Development of Magnetic Semiconductor Based on Spinel- Title type Oxide for Spintronics Application( Dissertation_全文 ) Author(s) Murase, Hideaki Citation 京都大学 Issue Date 2010-03-23 URL https://doi.org/10.14989/doctor.k15388 Right 許諾条件により要旨・本文は2011-03-31に公開 Type Thesis or Dissertation Textversion author Kyoto University Development of Magnetic Semiconductor Based on Spinel-type Oxide for Spintronics Application Hideaki Murase 2010 Contents General Introduction......................................................................................1 Chapter 1: Spintronics: spintronic applications and magnetic semiconductors ..............................................................................................8 Chapter 2: Magnetite-ulvöspinel solid solution ..........................................27 Chapter 3: Synthesis of magnetite-ulvöspinel solid solution thin films .....45 3.1 Magnetite-ulvöspinel solid solution thin films on α-Al 2O3(0001) substrates ...............................................................................................45 3.2 Magnetite-ulvöspinel solid solution thin films on MgO(100) substrates ...............................................................................................64 Chapter 4: Electronic and magnetic structure of magnetite-ulvöspinel solid solution thin films........................................................................................77 Summary....................................................................................................100 List of publications ....................................................................................103 Acknowledgements ...................................................................................104 General Introduction Electronics has developed and supported modern information technology, which has caused drastic changes in the quality of our daily life. Nowadays, it becomes feasible that information with very high-density is transmitted and processed in a very short time. The advancement in electronics has been accomplished by the development of various types of sophisticated devices which have been mainly made from semiconductors. Also, techniques to fabricate very tiny structures at nano- to micro-scales with precise dimensions have contributed to the invention of electronics devices. However, the technique for downsizing to obtain the devices like very large-scale integrated circuits is now facing such a problem that the size of the devices will reach the limit of the present fabrication technique very soon. Alternative way to resolve such an issue is to use multifunctional materials instead of prevalent semiconductors like Si and to utilize the spin, which is another degree of freedom of carriers. It is known that metal oxides are one of the multifunctional materials. The metal oxides have been also ubiquitous in many fields of technology and industry since early times. They possess remarkable physical properties such as ferroelectric, ferromagnetic, metallic, semiconducting, superconducting, pyroelectric, piezoelectric, and multiferroic ones. The discovery of superconductivity with high critical temperature (Tc) in layered perovskite-type cuprate oxides in 1986 [1] has opened up a new possibility of metal oxides from a point of view of both fundamental science and practical applications, and has also triggered findings of novel and exotic metal oxides involving high-Tc superconductors. For instance, in 1994, Jin et al. [2] observed ‘colossal magnetoresistance’ (CMR) in ferromagnetic perovskite of La 1−xCa xMnO 3. In 2004, multiferroic TbMn 2O5, in which an interaction between ferroelectricity and magnetism is so strong that electrical polarization can be reversed by an external 1 magnetic field, was reported [3]. Very recently, a new type of high-Tc superconductive properties were found in iron-based layered oxides, LaO 1–xFxFeAs, and intense studies have been carried out from both aspects of solid-state science and device applications [4,5]. Thus, metal oxides have been efficiently utilized in many fields of oxide electronics, and have potential applications in the future technologies involving new system of electronics. Among the metal oxides, perovskite-type oxides and the related compounds are of interest from both fundamental and technical aspects. Perovskite-type structure AB O3 can hold about 30 elements in the A sites and over half the periodic table in the B sites. Because of this malleable characteristic as host material, it is feasible to customize various physical properties. For instance, SrTi 1–xNb xO3 exhibits insulating, semiconducting, and metallic properties, depending on the concentration of Nb [6]. Incorporation of divalent cations such as Ca, Sr into LaMnO 