Microstructures and interface magnetic moments in MnVAl/Fe layered films showing exchange bias

窪田 崇秀*; 嶋田 雄介*; 土屋 朋生*; 吉川 智己*; 伊藤 啓太*; 竹田 幸治; 斎藤 祐児; 今野 豊彦*; 木村 昭夫*; 高梨 弘毅*

Nanomaterials (Internet), 11(7), p.1723_1 - 1723_11, 2021/07

https://doi.org/10.3390/nano11071723

Heusler alloys exhibit various magnetic properties. In this study, a layered sample consisting of a Heusler alloy, MnVAl and a ferromagnet, Fe, is selected as a material system exhibiting exchange bias. Although the fully ordered MnVAl is known as a ferrimagnet, the MnVAl/Fe layered structure exhibits exchange bias. The high-angle annular dark field STEM images demonstrated the formation of MnVAl clusters with the L2 phase distributed only around the interface to the Fe layer in the sample. Furthermore, the antiferromagnetic coupling between the Mn- and Fe-moments were observed in element specific hysteresis loops measured using XMCD. The locally ordered L2 phase and antiferromagnetic Mn-moments in the MnVAl layer are important for the exchange bias.

Insight into Kondo screening in the intermediate-valence compound SmOsSb uncovered by soft X-ray magnetic circular dichroism

斎藤 祐児; 藤原 秀紀*; 保井 晃*; 門野 利治*; 菅原 仁*; 菊地 大輔*; 佐藤 英行*; 菅 滋正*; 山崎 篤志*; 関山 明*; et al.

Physical Review B, 102(16), p.165152_1 - 165152_8, 2020/10

https://doi.org/10.1103/PhysRevB.102.165152

Through a high-precision soft X-ray magnetic circular dichroism (XMCD) study of the intermediate-valence compound SmOsSb, we show our successful approach of revealing unprecedented details of Kondo screening below a characteristic temperature of 20 K in the paramagnetic phase. The multiplet XMCD structure at the Sm edge sensitive to the 4 configuration enabled us to observe a clear difference between the temperature evolution above and below with one- and two-component behavior, respectively. Our findings are in strong contrast to the conventional Kondo crossover that coincides with the valence transition, but are qualitatively accounted for by theoretical XMCD predictions combined with the two-fluid phenomenology recently proposed. This work contributes to the large context of Kondo physics that is closely related to quantum criticality in heavy fermion systems.

Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator MnBiTe

Shikin, A. M.*; Estyunin, D. A.*; Klimovskikh, I. I.*; Filnov, S. O.*; Kumar, S.*; Schwier, E. F.*; 宮本 幸治*; 奥田 太一*; 木村 昭夫*; 黒田 健太*; et al.

Scientific Reports (Internet), 10, p.13226_1 - 13226_13, 2020/08

https://doi.org/10.1038/s41598-020-70089-9

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator MnBiTe and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures, light polarizations and photon energies. We have distinguished both large and reduced gaps at the DP in the ARPES dispersions, which remain open above the Nel temperature of = 24.5 K. We propose that the gap above remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the large gap sample and apparently significantly reduced effective magnetic moment for the reduced gap sample.

Element-specific density of states of CoMnGe revealed by resonant photoelectron spectroscopy

河野 嵩*; 鹿子木 将明*; 吉川 智己*; Wang, X.*; 角田 一樹*; 宮本 幸治*; 室 隆桂之*; 竹田 幸治; 斎藤 祐児; 後藤 一希*; et al.

Physical Review B, 100(16), p.165120_1 - 165120_6, 2019/10

https://doi.org/10.1103/PhysRevB.100.165120

Resonant photoelectron spectroscopy at the Co and Mn 2 core absorption edges of half-metallic CoMnGe has been performed to determine the element-specific density of states (DOS). A significant contribution of the Mn 3 partial DOS near the Fermi level () was clarified by measurement at the Mn 2 absorption edge. Further analysis by first-principles calculation revealed that it has symmetry, which must be responsible for the electrical conductivity along the line perpendicular to the film plane. The dominant normal Auger contribution observed at the Co 2 absorption edge indicates delocalization of photoexcited Co 3 electrons. The difference in the degrees of localization of the Mn 3 and Co 3 electrons in CoMnGe is explained by the first-principles calculation.

Electronic structure of the high- 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 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 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.

Negative Te spin polarization responsible for ferromagnetic order in the doped topological insulator V(SbBi)Te

Ye, M.*; Xu, T.*; Li, G.*; Qiao, S.*; 竹田 幸治; 斎藤 祐児; Zhu, S.-Y.*; Nurmamat, M.*; 角田 一樹*; 石田 行章*; et al.

