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論文

Origin of magnetically dead layers in spinel ferrites $$M$$Fe$$_2$$O$$_4$$ grown on Al$$_2$$O$$_3$$; Effects of postdeposition annealing studied by XMCD

野中 洋亮*; 若林 勇希*; 芝田 悟朗; 坂本 祥哉*; 池田 啓祐*; Chi, Z.*; Wan, Y.*; 鈴木 雅弘*; 田中 新*; 田中 雅明*; et al.

Physical Review Materials (Internet), 7(4), p.044413_1 - 044413_10, 2023/04

https://doi.org/10.1103/PhysRevMaterials.7.044413
 被引用回数：0 パーセンタイル：0(Materials Science, Multidisciplinary)

We study the electronic and magnetic states of as-grown and annealed $$M$$Fe$$_2$$O$$_4$$(111)/Al$$_2$$O3(111) ($$M =$$Co, Ni) thin films with various thicknesses grown on Si(111) substrates with the $$gamma$$-Al$$_2$$O$$_3$$(111)buffer layers by using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), to investigate magnetically dead layers in these films. Although the magnetically dead layers in the as-grown samples are formed near the interface with the Al$$_2$$O$$_3$$ buffer layer, we reveal that ferrimagnetic order is partially recovered by postdeposition annealing at 973 K for 48 hours in air. By analyzing the line shapes of the XAS and XMCD spectra, we conclude that, in the dead layers, there are a significant number of vacancies at the $$T_d$$ sites of the spinel structure, which may be the microscopic origin of the degraded ferrimagnetic order in the $$M$$Fe$$_2$$O$$_4$$ thin films.

論文

Mesospheric ionization during substorm growth phase

村瀬 清華*; 片岡 龍峰*; 西山 尚典*; 西村 耕司*; 橋本 大志*; 田中 良昌*; 門倉 昭*; 冨川 喜弘*; 堤 雅基*; 小川 泰信*; et al.

Journal of Space Weather and Space Climate (Internet), 12, p.18_1 - 18_16, 2022/06

https://doi.org/10.1051/swsc/2022012
 被引用回数：1 パーセンタイル：22.72(Astronomy & Astrophysics)

巨大な太陽フレアによってもたらされる太陽風により磁気圏内にエネルギーが溜まり、そのエネルギーが一気に解放されるサブストームが発生する。そのサブストームが発生する際、高エネルギー電子が大量に中間圏まで降り注ぐ事象(EEP)がしばしば観測されるが、その詳細な発生メカニズムは解明されていない。本研究では、あらせ衛星により観測された2つのEEPに対して、3次元グローバル電磁流体力学的(MHD)シミュレーションや放射線挙動解析コードPHITSを使った解析によりその発生メカニズムを検討した。その結果、カレントシート散乱とwave-particle散乱がEEPの初期及びサブストーム発生後に重要な役割を果たしていることが示唆された。

論文

Magnetic properties and electronic configurations of Mn ions in the diluted magnetic semiconductor Ba$$_{1-x}$$K$$_x$$(Zn$$_{1-y}$$Mn$$_y$$)$$_2$$As$$_2$$ studied by X-ray magnetic circular dichroism and resonant inelastic X-ray scattering

鈴木 博人*; Zhao, G.*; 岡本 淳*; 坂本 祥哉*; Chen, Z.-Y.*; 野中 洋亮*; 芝田 悟朗; Zhao, K.*; Chen, B.*; Wu, W.-B.*; et al.

Journal of the Physical Society of Japan, 91(6), p.064710_1 - 064710_5, 2022/06

https://doi.org/10.7566/JPSJ.91.064710
 被引用回数：0 パーセンタイル：0(Physics, Multidisciplinary)

The magnetic properties and the electronic excitations of the new diluted magnetic semiconductor Ba$$_{1-x}$$K$$_x$$(Zn$$_{1-y}$$Mn$$_y$$)$$_2$$As$$_2$$ have been studied by X-ray magnetic circular dichroism (XMCD) and resonant inelastic X-ray scattering (RIXS). The sum rule analysis of the XMCD spectra indicates that the Mn atoms are in the high-spin configurations of $$d^5$$, whereas the presence of competing ferromagnetic and antiferromagnetic interactions between the Mn ions reduces the net spin moment. Based on a comparison of the RIXS line shapes with those of Ga$$_{1-x}$$Mn$$_x$$As, it is concluded that the ground state of Mn in Ba$$_{1-x}$$K$$_x$$(Zn$$_{1-y}$$Mn$$_y$$)$$_2$$As$$_2$$ consists of both the $$3d^5 underline{L}$$ and $$3d^5$$ electron configurations.

