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Han, J.*; 内村 友宏*; 荒木 康史; Yoon, J.-Y.*; 竹内 祐太郎*; 山根 結太*; 金井 駿*; 家田 淳一; 大野 英男*; 深見 俊輔*
Nature Physics, 20(7), p.1110 - 1117, 2024/07
被引用回数：5 パーセンタイル：93.84(Physics, Multidisciplinary)量子状態の幾何学的構造を特徴づける基礎的な量として、「量子計量」と「ベリー曲率」が存在する。ベリー曲率は、凝縮系において多種の重要な現象に寄与することが知られている。一方、量子計量も新たなトポロジカル物理現象の起源となることが予想されているが、特に常温常圧下においては、その効果はほとんど探索されていない。本研究では、トポロジカルカイラル反強磁性体Mn
Snと白金の接合系を用い、界面でのナノスピン構造を介して、電子系の量子計量構造の制御に成功した。ベリー曲率ではなく量子計量特有の効果として、時間反転反対称でかつ電子散乱に対して頑強な、二次非線形ホール効果を測定した。更に、界面スピン構造は弱磁場及び界面スピン軌道相互作用によって急峻に変調されるため、量子計量構造も鋭敏に制御可能であることを示した。本研究成果により、量子状態に内在する量子計量構造を活用したトポロジカル物理現象が室温環境下でも可能になり、且つ応用面においても非線形素子を構築する助けとなることが期待される。
Sn nanodot佐藤 佑磨*; 竹内 祐太郎*; 山根 結太*; Yoon, J.-Y.*; 金井 駿*; 家田 淳一; 大野 英男*; 深見 俊輔*
Applied Physics Letters, 122(12), p.122404_1 - 122404_5, 2023/03
被引用回数：3 パーセンタイル：52.62(Physics, Applied)
-MnSn, an antiferromagnet having a non-collinear spin structure in a kagome lattice, has attracted great attention owing to various intriguing properties such as large anomalous Hall effect. Stability of magnetic state against thermal fluctuation, characterized in general by the thermal stability factor 
, has been well studied in ferromagnetic systems but not for antiferromagnets. Here we study of the antiferromagnetic MnSn nanodots as a function of their diameter 
. To obtain , we measure the switching probability as a function of pulse-field amplitude and analyze the results based on a model developed by accounting for two and six-fold magnetic anisotropies in the kagome plane. We observe no significant change in down to 
nm below which it decreases with . The obtained dependence is well explained by a single-domain and nucleation-mediated reversal models. These findings provide a basis to understand the thermal fluctuation and reversal mechanism of antiferromagnets for device application.
Sn thin films by magneto-optical Kerr effect内村 友宏*; Yoon, J.-Y.*; 佐藤 佑磨*; 竹内 祐太郎*; 金井 駿*; 武智 涼太*; 岸 桂輔*; 山根 結太*; DuttaGupta, S.*; 家田 淳一; et al.
Applied Physics Letters, 120(17), p.172405_1 - 172405_5, 2022/04
被引用回数：18 パーセンタイル：87.37(Physics, Applied)We perform a hysteresis-loop measurement and domain imaging for 
-oriented -Mn
Sn
thin films using magneto-optical Kerr effect (MOKE) and compare it with the anomalous Hall effect (AHE) measurement. We obtain a large Kerr rotation angle of 10 mdeg., comparable with bulk single-crystal MnSn. The composition 
dependence of AHE and MOKE shows a similar trend, suggesting the same origin, i.e., the non-vanishing Berry curvature in the momentum space. Magnetic domain observation at the saturated state shows that x dependence of AHE and MOKE is explained by an amount of reversible area that crucially depends on the crystalline structure of the film. Furthermore, in-depth observation of the reversal process reveals that the reversal starts with nucleation of sub-micrometer-scale domains dispersed in the film, followed by a domain expansion, where the domain wall preferentially propagates along the ![$$[11bar{2}0]$$](/search/images/ERNTCMK2MJQXE4ZSPUYF0JA.gif)
direction. Our study provides a basic understanding of the spatial evolution of the reversal of chiral-spin structure in non-collinear antiferromagnetic thin films.
竹内 祐太郎*; 山根 結太*; Yoon, J.-Y.*; 伊藤 隆一*; 陣内 佛霖*; 金井 駿*; 家田 淳一; 深見 俊輔*; 大野 英男*
Nature Materials, 20(10), p.1364 - 1370, 2021/10
被引用回数：112 パーセンタイル：98.77(Chemistry, Physical)Electrical manipulation of magnetic materials by current-induced spin torque constitutes the basis of spintronics. Recent studies have demonstrated electrical controls of ferromagnets and collinear antiferromagnets by spin-orbit torque (SOT). Here we show an unconventional response to SOT of a non-collinear antiferromagnet, which has recently attracted great attention owing to large anomalous Hall effect despite vanishingly small net magnetization. In heterostructures with epitaxial non-collinear antiferromagnet MnSn, we observe a characteristic fluctuation of Hall resistance, which is attributed to a persistent rotation of chiral-spin structure of MnSn driven by SOT. We find that level of the fluctuation that varies with sample size represents the number of magnetic domains of MnSn. In addition, MnSn thickness dependence of critical current reveals that SOT generated by small current density below 20 MA cm
effectively acts on the chiral-spin structure even in thick MnSn above 20 nm. The results provide unprecedented pathways of electrical manipulation of magnetic materials, offering new-concept spintronics devices with unconventional functionalities and low-power consumption.
