"published_papers" "Title(English)","Title(Japanese)","Author(s)(English)","Author(s)(Japanese)","Journal name(English)","Journal name(Japanese)","Volume","Number","First page","Last page","Date of publication","Peer reviewed/Not peer reviewed","Invited/Not invited","Publishing type","Language","ISSN","DOI","URL","Description(English)","Description(Japanese)" "Ultrafast spin-to-charge conversion in antiferromagnetic (111)-oriented L12-Mn3Ir","Ultrafast spin-to-charge conversion in antiferromagnetic (111)-oriented L12-Mn3Ir","Huiling Mao, Yuta Sasaki, Yuta Kobayashi, Shinji Isogami, Teruo Ono, Takahiro Moriyama, Yukiko K. Takahashi, Kihiro T. Yamada","Huiling Mao, Yuta Sasaki, Yuta Kobayashi, Shinji Isogami, Teruo Ono, Takahiro Moriyama, Yukiko K. Takahashi, Kihiro T. Yamada","Applied Physics Letters","Applied Physics Letters","123","21","","","2023-11-20","TRUE","","scientific_journal","eng","","10.1063/5.0168138","","Antiferromagnetic L12-Mn3Ir combines outstanding spin-transport properties with magnons in the terahertz (THz) frequency range. However, the THz radiation emitted by ultrafast spin-to-charge conversion via the inverse spin Hall effect remains unexplored. In this study, we measured the THz emission and transmission of a permalloy/(111)-oriented L12-Mn3Ir multilayer by THz time-domain spectroscopy. ","" "Probability of spin-orbit torque driven magnetization switching assisted by spin-transfer torque","Probability of spin-orbit torque driven magnetization switching assisted by spin-transfer torque","Tomohiro Taniguchi, Shinji Isogami, Shuji Okame, Katsuyuki Nakada, Eiji Komura, Tomoyuki Sasaki, Seiji Mitani, Masamitsu Hayashi","Tomohiro Taniguchi, Shinji Isogami, Shuji Okame, Katsuyuki Nakada, Eiji Komura, Tomoyuki Sasaki, Seiji Mitani, Masamitsu Hayashi","Physical Review B","Physical Review B","108","13","","","2023-10-01","TRUE","","scientific_journal","eng","","10.1103/physrevb.108.134431","","Spin-orbit torque (SOT) driven magnetization switching, assisted by spin-transfer torque (STT), enables field-free switching in ferromagnetic nanostructures and is expected to be a writing method for next-generation spintronic nonvolatile memory. The role of STT is to shift the magnetization pointing in an in-plane direction via the SOT to a different direction and ensure the switching. Here, we study the dependence of the switching probability on the STT strength using a numerical simulation of the Landau-Lifshitz-Gilbert (LLG) equation. While a monotonic increase of the switching probability with increasing STT strength is found in a relatively weak STT region, we find an unexpected increase in the error rate in a relatively large STT current close to a critical current. Based on the statistical analysis of the magnetization dynamics and solving the LLG equation analytically, we reveal that the origin of the switching error is the presence of an inactive region in the Bloch sphere where the magnetization dynamics becomes very slow compared with conventional SOT switching. Since this region exists far away from the initial state of the magnetization, a strong STT is necessary to reach the region. Accordingly, the switching error increases in a strong STT region. We also show that the issue can be solved by reducing the time only the STT is applied and/or enhancing the SOT strength.","" "Boron-induced magneto-optical Kerr spectra and dielectric tensors in ferrimagnetic (Mn4N)B antiperovskite thin films","Boron-induced magneto-optical Kerr spectra and dielectric tensors in ferrimagnetic (Mn4N)B antiperovskite thin films","Hotaka Sakaguchi, Shinji Isogami, Makoto Niimi, Takayuki Ishibashi","Hotaka Sakaguchi, Shinji Isogami, Makoto Niimi, Takayuki Ishibashi","Journal of Physics D: Applied Physics","Journal of Physics D: Applied Physics","56","36","365002","","2023-09-07","TRUE","","scientific_journal","eng","","10.1088/1361-6463/acd9d4","","B-induced electronic states of antiperovskite nitrides such as ferrimagnetic Mn4N were investigated via a combination of polar magneto-optical Kerr effect (p-MOKE) spectroscopy and spectroscopic ellipsometry (SE). The B content in the Mn4N film varied from 0 to 4.3 at.%, for which the crystal structure was maintained. The amplitude of p-MOKE spectra and the diagonal and off-diagonal dielectric tensors decreased with increasing B content, which is in agreement with magnetic properties such as magnetic anisotropy and saturation magnetization. These results were related to the lattice expansion and displacement of the charge density in Mn4N by B doping. However, the peak energy of Lorentz oscillator in the diagonal elements of dielectric tensors suggests that a dominant inter-band transition was independent of B content.","" "Introduction of VN underlayer and caplayer for preparation of Mn4N(001) single-crystal thin film with perpendicular magnetic anisotropy","Introduction of VN underlayer and caplayer for preparation of Mn4N(001) single-crystal thin film with perpendicular magnetic anisotropy","Kosuke Imamura, Mitsuru Ohtake, Shinji Isogami, Masaaki Futamoto, Tetsuroh Kawai, Fumiyoshi Kirino, Nobuyuki Inaba","Kosuke Imamura, Mitsuru