論文

Angular momentum transfer in multinucleon transfer channels of $$^{18}$$O + $$^{237}$$Np

田中 翔也*; 廣瀬 健太郎; 西尾 勝久; Kean, K. R.*; 牧井 宏之; Orlandi, R.; 塚田 和明; 有友 嘉浩*

Physical Review C, 105(2), p.L021602_1 - L021602_5, 2022/02

https://doi.org/10.1103/PhysRevC.105.L021602
 被引用回数：1 パーセンタイル：33.4(Physics, Nuclear)

The angular momentum of a primary excited compound nucleus produced in multinucleon transfer reaction is an important quantity to evaluate cross sections to synthesize neutron-rich heavy element nuclei as well as for surrogate reaction studies. The mechanism is, however, not enough understood due to the lack of detailed experimental data. In the present study, we determined the angular momentum of primary excited nuclei, $$^{237,238,239}$$Np, $$^{238,239,240}$$Pu, and $$^{239,240,241}$$Am, produced in the multi-nucleon transfer channels of the $$^{18}$$O + $$^{237}$$Np reaction. With this aim, angular distributions of fission fragments with respect to the axis perpendicular to the reaction plane were measured for each compound nucleus. The distributions show an anisotropy exhibiting an enhanced yield on the reaction plane. They are well reproduced by a saddle point model, from which the average angular momentum is derived in the model framework. The angular momentum increases with the compound-nucleus mass, thus the number of nucleons exchanged, but shows a saturating trend toward heavier compound nuclei. These results are the first ones to point to the dependence of the angular momentum on the transfer channels.

論文

Evolution of Fe 3$$d$$ impurity band state as the origin of high Curie temperature in the $$p$$-type ferromagnetic semiconductor (Ga,Fe)Sb

武田 崇仁*; 坂本 祥哉*; 荒木 恒星*; 藤澤 唯太*; Anh, L. D.*; Tu, N. T.*; 竹田 幸治; 藤森 伸一; 藤森 淳*; 田中 雅明*; et al.

Physical Review B, 102(24), p.245203_1 - 245203_8, 2020/12

https://doi.org/10.1103/PhysRevB.102.245203
 被引用回数：7 パーセンタイル：44.34(Materials Science, Multidisciplinary)

(Ga$$_{1-x}$$,Fe$$_x$$)Sb is one of the promising ferromagnetic semiconductors for spintronic device applications because its Curie temperature (Tc) is above 300 K when the Fe concentration $$x$$ is equal to or higher than 0.20. However, the origin of the high Tc in (Ga,Fe)Sb remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and first-principles calculations to investigate the x dependence of the Fe 3$$d$$ states in (Ga$$_{1-x}$$,Fe$$_x$$)Sb ($$x$$ = 0.05, 0.15, and 0.25) thin films. The observed Fe 2$$p$$-3$$d$$ RPES spectra reveal that the Fe-3$$d$$ impurity band (IB) crossing the Fermi level becomes broader with increasing $$x$$, which is qualitatively consistent with the picture of double-exchange interaction. Comparison between the obtained Fe-3$$d$$ partial density of states and the first-principles calculations suggests that the Fe-3$$d$$ IB originates from the minority-spin ($$downarrow$$) $$e$$ states. The results indicate that enhancement of the double-exchange interaction between $$e_{downarrow}$$ electrons with increasing $$x$$ is the origin of the high Tc in (Ga,Fe)Sb.

