In semiconductor-based spintronics devices, novel functionalities are expected by utilizing not only the charge but also spin of carriers. The realization of such devices relies both on the ability to inject a spin-polarized current into a semiconductor (SC) and to detect this current. One of the recent approaches is using ferromagnetic semiconductor (FMS) layers as a spin injector or a detector in device structures. However, FMSs have a disadvantage of low Curie temperature (TC). Another approach is using ferromagnetic metals (FM) as spin injecting and detecting layers. In this approach, however, interfacial compound layers are easily formed, thus the spin polarizations of carries at the interface is dramatically degrade. In 1990's, some magnetic Mn compounds such as MnAs, MnGa and MnAl were successfully grown on semiconductor substrates. Among them, the ferromagnetic metal MnAs is a promising material because its TC = 313 ~ 318°K and it can be epitaxially grown without interfacial reaction on many semiconductor substrates such as Si(001), Si(111), GaAs(001) and GaAs(111B), although the crystal structure of MnAs (NiAs-type hexagonal) is different from that of GaAs or Si. However, electrical probing spin injection from MnAs into SC still remains a challenging issue because it is not easy to grow MnAs / SC / MnAs heterostructures. Due to the difference of crystal structure between MnAs and SC, overgrowth of SC layers on MnAs is very difficult.

On the other hand, the GaAs:MnAs granular material which contains ferromagnetic MnAs nanoparticles embedded in a GaAs matrix has the advantage of good compatibility with III-V heterostructures and room-temperature ferromagnetism. The GaAs:MnAs granular material is formed by phase decomposition and phase separation in meta-stable GaMnAs alloy semiconductor annealed at 500-700°C . When annealed at around 500°C, zinc-blende (ZB) MnAs nanoparticles, which correspond to nano-scale areas with very high Mn concentration, are formed due to phase decomposition of GaMnAs. When annealed at higher temperature (> 550°C), the more stable hexagonal NiAs phase precipitates. Although the magneto-optical property of hexagonal or zinc-blende MnAs nanoparticles was intensively investigated, its spin dependent transport is little studied. If spin dependent transport of hexagonal or zinc-blende MnAs nanoparticles is realized, they can be used as spin injecting and spin detecting sources in FM / SM multi layers. Furthermore, because the size of MnAs nanoparticles is as small as 2-10 nm, the Coulomb Blockade (CB) effect is supposed to appear, thus MnAs nanoparticles can be used in single-electron devices. Finally, MnAs nanoparticles can also be used in many important parts of quantum computers using quantum dot systems, such as sources of local magnetic field, spin injectors, and spin detectors of the dot spins. Detailed investigations on the spin dependent transport as well as the charge effect in semiconductor nanostructures containing MnAs nanoparticles will be very important for understanding basic phenomena that can help us to develop new kinds of nano-scale spintronic devices.

This thesis presents studies of spin dependent transport phenomena in semiconductor based heterostructures consisting of MnAs thin film / III-V semiconductor / GaAs:MnAs granular layer, magnetic behaviors of MnAs nanoparticles, and device applications of ferromagnetic nanoparticles in active devices.

First, high quality single crystal magnetic tunnel junctions (MTJs) consisting of MnAs thin film / GaAs / AlAs / GaAs: hexagonal MnAs nanoparticles were grown by molecular beam epitaxy. TMR of those MTJs was clearly observed for the first time. We show that by using hexagonal MnAs nanoparticles as ferromagnetic electrodes, semiconductor based MTJs with high TMR ratio (18 % at 7 K), high V(half) (1200 mV at 7 K) and high operating temperature (~ 300 K), were obtained. The observed TMR ratio is the highest among MnAs-based MTJs. The TMR ratios were found to oscillate against the tunnel barrier thickness while the tunneling resistances follow the WKB approximation, revealing that there are some kinds of quantum effects in our single crystal MTJs. This result shows that hexagonal MnAs nanoparticles can be used as spin injecting and spin detecting sources in semiconductor based spintronics devices.