Ohtake, Shinji Isogami, Masaaki Futamoto, Tetsuroh Kawai, Fumiyoshi Kirino, Nobuyuki Inaba","AIP Advances","AIP Advances","13","2","","","2023-02-01","TRUE","","scientific_journal","eng","","10.1063/9.0000572","","A Mn4N thin film is prepared on MgO(001) single-crystal substrate by introducing VN underlayer and caplayer which are respectively expected to inhibit the oxidation from substrate and surface. The thin film is prepared by ultra-high vacuum radio-frequency magnetron sputtering. The growth, the structure, and the magnetic properties are investigated. A fully epitaxial VN/Mn4N/VN film with sharp interfaces is formed on the substrate. The Mn4N film has low out-of-plane and in-plane orientation dispersions of about 1degree and high N site order degree of 0.88. The lattice of Mn4N film is slightly deformed along the perpendicular direction (c/a = 0.9872) possibly due to accommodation of the lattice mismatch at Mn4N/VN interfaces. The film shows a low saturation magnetization of 85 kA/m (85 emu/cm3) and a strong perpendicular magnetic anisotropy. The present study has shown that introduction of VN underlayer and caplayer is useful for preparation of well-defined Mn4N thin films with perpendicular magnetic anisotropy.","" "Carbon-induced magnetic properties and anomalous Hall effect in Co2Mn2C thin films with L10-like structures","Carbon-induced magnetic properties and anomalous Hall effect in Co2Mn2C thin films with L10-like structures","Shinji Isogami, Yohei Kota, Hideyuki Yasufuku, Keiji Oyoshi, Masahiko Tanaka, Yukiko K. Takahashi","Shinji Isogami, Yohei Kota, Hideyuki Yasufuku, Keiji Oyoshi, Masahiko Tanaka, Yukiko K. Takahashi","Physical Review Materials","Physical Review Materials","7","1","","","2023-01-01","TRUE","","scientific_journal","eng","","10.1103/physrevmaterials.7.014411","","Co2Mn2C thin films were synthesized via vacuum carburization of the host CoMn alloy films based on a conventional gas–solid reaction to demonstrate the effect of C, a common light element, on the magnetic and spintronic materials. The crystal structure transitioned from the disordered face-centered cubic CoMn to the L10-like Co2Mn2C, for which the lattice constant increased from 0.356 to 0.378 nm. The C 1s X-ray photoemission spectra of the Co2Mn2C film indicated hybridization in C–Co and C–Mn and a homogeneous concentration of C in the film. The enhancement of both the saturation magnetization and the anomalous Hall conductivity was induced by C, attributing to the magnetic transition to the ferrimagnetic spin order. The surface flatness and high anomalous Hall conductivity are promising characteristics for spintronic applications such as the spin-anomalous Hall effect. The atomic- and spin-resolved density of states (DOS) via first-principles calculations revealed that face-centered Mn(II) and Co could be significantly influenced by C because of p–d hybridization, resulting in enhanced spin polarization of the DOS at the Fermi level of ~0.82. These results demonstrate that the use of C could be an essential way to boost material properties in the future.","" "Antiperovskite Magnetic Materials with 2p Light Elements for Future Practical Applications","Antiperovskite Magnetic Materials with 2p Light Elements for Future Practical Applications","Shinji Isogami, Yukiko K. Takahashi","Shinji Isogami, Yukiko K. Takahashi","Advanced Electronic Materials","Advanced Electronic Materials","9","1","","","2023-01-01","TRUE","","scientific_journal","eng","","10.1002/aelm.202200515","","Light elements having 2p electrons such as B, C, and N are common elements that have played an important role in various functional materials. In particular, nitrides have long been used in the development of semiconductors, superconductors, and magnets. More recently, Fe- and Mn-based antiperovskite-type compounds with light elements have attracted considerable attention in the field of spintronic engineering, because of the development of intriguing and practical applications making them one of the key materials for future devices. In this article, we first review the evolution of applications and which light elements are employed. We then individually highlight the representative applications and characteristics of the light elements, including magnetoresistive effects, magnetization switching, current-spin and thermoelectric conversions, magnetic anisotropy, and domain nucleation. Additionally, the crystal structure, fundamental properties, and theoretical study are addressed to enable a deeper understanding of the role of light elements in the unit cell. Beyond compounds with N, a demonstration using B and C is discussed to examine their effect on the magnetic structures of antiperovskite compounds. Finally, prospective and future strategies are discussed to build a platform of practical material based on light elements.","" "次世代スピントロニクス実用材料へ向けた逆ペロブスカイト型軽元素含有遷移金属膜の開発現状~Fe4N, Mn4N薄膜を例に~","次世代スピントロニクス実用材料へ向けた逆ペロブスカイト型軽元素含有遷移金属膜の開発現状~Fe4N, Mn4N薄膜を例に~","磯上 慎二","磯上 慎二","Magnetics Japan","Magnetics Japan","17","2","89","101","2022","TRUE","","scientific_journal","jpn","","","","","Transition metal nitrides such as Fe4N