論文

Measurement of fission-fragment mass distributions in the multinucleon transfer channels of the $$^{18}$$O+$$^{237}$$Np reaction

Vermeulen, M. J.; 西尾 勝久; 廣瀬 健太郎; Kean, K. R.; 牧井 宏之; Orlandi, R.; 塚田 和明; Tsekhanovich, I.*; Andreyev, A. N.; 石崎 翔馬*; et al.

Physical Review C, 102(5), p.054610_1 - 054610_11, 2020/11

https://doi.org/10.1103/PhysRevC.102.054610
 被引用回数：5 パーセンタイル：52.72(Physics, Nuclear)

Fission fragment mass distributions for 23 nuclei ($$^{234-237}$$U, $$^{236-239}$$Np, $$^{238-241}$$Pu, $$^{240-243}$$Am, $$^{242-245}$$Cm, and $$^{244-246}$$Bk) were measured using the multi-nucleon transfer approach in the reaction of $$^{18}$$O+$$^{237}$$Np, and their excitation-energy dependence was obtained up to a maximum of 70 MeV. Among them, the low energy fission of $$^{236}$$Np, $$^{238}$$Pu, and $$^{245}$$Cm is reported for the first time. The experimental data for all the studied nuclei were compared to the Langevin calculations. The calculation which takes into account the effects of multi-chance fission well reproduced the peak-to-valley ratio and mass-asymmetric peak positions of the distributions. The angular momenta given to the fissioning nucleus is also discussed.

論文

Transient ionization of the mesosphere during auroral breakup; Arase satellite and ground-based conjugate observations at Syowa Station

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

https://doi.org/10.1186/s40623-019-0989-7
 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。

論文

Effects of multichance fission on isotope dependence of fission fragment mass distributions at high energies

田中 翔也; 有友 嘉浩*; 宮本 裕也*; 廣瀬 健太郎; 西尾 勝久

Physical Review C, 100(6), p.064605_1 - 064605_6, 2019/12

https://doi.org/10.1103/PhysRevC.100.064605
 被引用回数：13 パーセンタイル：79.29(Physics, Nuclear)

散逸揺動定理を採用した動力学模型を使用し、$$^{234-240}$$U, $$^{236-242}$$Np, $$^{238-244}$$Pu ($$E^{*}$$=15-55MeV)という広い範囲の核種および励起エネルギーにおいて核分裂片質量分布の評価を行った。本研究の特色は、原子核が分裂へ至るまでに中性子を放出するという、マルチチャンス核分裂の概念を理論計算へ導入していることである。その結果、実験データを系統的に再現する計算結果を得られた。特に、励起エネルギーが高い領域においても核分裂片質量分布がふた山構造を維持する傾向や、原子核の初期状態(特に中性子数)に依存するという実験データの傾向を再現することに成功した。またこの傾向が、核分裂前の中性子放出確率を支配している複合核の中性子結合エネルギーと相関を持つことを示した。

論文

Electronic structure of the high-$$T_{rm C}$$ ferromagnetic semiconductor (Ga,Fe)Sb; X-ray magnetic circular dichroism and resonance photoemission spectroscopy studies

坂本 祥哉*; Tu, N. T.*; 竹田 幸治; 藤森 伸一; Hai, P. N.*; Anh, L. D.*; 若林 勇希*; 芝田 悟朗*; 堀尾 眞史*; 池田 啓祐*; et al.

Physical Review B, 100(3), p.035204_1 - 035204_8, 2019/07

https://doi.org/10.1103/PhysRevB.100.035204

The electronic structure and the magnetism of the ferromagnetic semiconductor (Ga,Fe)Sb, whose Curie temperature $$T_{rm C}$$ can exceed room temperature, were investigated by means of X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD), and resonance photoemission spectroscopy (RPES). The line-shape analyses of the XAS and XMCD spectra suggest that the ferromagnetism is of intrinsic origin. The orbital magnetic moments deduced using XMCD sum rules were found to be large, indicating that there is a considerable 3$$d^{6}$$ contribution to the ground state of Fe. From RPES, we observed a strong dispersive Auger peak and nondispersive resonantly enhanced peaks in the valence-band spectra. The latter is a fingerprint of the correlated nature of Fe 3$$d$$ electrons, whereas the former indicates their itinerant nature. It was also found that the Fe 3$$d$$ states have a finite contribution to the density of states at the Fermi energy. These states, presumably consisting of majority-spin $$p$$-$$d$$ hybridized states or minority-spin e states, would be responsible for the ferromagnetic order in this material.