3, i.e. La 0.7 A0.3 MnO 3, provides colossal magnetoresistance [1,7]. Also, Hg-Ba-Ca-Cu-O system with a layered perovskite-type structure shows high superconducting transition temperature above 130 K [8]. On the other hand, recent technical advances in thin film growth have allowed intense studies on superlattices that are inaccessible by the conventional chemical methods. The precise control of thin film growth process has led to the discovery of unique electronic structure and properties of an interface between perovskite-type oxides layers, for instance two-dimensional electron gas (2D) with high mobility at the interface between LaAlO 3 and SrTiO 3 [9], giant 2D-Seebeck coefficient of SrTiO 3-SrTi 0.8 Nb 0.2 O3 [10], and large magnetoresistance and magnetic hysteresis at the interface between LaAlO 3 and TiO 2-terminated SrTiO 3 [11] were prepared. Spinel-type oxides containing transition metal (TM) elements are as unique and intriguing as perovskite-type oxides from a viewpoint of structural and physical properties. In the spinel-type structure, there are two kinds of cation sites (tetrahedral and octahedral sites) in the cubic closest packing of oxides ions, where various TM ions can occupy, so that spinel-type oxides shows a wealth of the physical properties which 2 can be utilized in practical applications. For example, spinel-type oxides containing iron ions as a main constituent are well-known as ‘ferrite’, the properties of which have significantly contributed to the advancement in solid-state physics, in particular, in magnetism [12–14]. Ferrites are mainly categorized into two types based on their magnetic coercivity: soft and hard ferrites. The former has been primarily used in transformers, inductors, recording heads, and microwave devices, and the latter in permanent magnet motors and storage media in magnetic recording devices [15]. LiTi 2O4, which also has a spinel-type structure, is known as the first oxide superconductor with a relatively high critical temperature ( Tc ~ 12 K) and extended studies have been performed; for instant, studies about unusual characteristics in specific heat at the critical temperature [16,17]. Also, Cd 2SnO 4 is a transparent conductor, which can be used for high efficiency solar cells [18]. More recently, Yamasaki et al. have disclosed multiferroic behavior with a spontaneous magnetization in CoCr 2O4 crystal which has a conical spin structure [19]. The spinel-type compounds have been mainly examined in the form of bulk material for many decades. In contrast, there are only a few reports about the synthesis, structural characterization, and physical properties of thin film form of spinel-type compounds. Thin films are also important for the fabrication of sophisticated devices in electronics. On the other hand, semiconductors such as Si, Ge, GaAs, GaN, and SiC are essential for the fabrication of prevalent electronic devices (e.g., diodes and transistors), which mainly utilize only the charge of carriers. Recently, an attempt to use the spin of electrons and positive holes in addition to the charge has been made to gain new functionalities. This is a new emerging field of electronics called ‘spintronics’. Although a wide range of materials from metal to organic compounds have been studied to fabricate device in spintronics, magnetic oxide semiconductors have attracted considerable attention because of a variety of physical properties derived from the oxides as mentioned above. The main theme in the present thesis is the development of novel magnetic 3 semiconductor thin film which is applicable in spintronics. From a viewpoint of room-temperature ferromagnetism, easiness to control the carrier type, and high spin polarization, spinel-type TM oxides with intrinsic ferromagnetism are one of the promising materials for spintronic applications. Here, magnetite (Fe 3O4) and ulvöspinel (Fe 2TiO 4) solid solutions, (1–x)Fe 3O4·xFe 2TiO 4 (molar ratio: 0 < x < 1), are selected as a candidate material for semiconductor spintronics, and the possibilities of thin films of the solid solutions for spintronic applications are discussed. This thesis is composed of four chapters. The respective chapter outlines are described as follows. In chapter 1, the concept and history of spintronics are firstly described. Subsequently, main devices in spintronics technology are briefly overviewed, and then three types of magnetic semiconductors which can be candidate materials in semiconductor spintronics are summarized. In chapter 2, firstly, fundamental structure and properties as well as magnetic and transport functions applicable to spintronics for Fe 3O4 are summarized in order to understand the structure and properties of solid solutions based on Fe 3O4, and then the structural, electrical,