Controlled spin-polarized electron transport of oxide magnetic semiconductor films of hematite-ilumenite solid solutions by the exchange coupling

磁性半导体材料是下一代电子学材料，即自旋电子学材料。本研究提出了一种控制磁性半导体自旋极化输运的新技术，并试图实现这一技术。第一个课题是开发新型氧化物磁性半导体薄膜，重点是在室温下具有高自旋极化的赤铁矿-伊鲁明铁矿固溶体系统。由于有序-无序转变、Fe^2+/Fe^3+混合价态等因素的影响，制备具有良好磁性能的赤铁矿-钛铁矿固溶体薄膜存在诸多困难。该薄膜在室温下表现出磁电阻等磁性。第二个课题是利用反铁磁层的自旋反转来控制铁磁层的磁结构。只有外延赤铁矿薄膜表现出独特的自旋翻转转变。我们采用螺旋溅射法制备了赤铁矿/尖晶石铁氧体双层薄膜，并测量了自旋翻转转变前后的磁性。由于尖晶石铁氧体层具有较大的面内磁各向异性，赤铁矿层中的自旋翻转转变消失。我们首次发现了氧化物磁性层之间的90°交换耦合。不幸的是，我们无法实现所提出的自旋控制技术，但我们可以获得巨大的结果，有关制备技术的定义明确的氧化铁薄膜及其结构，磁性和电子特性。

Magnetic semiconductors are one of die promising electronics materials for the next generation, so-called spintronics materials. In this research we proposed a novel technique to control the spin-polarized transport of magnetic semiconductors and tried to realize it. The first subject was to develop thin films of new oxide magnetic semiconductors focused on a hematite-ilumenite solid solution system having a high spin polarization at room temperature. There had been many difficulties to prepare well-defined hematite ilumenite solid solution films having good magnetic semiconductive properties, because they had many controllable factors such as an order-disorder transition and Fe^<2+>/Fe^<3+> mixed valence states, etc. We found optimized preparation conditions for the solid solution films by helicon sputtering to control remanent oxygen pressure and substrate temperature precisely. The films showed magnetic semiconductive properties such as magnetoresistance at room temperature. The second subject was to control magnetic structures of ferromagnetic layers by means of a spin-flip transition of antiferromagnetic layers. Only epitaxial hematite films showed a unique spin-flip transition. We prepared hematite/spinel ferrite bilayered films by helicon sputtering and surveyed magnetic properties around the spin-flip transition. The spin-flip transition in a hematite layer disappeared because of a large in-plane magnetic anisotropy of a spinel ferrite layer. We first found the 90° exchange coupling between oxide magnetic layers. Unfortunately we could not realize the proposing spin control technique, but we could obtain huge results concerning preparation techniques for well-defined iron oxide films and their structural, magnetic and electronic properties.