論文

Electronic structure of the high-$$T_{rm C}$$ ferromagnetic semiconductor (Ga,Fe)Sb; X-ray magnetic circular dichroism and resonance photoemission spectroscopy studies

坂本 祥哉*; Tu, N. T.*; 竹田 幸治; 藤森 伸一; Hai, P. N.*; Anh, L. D.*; 若林 勇希*; 芝田 悟朗*; 堀尾 眞史*; 池田 啓祐*; et al.

Physical Review B, 100(3), p.035204_1 - 035204_8, 2019/07

https://doi.org/10.1103/PhysRevB.100.035204
 被引用回数：16 パーセンタイル：66.08(Materials Science, Multidisciplinary)

The electronic structure and the magnetism of the ferromagnetic semiconductor (Ga,Fe)Sb, whose Curie temperature $$T_{rm C}$$ can exceed room temperature, were investigated by means of X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD), and resonance photoemission spectroscopy (RPES). The line-shape analyses of the XAS and XMCD spectra suggest that the ferromagnetism is of intrinsic origin. The orbital magnetic moments deduced using XMCD sum rules were found to be large, indicating that there is a considerable 3$$d^{6}$$ contribution to the ground state of Fe. From RPES, we observed a strong dispersive Auger peak and nondispersive resonantly enhanced peaks in the valence-band spectra. The latter is a fingerprint of the correlated nature of Fe 3$$d$$ electrons, whereas the former indicates their itinerant nature. It was also found that the Fe 3$$d$$ states have a finite contribution to the density of states at the Fermi energy. These states would be responsible for the ferromagnetic order in this material.

論文

Origin of the dramatic change of fission mode in fermium isotopes investigated using Langevin equations

宮本 裕也*; 有友 嘉浩*; 田中 翔也; 廣瀬 健太郎; 西尾 勝久

Physical Review C, 99(5), p.051601_1 - 051601_7, 2019/05

https://doi.org/10.1103/PhysRevC.99.051601
 被引用回数：16 パーセンタイル：84.48(Physics, Nuclear)

The fission of even-even fermium nuclides $$^{250-260}$$Fm at low excitation energy was studied using Langevin equations of three-dimensional nuclear-shape parametrization. The mass distributions of fission fragments show a dramatic change from an asymmetric shape for the lighter fermium isotopes to sharp symmetric fission for the heavier isotopes. The time evolution of the nuclear shape on the potential surface reveals that the lighter fermium isotopes showing asymmetric fission are trapped in the second minimum for a substantial length of time before overcoming the second saddle point. This behavior changes dramatically for the compact symmetric fission found in the heavier neutron-rich fermium nuclei that disintegrate immediately after overcoming the first saddle point, without feeling the second barrier, resulting in a fission time two orders of magnitude shorter.

論文

Cation distribution and magnetic properties in ultrathin (Ni$$_{1-x}$$Co$$_{x}$$)Fe$$_{2}$$O$$_{4}$$ (x=0-1) layers on Si(111) studied by soft X-ray magnetic circular dichroism

若林 勇希*; 野中 洋亮*; 竹田 幸治; 坂本 祥哉*; 池田 啓祐*; Chi, Z.*; 芝田 悟朗*; 田中 新*; 斎藤 祐児; 山上 浩志; et al.

Physical Review Materials (Internet), 2(10), p.104416_1 - 104416_12, 2018/10

https://doi.org/10.1103/PhysRevMaterials.2.104416
 被引用回数：10 パーセンタイル：37.66(Materials Science, Multidisciplinary)

We study the electronic structure and magnetic properties of epitaxial (Ni$$_{1-x}$$Co$$_{x}$$)Fe$$_{2}$$O$$_{4}$$(111) layers with thicknesses $$d$$ = 1.7 - 5.2 nm grown on Al$$_{2}$$O$$_{3}$$(111)/Si(111) structures. We revealed the crystallographic (octahedral $$O_{h}$$ or tetrahedral $$T_{d}$$) sites and the valences of the Fe, Co, and Ni cations using experimental soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectra and configuration-interaction cluster-model calculation.

