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論文

Origin of magnetically dead layers in spinel ferrites FeO grown on AlO; 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 FeO(111)/AlO3(111) (Co, Ni) thin films with various thicknesses grown on Si(111) substrates with the -AlO(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 AlO 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 sites of the spinel structure, which may be the microscopic origin of the degraded ferrimagnetic order in the FeO thin films.

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 electrons, whereas the former indicates their itinerant nature. It was also found that the Fe 3 states have a finite contribution to the density of states at the Fermi energy. These states, presumably consisting of majority-spin - 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：65.43(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 electrons, whereas the former indicates their itinerant nature. It was also found that the Fe 3 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

https://doi.org/10.7566/JPSJ.87.105001

被引用回数：2 パーセンタイル：20.94(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

https://doi.org/10.1103/PhysRevMaterials.2.104416

被引用回数：10 パーセンタイル：37.21(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 = 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

https://doi.org/10.1103/PhysRevB.95.075203

被引用回数：9 パーセンタイル：42.46(Materials Science, Multidisciplinary)

Ge $_{1-x}$ Fe (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 components in the states at the Fermi level () in a wide region of momentum space, and the was located 0.35 eV above the valence-band maximum of the host Ge. Our calculation indicates that the is also within the deep acceptor-level impurity band induced by the strong -() hybridization. We conclude that the additional minority-spin 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：11 パーセンタイル：45.59(Materials Science, Multidisciplinary)

Ge $_{1_x}$ Mn (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(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 electrons are strongly hybridized with the Ge 4 states, and have a large orbital magnetic moment relative to the spin magnetic moment, namely $m_{rm orb}$ / $m_{rm spin}$ 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.