Sn thin filmsYoon, J.-Y.*; 竹内 祐太郎*; DuttaGupta, S.*; 山根 結太*; 金井 駿*; 家田 淳一; 大野 英男*; 深見 俊輔*
AIP Advances (Internet), 11(6), p.065318_1 - 065318_6, 2021/06
被引用回数：18 パーセンタイル：78.58(Nanoscience & Nanotechnology)We investigate the relationship between structural parameters, magnetic ordering, and the anomalous Hall effect (AHE) of MnSn (
) thin films annealed at various temperature 
. The crystal structure changes with and and at 
C near the stoichiometric composition (
) epitaxial single-phase 0
-MnSn(
) is obtained. At room temperature, a larger AHE is obtained when the single-phase epitaxial MnSn with the lattice constant closer to that of bulk is formed. The temperature dependence of the AHE shows different behaviors depending on and can be explained by considering the variation of magnetic ordering. A close inspection into the temperature and composition dependence suggests a variation of magnetic phase transition temperature with composition and/or a possible correlation between the AHE and Fermi level position with respect to the Weyl points. Our comprehensive study would provide the basis for utilizing the unique functionalities of non-collinear antiferromagnetic materials.
Han, J.*; 内村 友宏*; 荒木 康史; Yoon, J.-Y.*; 竹内 祐太郎*; 山根 結太*; 金井 駿*; 家田 淳一; 大野 英男*; 深見 俊輔*
no journal, ,
物質の特異な電子物性を実現する要素として、電子波動関数の幾何構造が注目されている。波数空間内の幾何構造はベリー曲率と量子計量という2つの量により特徴づけられる。ベリー曲率が異常ホール効果等の内因性効果の起源として活発に研究が行われている一方、量子計量については、電流ノイズや超流動・超伝導の不安定性等に寄与する理論予測はあるが、量子計量の効果を物質中で制御し観測することは困難であった。本講演では、カイラル反強磁性体MnSnにおける量子計量を室温下で制御し観測に成功した、我々の実験及び理論研究の成果について報告する。実験では(0001)配向のMnSnと重金属Ptのヘテロ構造を用い、面内磁場印加下で2次高調波成分の非線形ホール電圧を観測した。この非線形ホール効果は反強磁性転移温度直下の室温で強く現れ、磁場方向に追随して360
の周期性を示した(図参照)。我々はMnSn界面の強束縛模型を用いたモデル計算を行い、測定された非線形ホール効果が、磁場で制御を受けたMnSn電子の量子計量構造に起因することを示した。これは界面Dzyaloshinskii-Moriya相互作用によるスピン構造のカンティングにより現れる、空間反転対称性が強く破れた電子状態に由来する。この電子状態は電場の1次摂動で量子計量に比例したベリー曲率を得るため、電場の2次でホール電圧を示す。モデル計算では量子計量からの非線形ホール効果への寄与を他のメカニズムと比較することにより、測定結果が量子計量の効果であることを確かめた。本研究成果は非線形伝導のデバイス応用に向け、量子計量の制御の重要性を示唆するものである。
Han, J.*; 内村 友宏*; 荒木 康史; Yoon, J.-Y.*; 竹内 祐太郎*; 山根 結太*; 金井 駿*; 家田 淳一; 大野 英男*; 深見 俊輔*
no journal, ,
The quantum metric and Berry curvature are two fundamental and distinct factors that describe the geometry of quantum eigenstates. While the role of the Berry curvature in governing various condensed-matter states has been investigated extensively, the quantum metric, which was also predicted to induce topological phenomena of equal importance, has rarely been studied. Recently, a breakthrough has been made in observing the quantum-metric nonlinear transport in a van der Waals magnet, but the effect is limited at cryogenic temperature and is tuned by strong magnetic fields of several teslas. In our study, we demonstrate room-temperature manipulation of the quantum-metric structure of electronic states through its interplay with the interfacial spin texture in a topological chiral antiferromagnet/heavy metal Mn3Sn/Pt heterostructure, which is manifested in a time-reversal-odd second-order Hall effect (ScHE). We show the flexibility of controlling the quantum-metric structure with moderate magnetic fields and verify the quantum-metric origin of the observed ScHE by theoretical modeling. Our results open the possibility of building applicable nonlinear devices by harnessing the quantum-metric structure of electronic states.
