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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

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

 被引用回数:4 パーセンタイル:33.43(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

Local magnetic states of the weakly ferromagnetic iron-based superconductor Sr$$_{2}$$VFeAsO$$_{3-delta}$$ studied by X-ray magnetic circular dichroism

堀尾 眞史*; 竹田 幸治; 並木 宏允*; 片桐 隆雄*; 若林 勇希*; 坂本 祥哉*; 野中 洋亮*; 芝田 悟朗*; 池田 啓祐*; 斎藤 祐児; et al.

Journal of the Physical Society of Japan, 87(10), p.105001_1 - 105001_2, 2018/10

 被引用回数:1 パーセンタイル:77.53(Physics, Multidisciplinary)

We have performed X-ray magnetic circular dichroism (XMCD) measurements on the iron-based superconductor Sr$$_{2}$$VFeAsO$$_{3-delta}$$ to study the origin of weak ferromagnetism (WFM) reported for this compound. While Fe 3d electrons show a magnetic response similar to the other iron pnictides, signals from V 3d electrons remain finite at zero magnetic field and may be responsible for the WFM.

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

 被引用回数:3 パーセンタイル:70.78(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

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

 被引用回数:6 パーセンタイル:50.89(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

 被引用回数:6 パーセンタイル:50.89(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

Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning

Frandsen, B. A.*; Liu, L.*; Cheung, S. C.*; Guguchia, Z.*; Khasanov, R.*; Morenzoni, E.*; Munsie, T. J. S.*; Hallas, A. M.*; Wilson, M. N.*; Cai, Y.*; et al.

Nature Communications (Internet), 7, p.12519_1 - 12519_8, 2016/08

 被引用回数:13 パーセンタイル:33.61(Multidisciplinary Sciences)

RENiO$$_3$$ (RE = rare-earth element) and V$$_2$$O$$_3$$ are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO$$_3$$) or pressure (V$$_2$$O$$_3$$), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation experiments that the QPT in RENiO$$_3$$ and V$$_2$$O$$_3$$ is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition.

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

 被引用回数:11 パーセンタイル:36.98(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.

論文

Magnetization process of the $$n$$-type ferromagnetic semiconductor (In,Fe)As:Be studied by X-ray magnetic circular dichroism

坂本 祥哉*; Anh, L. D.*; Hai, P. N.*; 芝田 悟朗*; 竹田 幸治; 小林 正起*; 高橋 文雄*; 小出 常晴*; 田中 雅明*; 藤森 淳*

Physical Review B, 93(3), p.035203_1 - 035203_6, 2016/01

AA2016-0502.pdf:1.1MB

 被引用回数:8 パーセンタイル:47.49(Materials Science, Multidisciplinary)

In order to investigate the mechanism of ferromagnetic ordering in the new $$n$$-type magnetic semiconductor (In,Fe)As codoped with Be, we have performed X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) studies. The spectral line shapes suggest that the ferromagnetism is intrinsic, originating from Fe atoms incorporated into the zinc-blende-type InAs lattice. The magnetization curves of Fe measured by XMCD were well reproduced by the superposition of a Langevin function representing superparamagnetic (SPM) behavior of nanoscale ferromagnetic domains and a $$T$$-linear function representing Curie-Weiss paramagnetism even much above the Curie temperatures. The data at 20 K showed a deviation from the Langevin behavior, suggesting a gradual establishment of macroscopic ferromagnetism on lowering temperature. The existence of nanoscale ferromagnetic domains indicated by the SPM behavior suggests spatial fluctuations of Fe concentration on the nanoscale.

論文

Spin and orbital magnetic moments of Fe in the $$n$$-type ferromagnetic semiconductor (In,Fe)As

小林 正起*; Anh, L. D.*; Hai, P. N.*; 竹田 幸治; 坂本 祥哉*; 門野 利治*; 岡根 哲夫; 斎藤 祐児; 山上 浩志; 原田 慈久*; et al.

Applied Physics Letters, 105(3), p.032403_1 - 032403_4, 2014/07

AA2015-0418.pdf:0.82MB

 被引用回数:1 パーセンタイル:94.28(Physics, Applied)

The electronic and magnetic properties of Fe atoms in the ferromagnetic semiconductor (In,Fe)As codoped with Be have been studied by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Fe $$L_{2,3}$$ edge. The XAS and XMCD spectra showed simple spectral line shapes similar to Fe metal, but the ratio of the orbital and spin magnetic moments estimated using the XMCD sum rules was significantly larger than that of Fe metal, indicating a significant orbital moment of Fe 3$$d$$ electrons in (In,Fe)As:Be. The XMCD intensity as a function of magnetic field indicated hysteretic behavior of the superparamagnetic-like component due to discrete ferromagnetic domains. VC

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