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Lee, O.*; 山本 慧; 埋田 真樹; Zollitsch, C. W.*; Elyasi, M.*; 吉川 貴史*; 齊藤 英治; Bauer, G. E. W.*; 紅林 秀和*
Physical Review Letters, 130(4), p.046703_1 - 046703_6, 2023/01
被引用回数：5 パーセンタイル：80.44(Physics, Multidisciplinary)We experimentally and theoretically demonstrate that nonlinear spin-wave interactions suppress the hybrid magnon-photon quasiparticle or "magnon polariton" in microwave spectra of an yttrium iron garnet film detected by an on-chip split-ring resonator. We observe a strong coupling between the Kittel and microwave cavity modes in terms of an avoided crossing as a function of magnetic fields at low microwave input powers, but a complete closing of the gap at high powers. The experimental results are well explained by a theoretical model including the three-magnon decay of the Kittel magnon into spin waves. The gap closure originates from the saturation of the ferromagnetic resonance above the Suhl instability threshold by a coherent back reaction from the spin waves.
岡本 尚也*; 紅林 秀和*; Trypiniotis, T.*; Farrer, I.*; Ritchie, D. A.*; 齊藤 英治; Sinova, J.*; Ma
ek, J.*; Jungwirth, T.*; Barnes, C.*
Nature Materials, 13(10), p.932 - 937, 2014/10
被引用回数：47 パーセンタイル：85.62(Chemistry, Physical)Controlling spin-related material properties by electronic means is a key step towards future spintronic technologies. The spin Hall effect has become increasingly important for generating, detecting and using spin currents, but its strength-quantified in terms of the SHE angle-is ultimately fixed by the magnitude of the spin-orbit coupling present for any given material system. However, if the electrons generating the SHE can be controlled by populating different areas of the electronic structure with different SOC characteristic the SHE angle can be tuned directly within a single sample. Here we report the manipulation of the SHE in bulk GaAs at room temperature by means of an electrical intervalley transition induced in the conduction band. The spin Hall angle was determined by measuring an electromotive force driven by photoexcited spin-polarized electrons drifting through GaAs Hall bars. By controlling electron populations in different valleys, we manipulated the angle from 0.0005 to 0.02. This change by a factor of 40 is unprecedented in GaAs and the highest value achieved is comparable to that of the heavy metal Pt.
渡邉 峻一郎*; 安藤 和也*; Kang, K.*; Mooser, S.*; Vaynzof, Y.*; 紅林 秀和*; 齊藤 英治; Sirringhaus, H.*
Nature Physics, 10(4), p.308 - 313, 2014/04
被引用回数：168 パーセンタイル：97.83(Physics, Multidisciplinary)In spintronics, pure spin currents play a key role in transmitting, processing and storing information. A pure spin current is a flow of electron spin angular momentum without a simultaneous flow of charge current. It can be carried by conduction electrons or magnons and has been studied in many inorganic metals, semiconductors and insulators, but not yet in organic semiconductors. Charge carriers in 
-conjugated organic materials are localized spin-1/2 polarons which move by hopping, but the mechanisms of their spin transport and relaxation are not well understood. Here we use ferromagnetic resonance spin pumping in a ferromagnet/conjugated polymer/nonmagnetic spin-sink trilayer to demonstrate the ability of polarons to carry pure spin currents over hundreds of nanometres with long spin relaxation times of up to a millisecond and to exhibit Hanle precession. By systematically comparing charge and spin transport on the same trilayer we show that spin-orbit coupling mediates spin relaxation at room temperature.