Physical Review B, 99(14), p.144413_1 - 144413_7, 2019/04

https://doi.org/10.1103/PhysRevB.99.144413

We investigate the microscopic origin of ferromagnetism coupled with topological insulators in V-doped (Sb,Bi)Te employing X-ray magnetic circular dichroism and angle-resolved two-photon photoemission spectroscopies, combined with first-principles calculations. We found an magnetic moment at the Te site anti-parallel to that of the V and Sb sites, which plays a key role in the ferromagnetic order. We ascribe it to the hybridization between Te 5 and V 3 majority spin states at the Fermi energy, consistent with the Zener-type - exchange interaction scenario. The substitution of Bi for Sb suppresses the bulk ferromagnetism by introducing extra electron carriers in the majority spin channel of the Te states that compensates the antiparallel magnetic moment on the Te site. Our findings reveal important clues to designing magnetic topological insulators with higher Curie temperature that work under ambient conditions.

Manifestation of electron correlation effect in 5 states of uranium compounds revealed by 4-5 resonant photoelectron spectroscopy

藤森 伸一; 小畠 雅明; 竹田 幸治; 岡根 哲夫; 斎藤 祐児; 藤森 淳; 山上 浩志; 芳賀 芳範; 山本 悦嗣; 大貫 惇睦*

Physical Review B, 99(3), p.035109_1 - 035109_5, 2019/01

https://doi.org/10.1103/PhysRevB.99.035109

In the present study, we have elucidated the nature of the electron correlation effect in uranium compounds by imaging the partial density of states (pDOS) of typical itinerant, localized, and heavy fermion uranium compounds by using the 4-5 resonant photoemission spectroscopy. Obtained pDOS exhibit a systematic trend depending on the physical properties of compounds: Although the coherent peak at the Fermi level can be explained by the band-structure calculation, an incoherent peak emerges on the higher binding energy side () in the cases of localized and heavy fermion compounds. The intensity and energy position of the incoherent peak is increased and shifted to a higher binding energy as the localization of the state increases. These behaviors are consistent with the prediction of the Mott metal-insulator transition, suggesting that the Hubbard- type mechanism takes an essential role in the electronic structure of actinide materials.

Soft X-ray magnetic circular dichroism study on UGe

竹田 幸治; 岡根 哲夫; 斎藤 祐児; 山上 浩志; 山本 悦嗣; 芳賀 芳範

Progress in Nuclear Science and Technology (Internet), 5, p.171 - 174, 2018/11

In order to investigate the electronic and magnetic states of UGe element-specifically, we have performed soft X-ray magnetic circular dichroism experiments at the U N and Ge L absorption edges. We have detected the XMCD signals at both the U and Ge sites and observed clear hysteresis loops in the ferromagnetic (FM) state at T = 5.5 K. From the branching ratio B, it is found that the occupation number of 5f electrons (n) in UGe is close to 3. In addition, applying the magneto-optical sum rules analysis to the XMCD spectrum at the U N edges, the ratio of orbital magnetic moment to the spin magnetic moment (-M/M) is estimated to be 2.17. These values of B and -M/M are comparable compared with the results of other FM superconductors: URhGe and UCoGe

Cation distribution and magnetic properties in ultrathin (NiCo)FeO (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

We study the electronic structure and magnetic properties of epitaxial (NiCo)FeO(111) layers with thicknesses = 1.7 - 5.2 nm grown on AlO(111)/Si(111) structures. We revealed the crystallographic (octahedral or tetrahedral ) 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.

Element-specific observation of the ferromagnetic ordering process in UCoAl via soft X-ray magnetic circular dichroism

竹田 幸治; 斎藤 祐児; 岡根 哲夫; 山上 浩志; 松田 達磨*; 山本 悦嗣; 芳賀 芳範; 大貫 惇睦*

Physical Review B, 97(18), p.184414_1 - 184414_7, 2018/05

AA2018-0316.pdf:0.62MB

https://doi.org/10.1103/PhysRevB.97.184414

We have performed soft X-ray magnetic circular dichroism (XMCD) experiments on the itinerant-electron metamagnet UCoAl at the U 4-5 () and Co 2-3 () absorption edges in order to investigate the magnetic properties of the U 5 and Co 3 electrons separately. From the line shape of the XMCD spectrum, it is deduced that the orbital magnetic moment of the Co 3 electrons is unusually large. Through the systematic temperature ()- and magnetic field ()-dependent XMCD measurements, we have obtained two types of the magnetization curve as a function of and (M-H curve and M-T curve, respectively). The metamagnetic transition from a paramagnetic state to a field-induced ferromagnetic state was observed clearly under 15 K at . The value of the and its -dependence agree well between the U and Co sites, and the bulk magnetization. Whereas, we have discovered the remarkable differences in the M-H and M-T curves between the U and Co sites. The present findings show clearly that the role of the Co 3 electrons should be considered more carefully in order to understand the origin of the magnetic ordering in UCoAl.