論文

Determination of fusion barrier distributions from quasielastic scattering cross sections towards superheavy nuclei synthesis

田中 泰貴*; 成清 義博*; 森田 浩介*; 藤田 訓裕*; 加治 大哉*; 森本 幸司*; 山木 さやか*; 若林 泰生*; 田中 謙伍*; 武山 美麗*; et al.

Journal of the Physical Society of Japan, 87(1), p.014201_1 - 014201_9, 2018/01

https://doi.org/10.7566/JPSJ.87.014201
 被引用回数：18 パーセンタイル：74.47(Physics, Multidisciplinary)

ガス充填型反跳生成核分離装置GARISを用いて$$^{48}$$Ca + $$^{208}$$Pb, $$^{50}$$Ti + $$^{208}$$Pb, $$^{48}$$Ca + $$^{248}$$Cm反応系における準弾性散乱断面積の励起関数を測定した。これらのデータから融合障壁分布を導出し、チャンネル結合計算と比較した。$$^{48}$$Ca + $$^{208}$$Pb及び$$^{50}$$Ti + $$^{208}$$Pb反応の障壁分布のピークエネルギーはそれらの反応系における2中性子蒸発断面積のピークエネルギーと良く一致し、一方$$^{48}$$Ca + $$^{248}$$Cm反応の障壁分布のピークエネルギーは4中性子蒸発断面積のピークエネルギーより少し下に現れることが判った。この結果は超重核合成の際の最適ビームエネルギーの予測に役立つ情報を与える。

論文

Role of multichance fission in the description of fission-fragment mass distributions at high energies

廣瀬 健太郎; 西尾 勝久; 田中 翔也*; L$'e$guillon, R.*; 牧井 宏之; 西中 一朗*; Orlandi, R.; 塚田 和明; Smallcombe, J.*; Vermeulen, M. J.; et al.

Physical Review Letters, 119(22), p.222501_1 - 222501_6, 2017/12

https://doi.org/10.1103/PhysRevLett.119.222501
 被引用回数：50 パーセンタイル：91.08(Physics, Multidisciplinary)

JAEAタンデム加速器施設で行った$$^{18}$$O+$$^{238}$$U反応における多核子移行チャンネルを用いた実験により、$$^{237-240}$$U, $$^{239-242}$$Np、および$$^{241-244}$$Puの核分裂質量分布を励起エネルギー10$$sim$$60MeVにおいて測定した。これらのうち、$$^{240}$$U, $$^{240,241,242}$$Npのデータは本実験により初めて観測された。原子核の殻効果の減衰によって対称分裂すると予想されていた高励起エネルギーにおいても、質量分布が非対称を示すことがわかった。搖動散逸定理に基づく動力学モデル計算との比較から、この振る舞いはマルチチャンス核分裂によるものであることを明らかにした。

論文

Study of fission using multi-nucleon transfer reactions

西尾 勝久; 廣瀬 健太郎; Vermeulen, M. J.; 牧井 宏之; Orlandi, R.; 塚田 和明; 浅井 雅人; 豊嶋 厚史; 佐藤 哲也; 永目 諭一郎; et al.

EPJ Web of Conferences, 163, p.00041_1 - 00041_6, 2017/11

https://doi.org/10.1051/epjconf/201716300041
 被引用回数：1 パーセンタイル：61.21(Nuclear Science & Technology)

We are promoting a study of fission using multi-nucleon transfer (MNT) reactions, where excited states in neutron-rich actinide nuclei, which cannot be accessed by particle capture and/or fusion reactions, are populated. Also, the excited states in the fissioning nucleus are widely populated by the MNT reactions, from which effects of excitation energy on fission properties can be investigated. Experiments were carried out at the JAEA tandem facility in Tokai, Japan. We studied reactions using the $$^{18}$$O beam and several actinide target nuclei such as $$^{232}$$Th, $$^{238}$$U, $$^{237}$$Np, $$^{248}$$Cm. Ejectile nucleus was identified by a silicon $$Delta$$E-E telescope to identify transfer channel and hence the compound nucleus. Fission fragments were detected by multi-wire proportional counters, and fission fragment mass distributions (FFMDs) were measured for each isotope. Measured FFMDs are reproduced by a calculation based on the fluctuation-dissipation model, and importance of multi-chance fission concept is investigated. Fission fragment angular distribution relative to the recoil direction suggested the increase of the spin of the fissioning nucleus with the number of transferred nucleons.

