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Journal Articles

Symposium on "Science of nuclear fuel cycle and backend; Research and education" with a memory of Professor Joonhong Ahn, the University of California, Berkeley

Nakayama, Shinichi; Okumura, Masahiko*; Nagasaki, Shinya*; Enokida, Yoichi*; Umeki, Hiroyuki*; Takase, Hiroyasu*; Kawasaki, Daisuke*; Hasegawa, Shuichi*; Furuta, Kazuo*

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 23(2), p.131 - 148, 2016/12

http://nuce.aesj.or.jp/jnuce/vol23/Jnuce-Vol23-2-p131-148.pdf

A symposium "Science of nuclear fuel cycle and backend - Research and education -" was held at the Univer-sity of Tokyo in June 25, 2016. This aimed at developing the research on nuclear fuel cycle and backend. The time and the number of participants of the symposium were limited, but the active discussion was conducted, and the common perception for the future was shared among the experienced participants in those fields. This paper provides the discussions made in the symposium, and also, as a memory to Professor Ahn, the University of California, Berkeley, his prominent achievements in academic research and education.

Journal Articles

Theory of spin Hall magnetoresistance (SMR) and related phenomena

Chen, Y.-T.*; Takahashi, Saburo*; Nakayama, Hiroyasu*; Althammer, M.*; Goennenwein, S. T. B.*; Saito, Eiji; Bauer, G. E. W.*

Journal of Physics; Condensed Matter, 28(10), p.103004_1 - 103004_15, 2016/03

https://doi.org/10.1088/0953-8984/28/10/103004

Times Cited Count：81 Percentile：61.66(Physics, Condensed Matter)

We review the so-called spin Hall magnetoresistance (SMR) in bilayers of a magnetic insulator and a metal, in which spin currents are generated in the normal metal by the spin Hall effect. The associated angular momentum transfer to the ferromagnetic layer and thereby the electrical resistance is modulated by the angle between the applied current and the magnetization direction. The SMR provides a convenient tool to non-invasively measure the magnetization direction and spin-transfer torque to an insulator. We introduce the minimal theoretical instruments to calculate the SMR, i.e. spin diffusion theory and quantum mechanical boundary conditions. This leads to a small set of parameters that can be fitted to experiments. We discuss the limitations of the theory as well as alternative mechanisms such as the ferromagnetic proximity effect and Rashba spin-orbit torques, and point out new developments.

Journal Articles

Spin pumping blocked by single-layer graphene

Haku, Satoshi*; Tashiro, Takaharu*; Nakayama, Hiroyasu*; Ieda, Junichi; Entani, Shiro; Sakai, Seiji; Ando, Kazuya*

Applied Physics Express, 8(7), p.073009_1 - 073009_3, 2015/07

https://doi.org/10.7567/APEX.8.073009

Times Cited Count：4 Percentile：18.63(Physics, Applied)

We found that the spin pumping in a Ni $_{81}$ Fe $_{19}$ /Pt bilayer is strongly suppressed by inserting single-layer graphene (SLG) at the interface. The spin pumping in the Ni $_{81}$ Fe $_{19}$ /Pt bilayer enhances the magnetization damping of the ferromagnetic layer, which is quantified from microwave frequency dependence of ferromagnetic resonance linewidth. We show that the enhancement of the magnetization damping due to the spin pumping disappears in a Ni $_{81}$ Fe $_{19}$ /SLG/Pt trilayer. This result indicates that the spin pumping is blocked by the atomic monolayer, demonstrating the crucial role of the interfacial short-range spin-exchange coupling in the spin pumping in metallic systems.

Journal Articles

Interface-dependent magnetotransport properties for thin Pt films on ferrimagnetic Y $_{3}$ Fe $_{5}$ O $_{12}$

Shiomi, Yuki*; Otani, Takashi*; Iguchi, Satoshi*; Sasaki, Takahiko*; Qiu, Z.*; Nakayama, Hiroyasu*; Uchida, Kenichi*; Saito, Eiji

Applied Physics Letters, 104(24), p.242406_1 - 242406_5, 2014/06

https://doi.org/10.1063/1.4883898

Times Cited Count：28 Percentile：73.72(Physics, Applied)

Journal Articles

Spin mixing conductance at a well-controlled platinum/yttrium iron garnet interface

