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針井 一哉*; 埋田 真樹; 有沢 洋希*; 日置 友智*; 佐藤 奈々; 岡安 悟; 家田 淳一
Journal of the Physical Society of Japan, 92(7), p.073701_1 - 073701_4, 2023/07
被引用回数：1 パーセンタイル：61.99(Physics, Multidisciplinary)Magnetic property modification of an easy-plane magnetic insulator caused by an accumulation of nanocolumnar defects along the film normal was investigated by numerical simulations and heavy-ion beam irradiation experiments. Micromagnetic simulations suggest that depending on the density of the nonmagnetic nanocolumnar defects, the microstructure of the fragmented ferromagnetic domains is formed leading to magnetic coercivity enhancement as well as magnetization reduction. To prove this prediction, gold ions with 300 MeV were used for the irradiation to create amorphous nanocolumnar defects in crystalline bismuth-doped lutetium iron garnet (Bi:LuIG) films. As increasing the ion-irradiation dose, the modifications of the saturation magnetization and magnetic coercivity were observed in an uncorrelated manner; the enhancement of magnetic coercivity exhibits a fluence threshold whereas the decrease of saturation magnetization caused by ion-beam damage is monotonic with increasing beam fluence. These behaviors qualitatively agree with the numerical simulations and the models by continuum percolation theory. Because the irradiation effects are controlled by beam fluences, the present method has the potential to be a microstructuring technique for magnetic insulator.
埋田 真樹; 中堂 博之; 今井 正樹; 佐藤 奈々; 齊藤 英治
Review of Scientific Instruments, 94(6), p.063906_1 - 063906_8, 2023/06
被引用回数：0 パーセンタイル：0(Instruments & Instrumentation)We have developed experimental equipment for observing the Barnett effect, in which mechanical rotation magnetizes an object, at low temperatures. A sample in a rotor is rotated bidirectionally using a temperature-controlled high-pressure gas. The stray field generated from the sample due to the Barnett effect was detected using a fluxgate magnetic sensor with a sensitivity on the order of several picoteslas, even at low temperatures. By replacing the rotor with a solenoid coil, the magnetic susceptibility of the sample was estimated from the stray field to be of the same order of magnitude as that due to the Barnett effect. The Barnett field was estimated using the dipole model. To assess the performance of the setup at low temperatures, measurements were performed on commercial magnetite (Fe
O
) nanogranules. We confirmed the accordance of the 
factor between the experimental results using the present setup and those of our previous study performed at room temperature.
片岡 龍峰*; 西山 尚典*; 田中 良昌*; 門倉 昭*; 内田 ヘルベルト陽仁*; 海老原 祐輔*; 江尻 省*; 冨川 喜弘*; 堤 雅基*; 佐藤 薫*; et al.
Earth, Planets and Space (Internet), 71(1), p.9_1 - 9_10, 2019/12
被引用回数：8 パーセンタイル：40.25(Geosciences, Multidisciplinary)2017年6月30日22時21分から26分(世界時)にかけて、昭和基地にあるPANSYレーダーによってオーロラ爆発時の中間圏における過渡電離が観測された。通常、中間圏における過渡電離は100keV以上の高エネルギー電子が中間圏まで到達することにより引き起こされるが、同時間帯においてあらせ衛星が観測した100keV以上の電子フラックスは有意な上昇を示していなかった。このことから、本イベントは、10keV以下の低エネルギー電子が大量に熱圏に降り注ぐことにより発生したX線による電離であるとの仮説を立てた。この仮説の妥当性を検証するため、粒子・重イオン挙動解析コードPHITSを用いて様々なエネルギースペクトルを持つ電子が大気上空に進入した場合の電離分布を計算した。その結果、10keV以下の電子でも中間圏において十分な電離を引き起こすことが可能であることが分かり、仮説の妥当性が証明された。
佐藤 奈々; Schultheiss, K.*; K
rber, L.*; Puwenberg, N.*; M
hl, T.*; Awad, A. A.*; Arekapudi, S. S. P. K.*; Hellwig, O.*; Fassbender, J.*; Schultheiss, H.*
no journal, ,
In the last decade, two revolutionary concepts in nano magnetism emerged from research for advanced information processing and storage technologies. The first suggests the use of magnetic domain walls (DWs) in ferromagnetic nanowires to permanently store information in DW racetrack memories. The second proposes a hardware realisation of neuromorphic computing in nanomagnets using nonlinear magnetic oscillations in the GHz range. Both ideas originate from the transfer of angular momentum from conduction electrons to localised spins in ferromagnets, either to push data encoded in DWs along nanowires or to sustain magnetic oscillations in artificial neurones. Even though both concepts share a common ground, they live on very different time scales which rendered them incompatible so far. Here, we bridge both ideas by demonstrating the excitation of magnetic auto-oscillations inside nano-scale DWs using pure spin currents.