and Mn4N thin films, families of anti-perovskite type compounds with light elements, were used, and the validity against recent spintronic engineering is discussed. Inverse tunneling magnetoresistance and inverse current-induced magnetization switching were observed for magnetic tunnel junctions (MTJs) with a Fe4N electrode. On the basis of first-principles calculation, it was revealed that the perpendicular magnetic anisotropy for the Mn4N thin films originates from not only the spin-conservation process but also spin-flip process. The anomalous Nernst voltage for Mn4N thin films opens a new pathway for thermoelectric devices owing to its low saturation magnetization. The electrical switching for a (111)-oriented Mn4N single layer was efficient due to its non-collinear magnetic structure. A recent study demonstrated topological states such as skyrmions for Mn4N thin films. These results show the advantages of light elements for future practical spintronic materials." "Wide modulation of coercive fields in Mn4N ferrimagnetic thin films caused dominantly by dislocation microstructures","Wide modulation of coercive fields in Mn4N ferrimagnetic thin films caused dominantly by dislocation microstructures","Shinji Isogami, Mitsuru Ohtake, Yusuke Kozuka, Yukiko K. Takahashi","Shinji Isogami, Mitsuru Ohtake, Yusuke Kozuka, Yukiko K. Takahashi","Journal of Magnetism and Magnetic Materials","Journal of Magnetism and Magnetic Materials","560","","169642","","2022-10-01","TRUE","","scientific_journal","eng","","10.1016/j.jmmm.2022.169642","","Mn4N films with a maximum degree of order of nitrogen (S) were fabricated under three different substrate temperatures (Tsub), and the dominant factors were investigated for tuning the magnetic coercivity (Hc) in the films. It was revealed that Hc was strongly modulated from 10 kOe to 7.5 kOe for Tsub from 400 to 500 degree C, whereas the S, perpendicular magnetic anisotropy (PMA), and transport properties did not change. The number of dislocations and in-plane grain size (D) increased, whereas the residual strain decreased with increasing Tsub. Thus, we concluded that the Hc of the Mn4N film with maximum S is sensitive to the variation in dislocations, which are modulated by Tsub. Because Hc is linked to thermal stability and working power consumption, tunable Hc is an attractive phenomenon from the perspective of magnetic and spintronic devices using ferrimagnetic Mn4N films with high PMA and low saturation magnetization.","" "Effects of Composition on the Ordered Phase Formation in Mn-Al and Mn-Ge Alloy Thin Films Grown on Cr(001) Single-Crystal Underlayers","Effects of Composition on the Ordered Phase Formation in Mn-Al and Mn-Ge Alloy Thin Films Grown on Cr(001) Single-Crystal Underlayers","Shota Noro, Kotaro Nakano, Mitsuru Ohtake, Shinji Isogami, Masaaki Futamoto, Tetsuroh Kawai, Fumiyoshi Kirino, Nobuyuki Inaba","Shota Noro, Kotaro Nakano, Mitsuru Ohtake, Shinji Isogami, Masaaki Futamoto, Tetsuroh Kawai, Fumiyoshi Kirino, Nobuyuki Inaba","IEEE Transactions on Magnetics","IEEE Transactions on Magnetics","58","9","1","5","2022-09-01","TRUE","","scientific_journal","eng","","10.1109/tmag.2022.3151235","","MnxAl100–x and MnyGe100–y alloy epitaxial films of 20 nm thickness are prepared on Cr(001) single-crystal underlayers at 300 °C by varying the Mn content, x or y, from 0 to 100 at. %. The formation of crystallographic phase is systematically investigated as a function of composition by reflection high-energy electron and X-ray diffractions. The phases in MnxAl100–x and MnyGe100–y films are determined to vary in the orders of A1 => L10+A12 => L10 => A2 => A12 and A4 => tP => cP => D022 => A12 with increasing the Mn content, respectively. Metastable fct-based ordered L10 phase is formed in the MnxAl100–x films with x = 50–65. L10 single phase is obtained at an Mn-rich composition of x = 58, whereas bcc-based disordered A12 and A2 phases are respectively involved at the L10 stoichiometric composition of x = 50 and at a more Mn-rich composition of x = 65. Employment of a slight Mn-rich composition stabilizes L10 phase. On the other hand, metastable fct-based ordered D022 phase is formed in the MnyGe100–y films with y = 60–75. D022 single phase is obtained in the range from the stoichiometric composition of y = 75 to a slightly Mn-poor pomposition of y = 68. The c-axis orientation is normal to the film surface for both L10 and D022 crystals. The present study has clarified the crystal structure and growth of MnxAl100–x and MnyGe100–y films for the respective full compositional ranges and has shown that control of the composition is important to obtain L10 and D022 ordered Mn-based alloy films with perpendicular magnetic anisotropies.","" "Epitaxial Growth of γʹ-Fe4N Thin Films on MgO(001) Single-Crystal Substrates","MgO(001)単結晶基板上におけるγʹ-Fe4N薄膜のエピタキシャル成長","今村 光佑, 前田 悠良, 大竹 充, 磯上 慎二, 二本 正昭, 川井 哲郎, 桐野 文良, 稲葉 信幸","今村 光佑, 前田 悠良, 大竹 充, 磯上 慎二, 二本 正昭, 川井 哲郎, 桐野 文良, 稲葉 信幸","日本磁気学会論文特集号","日本磁気学会論文特集号","6","2","105","110","2022-09-01","TRUE","","scientific_journal","jpn","","10.20819/msjtmsj.22tr620","","","Fe-N thin films are prepared on MgO(001) single-crystal