Next, magnetic properties of hexagonal MnAs nanoparticles were studied by spin dependent transport measurements and Monte Carlo simulations. By utilizing the TMR effect in MnAs thin film / GaAs / AlAs / GaAs: hexagonal MnAs heterostructures, we observed rich magnetic behaviors of hexagonal MnAs nanoparticles system, such as their static and dynamic M-H characteristics, relaxation of magnetization, and blocking temperatures. By fitting the Monte Carlo simulations to the M-H data at 7 K, we estimated the anisotropy constant of hexagonal MnAs nanoparticles to be about 2.1×105 ~ 2.5×10(5) ergs/cc. Furthermore, hexagonal MnAs nanoparticles are found to have long relaxation times (several hundreds ms at 7 K for particles with φ = 5nm) as well as high blocking temperature (TB = 230 K for particles with φ = 5nm, and TB ~ 300 K for particles with φ = 10 nm). Those magnetic behaviors cannot be explained by Neel model for non-interacting particles. We suggested that the dipolar interaction is the origin of such long relaxation time and high blocking temperature.

Then, we studied the CB effect of double barrier MTJs consisting of hexagonal MnAs nanoparticles. The charging energy and the capacitance of hexagonal MnAs nanoparticle with φ = 5 nm are estimated by measuring the temperature dependence of conductance of GaAs:MnAs granular layers. The measured values are consistent with theoretical values and show dependence on the surrounding environment. The TMR oscillation of hexagonal MnAs based double barrier MTJs due to CB was observed for the first time both in vertical and lateral structures. The TMR oscillation curve observed in the lateral structure reveals that the spin-relaxation time of MnAs nanoparticles is as long as 10 μs.

We then propose a new type of spin device called single-electron spin transistor (SEST) that utilizes the TMR and CB effects. We show that by using SEST, reconfigurable circuits such as Tucker type inverter, AND/OR reconfigurable gate, reconfigurable logic gates for two input all symmetric Boolean functions and for two input asymmetric Boolean functions can be realized without using any floating gates. The proposed logic gates can provide nonvolatile and scalable reconfigurable hardware for future electronics.

Finally, electromotive force (emf), Coulomb blockade, and huge magnetoresistance in MTJ with zinc-blende MnAs nanoparticles are described. By detailed theoretical and experimental investigations, we show that the observed emf results from the conversion of the magnetic energy of ZB MnAs nano-magnets into electrical energy when these nano-magnets undergo magnetic quantum tunneling. Our results strongly suggest that Faraday's Law of induction must be generalized in order to account for purely spin effects in such magnetic nanostructures.

Although the subjects of this study are limited to nanoparticles of manganese arsenide compounds, our results provide the basic ideas and experimental foundations for any future studies of spin dependent transport phenomena in ferromagnetic semiconductor granular materials. Recently, during the quest for ferromagnetic semiconductors worldwide, many families of nanoparticles have been discovered. Most of them show room-temperature ferromagnetism. Thus, it is our hope that ferromagnetic nanoparticles embedded in semiconductor materials will be extensively studied and find their application in novel spin devices in near future.

The main achievements of this research are as follows: we experimentally observed the TMR and CB effects, as well as their related phenomena in III-V based MTJs containing MnAs nanoparticles, to better understand magnetic behaviors of MnAs nanoparticles, and to design new kind of devices utilizing the TMR as well as CB effects. A new spin effect, namely the electromotive force induced by a static magnetic field, has been discovered in MTJs with zinc-blende MnAs nanoparticles.

本論文は、「Spin dependent transport phenomena in III-V semiconductor heterostructures with ferromagnetic MnAs nano-scale particles(ナノスケール強磁性MnAs微粒子を含むIII-V半導体へテロ構造におけるスピン依存伝導現象)」と題し、英文で書かれている。本論文では、ナノスケール強磁性MnAs微粒子を含むIII-V族ベース半導体へテロ構造の成長、構造、磁性、およびスピン依存伝導とその応用に関する研究成果を記述しており、全8章から成る。

第1章は「Introduction」であり、スピントロニクスと強磁性半導体に関する研究の背景と状況を述べ、本論文の構成と目的を示している。その中で、本研究で用いるナノスケール強磁性MnAs微粒子をGaAs中に分散させたグラニュラー材料GaAs:MnAsの特色を述べている。