論文

Study of fission using multi-nucleon transfer reactions

西尾 勝久; 廣瀬 健太郎; L$'e$guillon, R.*; 牧井 宏之; Orlandi, R.; 塚田 和明; Smallcombe, J.*; 千葉 敏*; 有友 嘉浩*; 田中 翔也*; et al.

Proceedings of 6th International Conference on Fission and Properties of Neutron-rich Nuclei (ICFN-6), p.590 - 597, 2017/11

https://doi.org/10.1142/9789813229426_0101

We are promoting a fission study using multi-nucleon transfer reactions, where excited states in neutron-rich actinide nuclei, which cannot be accessed by particle capture and/or fusion reactions, are populated. This allows us to study fission in the new region of chart of nuclei. Also, the excited states in the fissioning nucleus are widely populated in the reactions, thus the effects of excitation energy on fission can be investigated. Experiments were carried out at the JAEA tandem facility in Tokai, Japan. We studied reactions using the $$^{18}$$O beam ($$sim$$9MeV/u) and several actinide target nuclei such as $$^{232}$$Th, $$^{238}$$U, $$^{237}$$Np, $$^{248}$$Cm. Ejectile nuclei and thus the transfer channels were identified by a newly developed silicon $$Delta$$E-E detectors. The produced nuclei in one experiment reached more than fifteen. Fission fragment mass distributions (FFMDs) were measured for each isotopes. Measured FFMDs are reproduced by a calculation based on the fluctuation-dissipation model (Langevin-type calculation), where effects of multi-chance fission were included. We also started to measure the prompt neutrons accompanied by fission.

論文

Experimental fission study using multi-nucleon transfer reactions

西尾 勝久; 廣瀬 健太郎; L$'e$guillon, R.*; 牧井 宏之; Orlandi, R.; 塚田 和明; Smallcombe, J.*; 千葉 敏*; 有友 嘉浩*; 田中 翔也*; et al.

EPJ Web of Conferences, 146, p.04009_1 - 04009_6, 2017/09

https://doi.org/10.1051/epjconf/201714604009
 被引用回数：3 パーセンタイル：86.43(Nuclear Science & Technology)

The objective of this work is to extend fission data for actinide nuclei not investigated so far in order to understand the fission mechanism, especially for neutron-rich nuclei. Multi-nucleon transfer reactions were used to populate the compound nuclei which cannot be accessed by particle capture or fusion reactions. The experiment was carried out at the tandem facility of Japan Atomic Energy Agency. In the multi-nucleon transfer reactions using $$^{18}$$O beam, a wide variety of nuclei around a target nucleus was produced. By identifying transfer channels, fission data of nuclei more than fifteen compound nuclei were generated in one experiment. Another feature of transfer reaction is that the excitation energies of a compound nucleus can be populated continuously from the ground state up to several tens MeV, allowing us to study the excitation energy dependence of fission properties. From the excitation function of fission probabilities, fission-barrier heights of neutron-rich actinide nuclei were obtained. Experiments were carried out in the reactions of $$^{18}$$O + $$^{238}$$U, $$^{232}$$Th, $$^{248}$$Cm, $$^{237}$$Np. The obtained fission fragment mass distributions were reproduced by a model calculation based on a fluctuation dissipation model.