substrates at temperatures ranging from room temperature to 600 °C by varying the ratio of N2 partial to total pressure in sputtering from 0 to 5%. The effects of substrate temperature and N2 partial pressure ratio on the formation of γʹ phase (sc-based L ’1 phase) are systematically investigated. Epitaxial α phase (bcc-based A2 phase) is formed in the films prepared by sputtering in pure Ar gas for all the investigated temperatures. On the other hand, the structure is delicately influenced by the substrate temperature when the films are prepared in Ar-N2 mixture gases. Epitaxial films are obtained at temperatures higher than 300 °C, whereas the films prepared at temperatures lower than 200 °C involve poly-crystals. When the N2 partial pressure ratio is 2.5%, the films prepared at temperatures ranging between 200 and 400 °C consist of a mixture of a and γʹ phases and the volume fraction of γʹ phase decreases with increasing the substrate temperature. When the N2 partial pressure ratio is 5%, γʹ single-crystal films are obtained at temperatures ranging from 300 to 400 °C. The N site ordering parameter increases from 0.9 to 1 as the substrate temperature increases from 200 to 400 °C. The films prepared at temperatures higher than 500 °C in the N2 partial pressure ratios of 2.5 and 5% primarily consist of a phase. The present study has shown that high substrate temperature prevents the formation of γʹ phase and employment of a moderate substrate temperature around 300–400 °C is suitable to obtain a γʹ single-crystal film." "Magnetization switching probability in the dynamical switching regime driven by spin-transfer torque","Magnetization switching probability in the dynamical switching regime driven by spin-transfer torque","Tomohiro Taniguchi, Shinji Isogami, Yohei Shiokawa, Yugo Ishitani, Eiji Komura, Tomoyuki Sasaki, Seiji Mitani, Masamitsu Hayashi","Tomohiro Taniguchi, Shinji Isogami, Yohei Shiokawa, Yugo Ishitani, Eiji Komura, Tomoyuki Sasaki, Seiji Mitani, Masamitsu Hayashi","Physical Review B","Physical Review B","106","10","","","2022-09-01","TRUE","","scientific_journal","eng","","10.1103/physrevb.106.104431","","We study magnetization switching in the dynamical regime for in-plane magnetized systems, useful for field-free spin orbit torque switching devices. We derive a formula for the switching probability, characterized by the thermal stability factor, critical current density and the linewidth of ferromagnetic resonance. The formula agrees well with numerical simulations based on the Landau- Lifshitz-Gilbert equation. To study the viability of the theory developed, the switching probability of an in-plane magnetized ferromagnet in three terminal spin orbit torque switching devices is measured. We find the transition width of the switching probability vs. current increases with decreasing pulse width. The shape of the probability density, the current derivative of the switching probability, changes with varying pulse width. These characteristics are in good agreement with the theory developed. From the analyses, we show that the dynamical and thermally activated switching regimes can be distinguished simply from the shape of the probability density. The formula therefore provides useful means to analyze the switching probability of two terminal spin torque and three terminal spin orbit torque switching devices.","" "Impact of B-doping on topological Hall resistivity in (111)- and (110)-oriented Mn4N single layers with the non-collinear spin structure","Impact of B-doping on topological Hall resistivity in (111)- and (110)-oriented Mn4N single layers with the non-collinear spin structure","Shinji Isogami, Mitsuru Ohtake, Yukiko K. Takahashi","Shinji Isogami, Mitsuru Ohtake, Yukiko K. Takahashi","Journal of Applied Physics","Journal of Applied Physics","131","7","","","2022-02-21","TRUE","","scientific_journal","eng","","10.1063/5.0083042","","Controllability of the topological Hall resistivity via the doping effect of light elements was investigated for the sputter-deposited (111)-oriented Mn4N single layer. The component of THE relative to the anomalous Hall resistivity for host Mn4N was found to increase with decreasing temperature. Boron (B), one of the 2p light elements acting as an interstitial impurity, was doped to the (111)-oriented Mn4N single layer. The microstrain, grain diameter, and surface roughness were found to decrease, resulting in the reduction of THE for all temperatures without a change in the antiperovskite bone structure of Mn4N. These results show a dilution effect in the spin frustration state with topological spin texture by B-doping. The effect of B on THE for a different orientation of (110) was similar to that of (111), while the enhancement of THE was observed by a higher amount of B. B-doping could, thus, be a promising approach to realize tailor-made spintronic devices based on the topological spin state owing to its material versatility.","" "Spin–orbit torque driven magnetization switching in W/CoFeB/MgO-based type-Y three terminal magnetic tunnel junctions","Spin–orbit torque driven magnetization switching in W/CoFeB/MgO-based