第2章は「Spin polarized tunneling in III-V based heterostructures with a ferromagnetic MnAs thin films and GaAs:MnAs nanoparticles」であり、MnAs/GaAs/AlAs/GaAs:MnAsからなるヘテロ構造において、トンネル磁気抵抗効果を低温から室温に至るまでの温度領域で観測し、GaAs:MnAs 中のMnAs微粒子がスピン注入源およびスピン検出器として働くことを示している。

第3章は「Magnetic properties of MnAs nanoparticles studied by tunneling and Monte Carlo simulations」であり、第3章と同様の磁性ヘテロ構造において、トンネル磁気抵抗効果の磁場依存性からマイナーループを測定し、GaAs:MnAsの磁化特性、磁化の緩和時間、ブロッキング温度(TB)、磁気異方性エネルギーなどをモンテカルロシミュレーションと比較することにより評価している。また、長い緩和時間と高いTBの原因は、磁気的な双極子相互作用であるとしている。

第4章は「Spin valve effect by ballistic transport in ferromagnetic metal (MnAs) / semiconductor (GaAs) hybrid heterostructures」であり、MnAs/GaAs/GaAs:MnAsからなるヘテロ構造において、バリスティック伝導による比較的大きなスピンバルブ効果を観測した結果について述べ、バリスティック伝導を用いればスピン注入における伝導率不整合の問題が解決できることを実験的に示唆している。

第5章は、「Coulomb blockade in III-V based heterostructures with a ferromagnetic MnAs thin and GaAs:MnAs nanoparticles」であり、MnAs/GaAs/AlAs/GaAs:MnAs/AlAs/p-GaAsから成る二重障壁トンネル接合(縦型デバイス)およびGaAs:MnAs薄膜表面にMnAs薄膜のソース・ドレイン電極を付けた横型デバイスにおいて、クーロンブロッケード(CB)によるTMRの振動現象を観測し、MnAs微粒子におけるスピン緩和時間が10μs以上と非常に長いことを示している。

第6章は「Device application of TMR and CB in nanostructures with ferromagnetic nanoparticles」であり、強磁性微粒子を含む単電子スピントランジスタ(SEST)と、SESTを用いたタッカー型インバータ、AND/OR回路、2入力対称関数、2入力非対称関数などの再構成可能な論理回路を提案し、その動作を数値シミュレーションにより示している。

第7章は「Electromotive force, Coulomb blockade, and huge magnetoresistance in magnetic tunnel junctions with Zinc-blende MnAs nanoparticles」であり、閃亜鉛鉱型結晶構造をもつMnAs (ZB-MnAs)微粒子を含むヘテロ構造MnAs/GaAs/AlAs/GaAs:ZB-MnAsにおいて、静磁場による起電力の発生とCB効果による100,000%を超えるきわめて大きな磁気抵抗効果の観測について述べている。観測された静磁場による起電力は、古典的な電磁気学で説明できるものではなく、巨視的な量子トンネリングによってMnAs微粒子の磁化が回転し、同時に電子スピンが反転しながらトンネルするというコトンネリングが起こることによって、磁気的なゼーマンエネルギーが電子系の電気的なエネルギーに転換される、というモデルで説明している。このような磁性ナノ構造で観測される量子力学的なスピンの効果を説明するためには、ファラデーの電磁誘導の法則を拡張する必要があることを示している。

第8章は「Concluding remarks and outlook」であり、本論文で得られた結果のまとめと今後の展望を述べている。

以上これを要するに、本論文は、ナノスケール強磁性MnAs微粒子を含むIII-V半導体へテロ構造におけるスピン依存伝導現象を中心とする物性を実験的に明らかにし、スピン依存トンネル現象を観測することにより、III-V族半導体へテロ構造中のMnAs微粒子がスピン注入源および検出器になること、クーロンブロッケード効果とトンネル磁気抵抗効果が同時に起こりうること、巨大な磁気抵抗と静磁場による起電力が生じうることなど、新しい知見を示したもので、電子工学およびスピントロニクスの発展のために寄与するところが少なくない。

よって本論文は、博士(工学)の学位請求論文として合格と認められる。