the orbital ordering directly.We have succeeded to observe many types of charge, spin, and orbital ordering states in perovskite oxides as well as a f-electron systems by using the resonant x-ray scattering. Recent great achievements are an observation of short-range ordering of charge and orbital and a observation of orbital-superlattice in an artificial lattice. The mechanism of the charge and orbital ordering has been elucidated by the observation of the short-range order. Meanwhile, it is expected that we can find a way of making electric devices with an artificial lattice by the observation of the orbital-superlattice.