埋田 真樹; 中堂 博之; 今井 正樹; 松尾 衛*; 佐藤 奈々; 前川 禎通*; 齊藤 英治*
no journal, ,
ブラウン運動は熱揺らぎによって生じる巨視的な物理量が不規則に変化する現象であり、例えば有限温度の液体中に浮かぶ微粒子に媒質の分子が衝突することで生じる。従来、標準的なブラウン運動に基づく確率変数としての物理量の平均は0であり、主に拡散やエネルギー散逸の観点から議論されてきた。一方、非慣性系上で運動する物体には慣性力であるコリオリ力が働くため、時間反転対称性の破れが実現する。よって回転する系のブラウン運動は、有限の統計平均値を示す物理量を伴うことが期待される。本発表ではバーネット効果を用いた回転液体中の微粒子の角速度の観測について報告する。バーネット効果は力学的に回転する物体中の電子スピンに慣性磁場(バーネット磁場)が働くことで磁化する現象であり、バーネット磁場の大きさから液中の微粒子の角速度を見積もることができる。試料には液体溶媒中に磁性ナノ粒子がコロイド状に懸濁した機能性材料である磁性流体を用いた。初めにエポキシ樹脂のカプセルに試料を封入し、液状の磁性流体を従来の固体試料と同程度の角速度で高速回転させることに成功した。次に試料中の電子スピンに働くバーネット磁場を換算したところ、溶媒を除いた粉末固体と比較して約2倍の大きさのバーネット磁場が働いていることを発見した。この結果は液中の磁性ナノ粒子が試料の回転に対して付加的な角速度をもつことを示唆する。この付加的回転の起源について、系統測定の結果を交え慣性力と流体力学の観点から議論を行う。
埋田 真樹; 中堂 博之; 今井 正樹; 松尾 衛; 佐藤 奈々; 前川 禎通*; 齊藤 英治
no journal, ,
Gyromagnetic effects can relate motions including rotation and vibration with the magnetic moment and currently gain renewed attention due to the potential applications in micro-electromechanical systems (MEMS). The Barnett effect is one of the gyromagnetic phenomena in which an object is magnetized when it is rotated. Precise measurement of the inertial field acting on an electron, i.e. Barnett field 
, allows us to obtain not only the gyromagnetic properties of electron spin but also the angular velocity of the rotated samples in solid and liquid states. Here we present the first observation of the Barnett effect in ferrofluids. Ferrofluid is a novel kind of functional material with a stable colloidal suspension of fine particles in a liquid solvent that exhibits both magnetic and fluid properties. Magnetic measurement shows contained FeO fine particles are super-paramagnetic and have large magnetic susceptibility, whose property is suitable for the measurement of the Barnett effect. By rotating the ferrofluid at high speed up to 1.8 kHz we found that arises with almost twice larger the magnitude than that in a solid state as shown in Figure 1. The result at low temperatures suggests that the enhancement of is due to the Brownian motion of the fine particles. We discuss the details of this enhancement in relation to fluid dynamics and an inertial force with additional results.