type-Y three terminal magnetic tunnel junctions","Shinji Isogami, Yohei Shiokawa, Atsushi Tsumita, Eiji Komura, Yugo Ishitani, Kosuke Hamanaka, Tomohiro Taniguchi, Seiji Mitani, Tomoyuki Sasaki, Masamitsu Hayashi","Shinji Isogami, Yohei Shiokawa, Atsushi Tsumita, Eiji Komura, Yugo Ishitani, Kosuke Hamanaka, Tomohiro Taniguchi, Seiji Mitani, Tomoyuki Sasaki, Masamitsu Hayashi","Scientific Reports","Scientific Reports","11","1","","","2021-12-01","TRUE","","scientific_journal","eng","","10.1038/s41598-021-95422-8","","We have studied current induced magnetization switching in W/CoFeB/MgO based three terminal magnetic tunnel junctions. The switching driven by spin—orbit torque (SOT) is evaluated in the so-called type-Y structure, in which the magnetic easy-axis of the CoFeB layer lies in the film plane and is orthogonal to the current flow. The effective spin Hall angle estimated from the bias field dependence of critical current (Ic) is ~ 0.07. The field and current dependence of the switching probability are studied. The field and DC current induced switching can be described using a model based on thermally assisted magnetization switching. In contrast, the 50 ns long pulse current dependence of the switching probability shows significant deviation from the model, even if contribution from the field-like torque is included. The deviation is particularly evident when the threshold switching current is larger. These results show that conventional thermally assisted magnetization switching model cannot be used to describe SOT induced switching using short current pulses.","" "Efficient current-driven magnetization switching owing to isotropic magnetism in a highly symmetric 111-oriented Mn4N epitaxial single layer","Efficient current-driven magnetization switching owing to isotropic magnetism in a highly symmetric 111-oriented Mn4N epitaxial single layer","Shinji Isogami, Nagalingam Rajamanickam, Yusuke Kozuka, Yukiko K. Takahashi","Shinji Isogami, Nagalingam Rajamanickam, Yusuke Kozuka, Yukiko K. Takahashi","AIP Advances","AIP Advances","11","10","105314","","2021-10-01","TRUE","","scientific_journal","eng","","10.1063/5.0062253","","We investigated in-plane current-induced magnetization switching in a Mn4N epitaxial single layer. Efficient magnetization switching was detected via the measurement of anomalous Hall resistivity after the application of current pulses, with a duration of 1 s, to the 111-oriented Mn4Nfilm compared with a reference 001-oriented Mn4N film. The threshold current density of magnetization switching, 1 MA/cm2, was relatively low compared with that reported for magnetic tunnel junctions and/or ferromagnet/heavy metal bilayer systems. The relatively low current density in the 111-oriented film was attributed to the low magnetic anisotropy on the (111) plane of Mn4N owing to the isotropic crystal symmetry as revealed by x-ray diffraction and transmission electron microscopy as a reduced switching barrier boosts the probability of magnetization switching. It was concluded that manipulation of the magnetic anisotropy based on the crystal orientation is one of the promising approaches to develop materials suitable for application in highly efficient switching devices with Mn4N layers.","" "Magnetic Properties in CH3NH3PbI3 Perovskite Thin Films by Mn Doping","Magnetic Properties in CH3NH3PbI3 Perovskite Thin Films by Mn Doping","Nagalingam Rajamanickam, Towhid H. Chowdhury, Shinji Isogami, Ashraful Islam","Nagalingam Rajamanickam, Towhid H. Chowdhury, Shinji Isogami, Ashraful Islam","The Journal of Physical Chemistry C","The Journal of Physical Chemistry C","125","36","20104","20112","2021-09-16","TRUE","","scientific_journal","eng","","10.1021/acs.jpcc.1c06558","","In recent years, perovskite halide compounds have attracted attention for the fabrication of highly efficient solar cells, light-emitting diodes, and X-ray detection. However, a comprehensive understanding of their microscopic origins has not been fully explored. In this work, the effect of Mn doping in organic−inorganic perovskite semiconductor methylammonium lead iodide (CH3NH3PbI3) has been studied. The existence of magnetism in CH3NH3PbI3 (MAPbI3) has been confirmed by magnetization measurements at room temperature. A drastic enhancement in magnetic moment is obtained in Mn (15%)-doped MAPbI3.","" "Observation of Nonlinear Spin-Charge Conversion in the Thin Film of Nominally Centrosymmetric Dirac Semimetal SrIrO3 at Room Temperature","Observation of Nonlinear Spin-Charge Conversion in the Thin Film of Nominally Centrosymmetric Dirac Semimetal SrIrO3 at Room Temperature","Y. Kozuka, S. Isogami, K. Masuda, Y. Miura, Saikat Das, J. Fujioka, T. Ohkubo, S. Kasai","Y. Kozuka, S. Isogami, K. Masuda, Y. Miura, Saikat Das, J. Fujioka, T. Ohkubo, S. Kasai","Physical Review Letters","Physical Review Letters","126","23","","","2021-06-01","TRUE","","scientific_journal","eng","","10.1103/physrevlett.126.236801","","Spin-charge conversion via spin-orbit interaction is one of the core concepts in the current spintronics research. The efficiency of the interconversion between charge and spin current is