Qiu, Z.*; Ando, Kazuya*; Uchida, Kenichi*; Kajiwara, Yosuke*; Takahashi, Ryo*; Nakayama, Hiroyasu*; An, Toshu*; Fujikawa, Yasunori*; Saito, Eiji

Applied Physics Letters, 103(9), p.092404_1 - 092404_4, 2013/08

https://doi.org/10.1063/1.4819460

Times Cited Count：115 Percentile：95.84(Physics, Applied)

A platinum (Pt)/yttrium iron garnet (YIG) bilayer system with a well-controlled interface has been developed; spin mixing conductance at the Pt/YIG interface has been studied. A clear interface with good crystal perfection is experimentally demonstrated to be one of the important factors for an ultimate spin mixing conductance. The spin mixing conductance is obtained to be 1.310 $^{18}$ m $^{-2}$ at the well-controlled Pt/YIG interface, which is close to a theoretical prediction.

Journal Articles

Quantitative study of the spin Hall magnetoresistance in ferromagnetic insulator/normal metal hybrids

Althammer, M.*; Meyer, S.*; Nakayama, Hiroyasu*; Schreier, M.*; Altmannshofer, S.*; Weiler, M.*; Huebl, H.*; Geprgs, S.*; Opel, M.*; Gross, R.*; et al.

Physical Review B, 87(22), p.224401_1 - 224401_15, 2013/06

https://doi.org/10.1103/PhysRevB.87.224401

Times Cited Count：416 Percentile：99.41(Materials Science, Multidisciplinary)

We experimentally investigate and quantitatively analyze the spin Hall magnetoresistance (SMR) effect in ferromagnetic insulator (FI)/Pt and FI/nonmagnetic metal/Pt hybrid structures. For the FI, we use either YIG, nickel ferrite, or magnetite and for the nonmagnet, Cu or Au. The SMR is theoretically ascribed to the combined action of spin Hall and inverse spin Hall effect in the Pt top layer. It therefore should characteristically depend upon the orientation of the magnetization in the adjacent ferromagnet and prevail even if an additional, nonmagnetic metal layer is inserted between Pt and the ferromagnet. Our experimental data corroborate these theoretical conjectures. Using the SMR theory to analyze our data, we extract the spin Hall angle and the spin diffusion length in Pt. For a spin-mixing conductance of $4times 10^{14}Omega^{-1}$ m $^{-2}$ , we obtain a spin Hall angle of 0.11 0.08 and a spin diffusion length of (1.5 0.5) nm for Pt in our samples.

Journal Articles

Theory of spin Hall magnetoresistance

Chen, Y.-T.*; Takahashi, Saburo*; Nakayama, Hiroyasu*; Althammer, M.*; Goennenwein, S. T. B.*; Saito, Eiji; Bauer, G. E. W.*

Physical Review B, 87(14), p.144411_1 - 144411_9, 2013/04

https://doi.org/10.1103/PhysRevB.87.144411

Times Cited Count：614 Percentile：99.7(Materials Science, Multidisciplinary)

We present a theory of the spin Hall magnetoresistance (SMR) in multilayers made from an insulating ferromagnet F, such as yttrium iron garnet (YIG), and a normal metal N with spin-orbit interactions, such as platinum (Pt). The SMR is induced by the simultaneous action of spin Hall and inverse spin Hall effects and therefore a nonequilibrium proximity phenomenon. We compute the SMR in F|N and F|N|F layered systems, treating N by spin-diffusion theory with quantum mechanical boundary conditions at the interfaces in terms of the spin-mixing conductance. Our results explain the experimentally observed spin Hall magnetoresistance in N|F bilayers. For F|N|F spin valves we predict an enhanced SMR amplitude when magnetizations are collinear. The SMR and the spin-transfer torques in these trilayers can be controlled by the magnetic configuration.

Journal Articles

Interface induced inverse spin Hall effect in bismuth/permalloy bilayer

Hou, D.*; Qiu, Z.*; Harii, Kazuya; Kajiwara, Yosuke*; Uchida, Kenichi*; Fujikawa, Yasunori*; Nakayama, Hiroyasu*; Yoshino, Tatsuro*; An, Toshu*; Ando, Kazuya*; et al.