論文

Origin of robust nanoscale ferromagnetism in Fe-doped Ge revealed by angle-resolved photoemission spectroscopy and first-principles calculation

坂本 祥哉*; 若林 勇希*; 竹田 幸治; 藤森 伸一; 鈴木 博人*; 伴 芳祐*; 山上 浩志; 田中 雅明*; 大矢 忍*; 藤森 淳*

Physical Review B, 95(7), p.075203_1 - 075203_5, 2017/02

https://doi.org/10.1103/PhysRevB.95.075203
 被引用回数：9 パーセンタイル：42.75(Materials Science, Multidisciplinary)

Ge$$_{1-x}$$Fe$$_x$$ (Ge:Fe) shows ferromagnetic behavior up to a relatively high temperature of 210 K and hence is a promising material for spintronic applications compatible with Si technology. We have studied its underlying electronic structure by soft X-ray angle-resolved photoemission spectroscopy measurements and first-principles supercell calculation. We observed finite Fe 3$$d$$ components in the states at the Fermi level ($$Erm_F$$) in a wide region of momentum space, and the $$Erm_F$$ was located $$sim$$0.35 eV above the valence-band maximum of the host Ge. Our calculation indicates that the $$Erm_F$$ is also within the deep acceptor-level impurity band induced by the strong $$p$$-$$d$$($$t_2$$) hybridization. We conclude that the additional minority-spin $$d(e)$$ electron characteristic of the Fe$$^{2+}$$ state is responsible for the short-range ferromagnetic coupling between Fe atoms.

論文

Origin of the large positive magnetoresistance of Ge$$_{1-x}$$Mn$$_{x}$$ granular thin films

若林 勇希*; 秋山 了太*; 竹田 幸治; 堀尾 眞史*; 芝田 悟朗*; 坂本 祥哉*; 伴 芳祐*; 斎藤 祐児; 山上 浩志; 藤森 淳*; et al.

Physical Review B, 95(1), p.014417_1 - 014417_6, 2017/01

https://doi.org/10.1103/PhysRevB.95.014417
 被引用回数：10 パーセンタイル：45.87(Materials Science, Multidisciplinary)

Ge$$_{1_x}$$Mn$$_x$$ (GeMn) granular thin films are a unique and promising material for spintronic applications owing to their large positive magnetoresistance (MR). The microscopic origin of the MR has not yet been clarified. Here, we develop a method to separately investigate the magnetic properties of the nanoparticles and the matrix, utilizing the extremely high sensitivity of X-ray magnetic circular dichroism (XMCD) to the local magnetic state of each atom. We find that the MR ratio is proportional to the product of the magnetizations originating from the nanoparticles and the matrix. This result indicates that the spin-polarized holes in the nanoparticles penetrate into the matrix and that these holes undergo first order magnetic scattering by the paramagnetic Mn atoms in the matrix, which induces the large MR.

論文

Room-temperature local ferromagnetism and its nanoscale expansion in the ferromagnetic semiconductor Ge$$_{1-x}$$Fe$$_{x}$$

若林 勇希*; 坂本 祥哉*; 竹田 幸治; 石上 啓介*; 高橋 文雄*; 斎藤 祐児; 山上 浩志; 藤森 淳*; 田中 雅明*; 大矢 忍*

Scientific Reports (Internet), 6, p.23295_1 - 23295_9, 2016/03

AA2016-0503.pdf:1.66MB

https://doi.org/10.1038/srep23295
 被引用回数：20 パーセンタイル：66.31(Multidisciplinary Sciences)

We investigate the local electronic structure and magnetic properties of the group-IV-based ferromagnetic semiconductor, Ge$$_{1-x}$$Fe$$_{x}$$ (GeFe), using soft X-ray magnetic circular dichroism. Our results show that the doped Fe 3$$d$$ electrons are strongly hybridized with the Ge 4$$p$$ states, and have a large orbital magnetic moment relative to the spin magnetic moment, namely $$m_{rm orb}$$/$$m_{rm spin}$$ $$approx$$ 0.1. We find that nanoscale local ferromagnetic regions, which are formed through ferromagnetic exchange interactions in the high-Fe-content regions of the GeFe films, exist even at room temperature, well above the Curie temperature of 20 - 100K. We observe the intriguing nanoscale expansion of the local ferromagnetic regions with decreasing temperature, followed by a transition of the entire film into a ferromagnetic state at the Curie temperature.

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