estimated based on Berry curvature of Bloch wavefunction in the linear-response regime. Beyond the linear regime, nonlinear spin-charge conversion in the higher-order electric field terms has recently been demonstrated in noncentrosymmetric materials with nontrivial spin texture in the momentum space. Here we report the observation of the nonlinear charge-spin conversion in a nominally centrosymmetric oxide material, SrIrO3, by breaking inversion symmetry at the interface. A large second-order magnetoelectric coefficient is observed at room temperature because of the antisymmetric spin-orbit interaction at the interface of Dirac semimetallic bands, which is subject to the symmetry constraint of the substrates. Our study suggests that nonlinear spin-charge conversion can be induced in many materials with strong spin-orbit interaction at the interface by breaking the local inversion symmetry to give rise to spin splitting in otherwise spin degenerate systems.","" "Long-term outdoor observation of geomagnetic fields using tunneling magnetoresistance-based magnetometer and detection of long-period oscillations of geomagnetic pulsations: prospects for earthquake disaster prevention","Long-term outdoor observation of geomagnetic fields using tunneling magnetoresistance-based magnetometer and detection of long-period oscillations of geomagnetic pulsations: prospects for earthquake disaster prevention","Shinji Isogami, Yuto Oishi, Chikara Furukawa, Nobuhito Ohnishi, Kan Okubo","Shinji Isogami, Yuto Oishi, Chikara Furukawa, Nobuhito Ohnishi, Kan Okubo","Applied Physics Express","Applied Physics Express","14","5","056502","","2021-05-01","TRUE","","scientific_journal","eng","","10.35848/1882-0786/abf66a","","We investigated the appropriateness of a tunneling magnetoresistance (TMR)-based magnetometer for detecting geomagnetic fields with long period oscillations through observation of the geomagnetic pulsation (Pi). Pi was not sufficiently observed in the raw outputs due to thermal noise; a bandwidth-limitation treatment helped extract Pi without data stacking. The detectable intensity was reduced to 1 nT with a period of 30 s, but detection was impossible for 100 s. The frequency characteristics revealed that reducing the cutoff frequency of a TMR magnetometer is a promising approach for detecting field oscillations occurring over long periods, such as 100 s.","" "Anomalous Hall and Nernst effects in ferrimagnetic Mn4N films: Possible interpretations and prospects for enhancement","Anomalous Hall and Nernst effects in ferrimagnetic Mn4N films: Possible interpretations and prospects for enhancement","Shinji Isogami, Keisuke Masuda, Yoshio Miura, Nagalingam Rajamanickam, Yuya Sakuraba","Shinji Isogami, Keisuke Masuda, Yoshio Miura, Nagalingam Rajamanickam, Yuya Sakuraba","Applied Physics Letters","Applied Physics Letters","118","9","092407","","2021-03-01","TRUE","","scientific_journal","eng","","10.1063/5.0039569","","Ferrimagnetic Mn4N is a promising material for heat flux sensors, based on the anomalous Nernst effect (ANE), because of its sizeable uniaxial magnetic anisotropy (Ku) and low saturation magnetization (Ms). We experimentally and theoretically investigated the ANE and anomalous Hall effect in sputter-deposited Mn4N films. It was revealed that the observed negative anomalous Hall conductivity (σxy) could be explained by two different coexisting magnetic structures, that is, a dominant magnetic structure with high Ku, contaminated by another structure with negligible Ku, owing to an imperfect degree of order of N. The observed transverse thermoelectric power (S_ANE) of 0.5 uV/K at 300 K yielded a transverse thermoelectric coefficient (αxy) of 0.34 A/(m K), which was smaller than the value predicted from the first-principles calculation. The interpretation for αxy based on the first-principles calculations led us to conclude that the realization of single magnetic structure with high Ku and optimal adjustment of the Fermi level are promising approaches to enhance S_ANE in Mn4N through the sign reversal of σxy and the enlargement of αxy up to a theoretical value of 1.77 A/(m K).","" "Effect of Co doping for improved photovoltaic performance of dye-sensitized solar cells in BaSnO3 nanostructures","Effect of Co doping for improved photovoltaic performance of dye-sensitized solar cells in BaSnO3 nanostructures","Nagalingam Rajamanickam, Shinji Isogami, Kathirvel Ramachandran","Nagalingam Rajamanickam, Shinji Isogami, Kathirvel Ramachandran","Materials Letters","Materials Letters","275","","128139","","2020-09-01","TRUE","","scientific_journal","eng","","10.1016/j.matlet.2020.128139","","太陽電池膜として注目を集めるペロブスカイト酸化物BaSnO3の結晶に強磁性元素Coが相分離することなく置換添加されることを確かめた.本材料において置換前に対し,光電変換効率の増大を達成した.これは,BaSnO3:Co層と電子/ホール透過層との格子ミスフィットが調整された結果,インピーダンスが低下したことが原因であることを明らかとした.近年,ペロブスカイト型太陽電池膜への第三元素添加による磁気特性開拓に関する研究報告が増加している.