Applied Physics Letters, 101(4), p.042403_1 - 042403_4, 2012/07

https://doi.org/10.1063/1.4738786

Times Cited Count：76 Percentile：92.18(Physics, Applied)

Inverse spin Hall effect has been investigated in bismuth(Bi)/permalloy(Py) bilayer films by using the spin pumping at room temperature. From the ferromagnetic-resonance-spectrum linewidth data, Bi is proved to be a good spin sink in our structure. We measured inverse spin Hall voltage and conductance of the Bi/Py bilayer and found that the inverse spin Hall current, , decreases with increasing the Bi thickness, which is in contrast to the former understanding in similar bilayer systems, e.g., Pt/Py. We constructed a model to explain the thickness dependence of quantitatively, in which spin transport modulation near Bi/Py interface is considered.

Journal Articles

Geometry dependence on inverse spin Hall effect induced by spin pumping in Ni $_{81}$ Fe $_{19}$ /Pt films

Nakayama, Hiroyasu*; Ando, Kazuya*; Harii, Kazuya; Yoshino, Tatsuro*; Takahashi, Ryo*; Kajiwara, Yosuke*; Uchida, Kenichi*; Fujikawa, Yasunori*; Saito, Eiji

Physical Review B, 85(14), p.144408_1 - 144408_7, 2012/04

https://doi.org/10.1103/PhysRevB.85.144408

Times Cited Count：190 Percentile：98(Materials Science, Multidisciplinary)

Geometric effects on the inverse spin Hall effect (ISHE) induced by the spin pumping driven by the ferromagnetic resonance (FMR) have been investigated quantitatively. We measured the FMR spectra and the electromotive force induced by the ISHE with changing the size and the thickness in Ni $_{81}$ Fe $_{19}$ /Pt films. The intensity of generated charge currents due to the ISHE changes systematically with changing the film geometry. The experimental results show a clear difference between Ni $_{81}$ Fe $_{19}$ and Pt thickness dependence of the ISHE induced by the spin pumping.

Journal Articles

Acoustic spin pumping; Direct generation of spin currents from sound waves in Pt/Y $_{3}$ Fe $_{5}$ O $_{12}$ hybrid structures

Uchida, Kenichi*; Adachi, Hiroto; An, Toshu*; Nakayama, Hiroyasu*; Toda, Masaya*; Hillebrands, B.*; Maekawa, Sadamichi; Saito, Eiji

Journal of Applied Physics, 111(5), p.053903_1 - 053903_8, 2012/03

https://doi.org/10.1063/1.3688332

Times Cited Count：27 Percentile：71.79(Physics, Applied)

Using a Pt/Y $_{3}$ Fe $_{5}$ O $_{12}$ (YIG) hybrid structure attached to a piezoelectric actuator, we demonstrate the generation of spin currents from sound waves. This "acoustic spin pumping" is caused by the sound wave generated by the piezoelectric actuator, which then modulates the distribution function of magnons in the YIG layer and results in a pure-spin-current injection into the Pt layer across the Pt/YIG interface. In the Pt layer, this injected spin current is converted into an electric voltage due to the inverse spin-Hall effect. The ISHE induced by the acoustic spin pumping is detected by measuring a voltage in the Pt layer at the piezoelectric resonance frequency of the actuator coupled with the Pt/YIG system. Our model calculation based on the linear response theory provides us with a qualitative and quantitative understanding of the acoustic spin pumping in the Pt/YIG system.

Journal Articles

Inverse spin-Hall effect induced by spin pumping in metallic systems

Ando, Kazuya*; Takahashi, Saburo; Ieda, Junichi; Kajiwara, Yosuke*; Nakayama, Hiroyasu*; Yoshino, Tatsuro*; Harii, Kazuya*; Fujikawa, Yasunori*; Matsuo, Mamoru*; Maekawa, Sadamichi; et al.