安藤 和也*; 高橋 三郎; 家田 淳一; 紅林 秀和*; Trypiniotis, T.*; Barnes, C. H. W.*; 前川 禎通; 齊藤 英治
Nature Materials, 10(9), p.655 - 659, 2011/09
被引用回数：249 パーセンタイル：98.67(Chemistry, Physical)Injection of spin currents into solids is crucial for exploring spin physics and spintronics. There has been significant progress in recent years in spin injection into high-resistivity materials, for example, semiconductors and organic materials, which uses tunnel barriers to circumvent the impedance mismatch; the impedance mismatch between ferromagnetic metals and high-resistivity materials drastically limits the spin-injection efficiency. However, because of this problem, there is no route for spin injection into these materials through low-resistivity interfaces, that is, Ohmic contacts, even though this promises an easy and versatile pathway for spin injection without the need for growing high-quality tunnel barriers. Here we show experimental evidence that spin pumping enables spin injection free from this condition; room-temperature spin injection into GaAs from Ni
Fe
through an Ohmic contact is demonstrated through dynamical spin exchange. Furthermore, we demonstrate that this exchange can be controlled electrically by applying a bias voltage across a NiFe/GaAs interface, enabling electric tuning of the spin-pumping efficiency.
山本 慧; 紅林 秀和*
no journal, ,
Magnetic materials are good model systems for studying non-linear dynamics. Since its discovery in 1950s, the instability of ferromagnetic resonance (FMR) under high-power microwave driving has led to the seminal work by Suhl that identified its mechanism [1], and the subsequent development revealed a rich array of phenomena including auto-oscillations and chaos [2]. The first-order Suhl instability, also known as three-magnon instability, has the lowest threshold power among all the known non-linear phenomena in ferromagnetic dynamics. Nevertheless, it has attracted comparatively little attention so far, perhaps because it requires a pair of spin wave modes whose eigen frequency is half of the FMR frequency. This condition is almost automatically satisfied, however, in film geometry with a low static magnetic field, a setup that is prevalent in spintronics. In this work, we study the three-magnon instability when the ferromagnetic film is placed in a microwave cavity. In spin cavitronics, one of the aims is to couple the FMR mode and cavity photons strongly so as to enhance the coherence of the former for potential quantum magnonic applications. Understanding the effect of non-linearity is important to control such devices since a strong coupling tends to induce a large FMR mode amplitude, which in turn may set off the spin wave instability. We show, both theoretically and experimentally, that the three-magnon instability generally blurs the anti-crossing of FMR mode and cavity photon. It does not completely wipe out the magnon-photon hybridization, however. Sufficiently strong input power sees a coalescence of the split resonance peaks into one, which still maintains the resonant Lorentzian shape (Fig. 1). We discuss the dependence of line shape on temperature and phase detuning between the magnetic film and the cavity.
山本 慧; 紅林 秀和*
no journal, ,
マイクロ波空洞共振モードと強磁性共鳴の結合は、磁気ダイナミクスの量子性を調べるためのプラットフォームとして興味を集めている。磁気励起の量子制御にはスピン波の非線形性が本質的に重要であるが、それが空洞共振との結合に与える影響についてはほとんど調べられていない。本研究発表では、強磁性共鳴の振幅が3マグノン分裂過程に起因する不安定性を引き起こす程度に大きい領域で、空洞共振モードと強磁性共鳴モードの混成に生じる変化について理論・実験の両面から述べる。非線形不安定性によって励起されたスピン波対の位相が期待値を獲得し、その反作用によって強磁性共鳴モードの振幅は飽和する。強磁性共鳴の飽和は空洞共振モードとの混成を阻害し、結果として共鳴吸収線に現れる反交差によるギャップが強励起によって消失することを明らかにする。
山本 慧; 紅林 秀和*
no journal, ,
Motivated by the importance of non-linearity in general in quantum manipulation of cavity photons, we study the non-linear spin wave instability when a magnetic film is strongly coupled to a microwave resonator mode. We show both experimentally and theoretically that when the input driving power exceeds a certain threshold, the first-order Suhl instability for the spin waves manifests itself as an apparent closure of the anti-crossing gap in the microwave reflection spectrum of the coupled system. We identify its mechanism to be the blocking of hybridisation between the Kittel mode magnon and the resonator photon by a coherent back reaction from the unstable magnon pair.