本論文は3d遷移金属磁性元素の置換型添加による影響を調べたものであり,主眼となっていた光電変換効率のみならず,低飽和磁化,垂直磁気異方性を有する今後の新しい磁性材料として発展する可能性が示唆される.","" "Spin-orbit torques in antiperovskite Fe4N pseudo-single-crystal films with negative spin polarization","Spin-orbit torques in antiperovskite Fe4N pseudo-single-crystal films with negative spin polarization","Shinji Isogami","Shinji Isogami","Journal of Magnetism and Magnetic Materials","Journal of Magnetism and Magnetic Materials","501","","166400","","2020-05-01","TRUE","","scientific_journal","eng","","10.1016/j.jmmm.2020.166400","","スピン軌道トルク(SOT)の自由度を拡張する研究が近年高い注目を集めている.本研究ではFe4N/Wヘテロ接合膜におけるSOT有効磁場および効率をプレーナーホール効果の二次高調成分測定によって評価した.また異方性磁気抵抗効果の実験からFe4Nは負のスピン分極率を有していることを確認した.SOT成分を含む二次高調成分は温度低減によって符号反転が確認されたが,この要因はFe4Nの異常ネルンスト信号のオフセットであることが判明し,結論としてSOTの符号は,強磁性材料のスピン分極率の符号に依存しないことを明らかとした.","" "Contributions of magnetic structure and nitrogen to perpendicular magnetocrystalline anisotropy in antiperovskite ɛ−Mn4N","Contributions of magnetic structure and nitrogen to perpendicular magnetocrystalline anisotropy in antiperovskite ɛ−Mn4N","Shinji Isogami, Keisuke Masuda, Yoshio Miura","Shinji Isogami, Keisuke Masuda, Yoshio Miura","Physical Review Materials","Physical Review Materials","4","1","","","2020-01-01","TRUE","","scientific_journal","eng","","10.1103/physrevmaterials.4.014406","","MgO(001)単結晶基板上にフェリ磁性のMn4Nエピタキシャル結晶膜を作製し,PMAの起源を実験,理論計算両方から追及した.その結果,KuとMsの低減の主な原因が窒素欠損であることを明らかにした.PMAを示すMn4Nの磁気構造は“type B”と呼ばれるL10規則型に類似したコリニアのフェリ構造であることを明らかとした.二次摂動解析によって,スピン保存項,スピン反転項のいずれもPMAに寄与していること,また,それらはMnサイト依存性を持つことを明らかとした.Mn基窒化物フェリ磁性体のPMA起源の研究は初めての試みであり,今後のPMAを有する同種材料開発指針となることが期待される.","" "Impact of oxygen interdiffusion on spin-to-charge conversion at nonmagnetic metal/Bi oxide interfaces","Impact of oxygen interdiffusion on spin-to-charge conversion at nonmagnetic metal/Bi oxide interfaces","S. Sugimoto, J. Uzuhashi, S. Isogami, T. Ohkubo, Y. K. Takahashi, S. Kasai, K. Hono","S. Sugimoto, J. Uzuhashi, S. Isogami, T. Ohkubo, Y. K. Takahashi, S. Kasai, K. Hono","Physical Review Materials","Physical Review Materials","3","10","","","2019-10-01","TRUE","","scientific_journal","eng","","10.1103/physrevmaterials.3.104410","","Spin-to-charge conversion at metal/oxide interfaces with stacked structures of permalloy/(Cu or Ag)/Bi2O3 were systematically investigated by using the spin-pumping technique and cross-sectional transmission electron microscopy. Although the transport measurement reproduced the results in previous studies, the interfacial structure of (Cu or Ag)/Bi2O3 was found to change depending on the crystalline orientation of the Cu or Ag underlayer. While the Ag/Bi2O3 stacks with Ag(111) had steep interfaces, the formation of a nanometer-scale Cu-O layer can be found in the Cu/Bi2O3 interface, which should be the main origin of the sign inversion of the conversion coefficients between Cu/Bi2O3 and Ag/Bi2O3. This study stresses the importance of nanostructure identification for discussing spin-to-charge conversions at metal/oxide interfaces.","" "Strain relaxation in epitaxial γ′-Fe4N ultrathin films","極薄Fe4Nエピタキシャル膜の歪み緩和","Ippei Suzuki, Jun Uzuhashi, Tadakatsu Ohkubo, Shinji Isogami","Ippei Suzuki, Jun Uzuhashi, Tadakatsu Ohkubo, Shinji Isogami","Materials Research Express","Materials Research Express","6","10","106446","","2019-09-13","TRUE","","scientific_journal","eng","","10.1088/2053-1591/ab41b9","","負に完全スピン分極すると理論予測されている窒化鉄Fe4N薄膜は,逆TMR効果やAMR効果などのスピン輸送特性において従来にない現象が多く報告されている.今後のスピン軌道トルク測定やこれによる磁化反転検証実験に先立ち,極薄Fe4N膜における材料特性や構造評価を調べておく必要がある.本研究ではXRD, TEMによる解析をおこない,異なる格子定数をもつ基板上に成長する極薄Fe4N結晶膜の歪み緩和の評価を行った.FMRおよびAMR測定に及ぼす影響も明確化し,今後のスピン軌道トルク磁化反転に対して有効な基板,歪み状態を議論した.","" "Magnetic and magneto-transport properties of Mn4N thin films by Ni substitution and their possibility of magnetic compensation","Magnetic and magneto-transport properties of Mn4N thin films by Ni substitution and their possibility of magnetic compensation","Taro Komori, Toshiki Gushi, Akihito Anzai, Laurent Vila, Jean-Philippe Attané, Stefania Pizzini, Jan Vogel, Shinji Isogami, Kaoru Toko, Takashi Suemasu","Taro Komori, Toshiki Gushi, Akihito Anzai, Laurent Vila, Jean-Philippe Attané, Stefania Pizzini, Jan Vogel, Shinji Isogami, Kaoru Toko, Takashi Suemasu","Journal of Applied Physics","Journal of Applied Physics","125","21","213902","","2019-06-07","TRUE","","scientific_journal","eng","","10.1063/1.5089869","","垂直磁気異方性を有するフェリ磁性のMn4Nエピタキシャル結晶膜に0.1~0.3at.%の微量のNiを添加(Mn元素との置換)し新規磁気特性の導出を試みた.異常ホール効果,磁気光学Kerr効果の符号がNi0.25at.%程度の置換量にて反転することを見出した.飽和磁化の値がその置換量付近で極小となることがわかり,コリニアのスピン構造を仮定した第一原理計算の結果と定量的にも一致することを確かめた.また飽和磁化の温度依存性の実験結果と上記,種々の実験結果を総合して考察したところ補償点の存在が示唆されることが判った.","" "Crystalline-structure-dependent magnetoresistance in ferromagnetic metal/conducting amorphous oxide heterostructures","Crystalline-structure-dependent magnetoresistance in ferromagnetic metal/conducting amorphous oxide heterostructures","Shinji Isogami, Jun Uzuhashi, Tadakatsu Ohkubo, Masamitsu Hayashi","Shinji Isogami, Jun Uzuhashi, Tadakatsu Ohkubo, Masamitsu Hayashi","Physical Review Materials","Physical Review Materials","3","2","","","2019-02-01","TRUE","","scientific_journal","eng","","10.1103/physrevmaterials.3.024408","","熱酸化膜付きSi基板/ IWO / FM層(Co, FeCo, CoFeB, Cu/Co)/ Cap層におけるy-z面内のMR測定と断面TEM観察を行った.IWO膜厚に対するMR変化率は,Cu膜厚がゼロ(IWO/Co, FeCo/cap)の時,IWO膜厚が3nmを越える領域で- 0.2 %一定値となった.それ以外の積層膜ではほぼ0%の値であった.コンダクタンスのIWO膜厚依存性から,検出されたMR変化率はFM層の異方性MRであることが判った.STEM像ならびにナノビーム電子線回折パターンを解析したところ,MR変化率を示す積層膜のFM層(Co, FeCo)では支配的な配向方向をもった結晶粒が確認された.