Journal of Applied Physics, 109(10), p.103913_1 - 103913_11, 2011/05

https://doi.org/10.1063/1.3587173

Times Cited Count：428 Percentile：99.49(Physics, Applied)

The inverse spin-Hall effect (ISHE) induced by the spin pumping has been investigated systematically in simple ferromagnetic/paramagnetic bilayer systems. The spin pumping driven by ferromagnetic resonance injects a spin current into the paramagnetic layer, which gives rise to an electromotive force transverse to the spin current using the ISHE in the paramagnetic layer. In a Ni $_{81}$ Fe $_{19}$ /Pt film, we found an electromotive force perpendicular to the applied magnetic field at the ferromagnetic resonance condition. The spectral shape of the electromotive force is well reproduced using a simple Lorentz function, indicating that the electromotive force is due entirely to the ISHE induced by the spin pumping; the extrinsic magnetogalvanic effects are eliminated in this measurement. The electromotive force varies systematically by changing the microwave power, magnetic-field angle, and film size, which are well reproduced by a calculation based on the Landau-Lifshitz-Gilbert equation combined with the models of the ISHE and spin pumping. The electromotive force was observed also in a Pt/YFeGaO $_{12}$ film, in which the metallic Ni $_{81}$ Fe $_{19}$ layer is replaced by an insulating YFeGaO $_{12}$ layer, supporting that the spin-pumping-induced ISHE is responsible for the observed electromotive force.

Journal Articles

Universality of the spin pumping in metallic bilayer films

Yoshino, Tatsuro*; Ando, Kazuya*; Harii, Kazuya*; Nakayama, Hiroyasu*; Kajiwara, Yosuke*; Saito, Eiji

Applied Physics Letters, 98(3), p.132503_1 - 132503_3, 2011/03

https://doi.org/10.1063/1.3571556

Times Cited Count：33 Percentile：76.99(Physics, Applied)

We show a clear guideline for generating a large spin current using the spin pumping in metallic bilayer films. We measured spin currents generated by the spin pumping in Ni $_{1-x}$ Fe $_{x}$ /Pt bilayer films using the inverse spin-Hall effect (ISHE). The magnitude of the ISHE signals are well reproduced by a calculation based on the model of the spin pumping. The result shows that the amplitude of a spin current is universally determined by the product of the saturation magnetization, the additional damping constant, and the solid angle of the magnetization precession.

Journal Articles

Inverse spin-Hall effect induced by spin pumping in different size Ni $_{81}$ Fe $_{19}$ /Pt films

Nakayama, Hiroyasu*; Ando, Kazuya*; Harii, Kazuya*; Fujikawa, Yasunori*; Kajiwara, Yosuke*; Yoshino, Tatsuro*; Saito, Eiji

Journal of Physics; Conference Series, 266, p.012100_1 - 012100_4, 2011/01

https://doi.org/10.1088/1742-6596/266/1/012100

Times Cited Count：8 Percentile：95.73(Materials Science, Multidisciplinary)

The inverse spin-Hall effect, conversion of spin currents into electric voltage, has been investigated in Ni $_{81}$ Fe $_{19}$ /Pt bilayer films with different length and width. We measured the electric voltage generated by the inverse spin-Hall effect induced by the spin pumping at room temperature. The experimental results indicate that the electric voltage induced by the inverse spin-Hall effect is proportional to the length of Ni $_{81}$ Fe $_{19}$ /Pt bilayer films. In contrast, the electric voltage exhibits almost no variation by changing the width of the Ni $_{81}$ Fe $_{19}$ /Pt films. These results are consistent with the inverse spin-Hall effect model.

Journal Articles

Quantifying spin mixing conductance in /Pt ( = Ni, Fe, and Ni $_{81}$ Fe $_{19}$ ) bilayer film

Yoshino, Tatsuro*; Ando, Kazuya*; Harii, Kazuya*; Nakayama, Hiroyasu*; Kajiwara, Yosuke*; Saito, Eiji

Journal of Physics; Conference Series, 266, p.012115_1 - 012115_4, 2011/01

https://doi.org/10.1088/1742-6596/266/1/012115

Times Cited Count：11 Percentile：97.46(Materials Science, Multidisciplinary)

The spin-mixing conductances in /Pt ( = Ni, Fe, and Ni $_{81}$ Fe $_{19}$ ) bilayer films were quantified from the peak-to-peak linewidth of ferromagnetic resonance (FMR) spectra based on the model of the spin pumping. When the Pt layer is attached to the layer, we found the enhancement of the FMR linewidth due to the spin pumping. The experimental results show that the spin-mixing conductances in /Pt ( = Ni, Fe, and Ni $_{81}$ Fe $_{19}$ ) bilayer films have the same order of magnitude, showing that spin injection efficiency in the spin pumping is almost identical in these films.

Journal Articles