一方でMR変化率を示さない積層膜のFM層(Cu/Co, CoFeB)では支配的な配向をもたずランダムな結晶粒もしくは,アモルファス構造が確認された.これまでGeometrical MR効果が報告されていたのに対し,本論文の成果は,結晶質であっても配向性に依存する詳細をTEM像から更に明確化した点において新しい.","" "Temperature independent, wide modulation of anomalous Hall effect by Mn doping in Fe4− x Mn x N pseudo-single-crystal films","Temperature independent, wide modulation of anomalous Hall effect by Mn doping in Fe4− x Mn x N pseudo-single-crystal films","Shinji Isogami, Akihito Anzai, Toshiki Gushi, Taro Komori, Takashi Suemasu","Shinji Isogami, Akihito Anzai, Toshiki Gushi, Taro Komori, Takashi Suemasu","Japanese Journal of Applied Physics","Japanese Journal of Applied Physics","57","12","120305","","2018-12-01","TRUE","","scientific_journal","eng","","10.7567/jjap.57.120305","","MgO単結晶基板上にFe4-xMnxN (x=0, 1, 2, 3, 4)のエピタキシャル膜をMBE装置にて作製した.out-of-plane XRD解析から見積った規則度は0.89から0.99であり,飽和磁化はxの増加に伴って単調に減少することを確かめた.異常ホール抵抗率の測定温度依存性を評価したところ,x=0と4の組成では低温となるに従い,絶対値が低減した.一方で高規則性結晶であるにも関わらずx=1, 2, 3では温度に依存せずほぼ一定の値が確認された.さらに,x=0と4では符号の反転,x=3ではほぼゼロの値となる特性が確認された.スキュー散乱とサイドジャンプを用いたスケーリング則を用いた解析すると,それぞれの起源に符号の異なる寄与が含まれることが判った.","" "Transition from minority to majority spin transport in iron-manganese nitride Fe4−xMnxN films with increasing x","Transition from minority to majority spin transport in iron-manganese nitride Fe4−xMnxN films with increasing x","Akihito Anzai, Toshiki Gushi, Taro Komori, Syuta Honda, Shinji Isogami, Takashi Suemasu","Akihito Anzai, Toshiki Gushi, Taro Komori, Syuta Honda, Shinji Isogami, Takashi Suemasu","Journal of Applied Physics","Journal of Applied Physics","124","12","123905","","2018-09-28","TRUE","","scientific_journal","eng","","10.1063/1.5051721","","30nm膜厚のFe4-xMnxN(x=0,1,2,3,4)エピタキシャル膜をMgO基板上にMBE装置にて作製した.AMR効果の測定を300Kから10Kにかけて行ったところ,x組成が増大するに従い,AMR変化率の符号が(負,正,正,負,負)のように変化した.第一原理計算でスピン分解状態密度を求めたところ,スピン分極率の符号は(負,正,正,正,正)であることが示唆された.以上の関係性を考慮すると,それぞれの組成における伝導電子のスピン分極率は,(負,負,負,正,正)であることが判り,FeN中におけるMn組成の増大は多数スピン伝導性をエンハンスする効果があることを明らかにした.","" "Strain Mediated in-Plane Uniaxial Magnetic Anisotropy in Amorphous CoFeB Films Based on Structural Phase Transitions of BaTiO3 Single-Crystal Substrates","Strain Mediated in-Plane Uniaxial Magnetic Anisotropy in Amorphous CoFeB Films Based on Structural Phase Transitions of BaTiO3 Single-Crystal Substrates","Shinji Isogami, Tomoyasu Taniyama","Shinji Isogami, Tomoyasu Taniyama","physica status solidi (a)","physica status solidi (a)","215","6","1700762","","2018-03-01","TRUE","","scientific_journal","eng","","10.1002/pssa.201700762","","(前任地での研究成果)BaTiO3単結晶基板上にCoFeBアモルファスまたは微結晶薄膜を作製し,BaTiO3単結晶の正方晶系から斜方晶系への構造相転移(転移温度278 K)の過程で誘起されるCoFeB薄膜の面内磁気異方性を評価した.その結果,面内磁気異方性エネルギーが3.8×10^4 erg/cm^3から7.4×10^6 erg/cm^3へと劇的に増大した.対比実験として同時に行ったSi基板上の場合はほぼ変化しなかったこと,別途評価したCoFeB薄膜の磁歪定数を用いた磁気異方性の見積り結果を根拠として,得られた劇的な増大は磁気弾性効果で説明がつくことを確認した.またBaTiO3基板とCoFeB層との間にTa薄膜を挿入した場合の結果をあわせて,基板から付与される歪み効果の制御に関して従来のエピタキシャルFe薄膜との比較をしながら違いを議論した.","" "High-resolution geomagnetic observation system using HTS-SQUID","High-resolution geomagnetic observation system using HTS-SQUID","Yuta Katori, Shinji Isogami, Tsunehiro Hato, Akira Tsukamoto, Keiichi Tanabe, Nobuhito Ohnishi, Chikara Furukawa, Kan Okubo","Yuta Katori, Shinji Isogami, Tsunehiro Hato, Akira Tsukamoto, Keiichi Tanabe, Nobuhito Ohnishi, Chikara Furukawa, Kan Okubo","IEICE Communications Express","IEICE Communications Express","7","3","71","76","2017-12-21","TRUE","","scientific_journal","eng","","10.1587/comex.2017xbl0168","","我々は岩手・宮城内陸地震の強震が到達する数秒前において,地磁気が数ピコテスラ領域で変動する前駆現象を世界に先駆けて発表している.理論モデルによる考察によって,その原理が断層運動が発生する瞬間に起きるピエゾ効果であると推察されている.本研究では数ピコテスラ領域の微弱磁場強度を高感度で検出するために,屋外用の高温超伝導SQUID磁力計を開発した.東日本大震災後に余震頻発地帯となった福島県いわき市へ磁力計を設置し,いくつかの地震サンプルで実際に観測されたデータの検証を行った.その結果,従来のフラックスゲート,オーバーハウザー磁力計では不可能な程度に微弱な前駆現象の検出に成功した.","" "Real time and wide range observation of dynamical magnetic field via magneto-optical effect","磁気光学効果による動的磁場の広範囲かつ実時間観察","中村泰貴, 磯上 慎二, 豊島 晋","中村泰貴, 磯上 慎二, 豊島 晋","日本分光学会誌「光学」","日本分光学会誌「光学」","47","12","505","509","2017-12-01","TRUE","","scientific_journal","jpn","","","","","(前任地での研究成果)静的磁場を光でイメージングする技術は既に確立されている.それに対し本研究では永久磁石回転機構モデルと画像処理プログラムを試作することで,動的磁場を磁気光学像としてリアルタイム観察することに成功した.そのために必要であった知見,デモンストレーション結果をまとめた.具体的には回転軸の偏振を極力排除する構造工夫と共に,適正な電子回路の構築をとおして回転周期は1.3秒から10秒まで可変とした.回転する漏洩磁場全体のリアルタイム観察にはネオジウム永久磁石とビスマス置換型磁性ガーネット転写膜を用いた.また磁気光学画像をデータとして取り込み,磁場の極性の違いを輝度コントラストとして明瞭に示すことのできる画像処理プログラムを構築した.今後の展望としては,モーターなど高周波領域で動作する装置の電磁界解析や,立体的磁場分布構造の瞬時観察技術への展開が挙げられる." "Dependence of anomalous Nernst effect on crystal orientation in highly ordered γ′-Fe4N films with anti-perovskite structure","γ’- Fe4N薄膜における異常ネルンスト効果の結晶方位依存性","Shinji Isogami, Koki Takanashi, Masaki Mizuguchi","Shinji Isogami, Koki Takanashi, Masaki Mizuguchi","Applied Physics Express","Applied Physics Express","10","7","073005","","2017-07-01","TRUE","","scientific_journal","eng","","10.7567/apex.10.073005","","(前任地での研究成果)γ’相Fe4N疑単結晶薄膜をMgO単結晶基板上に作製し,室温の異常ネルンスト効果を評価した.その結果,面内に温度勾配がある場合の異常ネルンスト係数がFe4N結晶方位によって明瞭に異なることが確認された.これは面直に温度勾配がある場合も同様であった.一方で,対比実験として同時に評価したA2相Fe-Al合金薄膜においては,Fe4Nのような結晶方位依存性は確認されなかった.MnGa合金系,FePt合金系,さらにCo/Pt多層膜系など,過去のいずれの文献においても結晶方位依存性は確認されていないことから,今回の結果は,窒素原子と軌道混成した鉄の特異的なスピン伝導物性であることを議論した.今後の展望として,窒化物遷移金属材料の特異的な物性の追求,あるいは優れた(デバイス活用可能な)材料物性の人工的導出などへの展開が期待される.",""