研究者詳細 - 林　宏樹

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Observation of long-range orbital transport and giant orbital torque

Hayashi H., Jo D., Go D., Gao T., Haku S., Mokrousov Y., Lee H.W., Ando K.

Communications Physics （Communications Physics） 6 （ 1 ） 2023年12月

　概要を見る

Modern spintronics relies on the generation of spin currents through spin-orbit coupling. The spin-current generation has been believed to be triggered by current-induced orbital dynamics, which governs the angular momentum transfer from the lattice to the electrons in solids. The fundamental role of the orbital response in the angular momentum dynamics suggests the importance of the orbital counterpart of spin currents: orbital currents. However, evidence for its existence has been elusive. Here, we demonstrate the generation of giant orbital currents and uncover fundamental features of the orbital response. We experimentally and theoretically show that orbital currents propagate over longer distances than spin currents by more than an order of magnitude in a ferromagnet and nonmagnets. Furthermore, we find that the orbital current enables electric manipulation of magnetization with efficiencies significantly higher than the spin counterpart. These findings open the door to orbitronics that exploits orbital transport and spin-orbital coupled dynamics in solid-state devices.
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Nonlocal orbital torques in magnetic multilayers

Taniguchi M., Hayashi H., Soya N., Ando K.

Applied Physics Express （Applied Physics Express） 16 （ 4 ） 2023年04月

ISSN 18820778

　概要を見る

We investigate current-induced torques in Ni/Ti/Fe/Ti multilayers. In the multilayers, we find that the damping-like torque acting on the Ni magnetization increases with the thickness of the bottom Ti layer, despite the negligible spin Hall conductivity of Ti and the presence of the Fe interlayer that effectively absorbs the transverse spins. The nonlocal nature of the observed torque is consistent with the orbital torque arising from the orbital Hall effect in the Ti layer and orbital transport through the Fe layer. This observation highlights the unique features of the orbital currents, offering enhanced flexibility in the design of spintronic devices.
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Crossover of the intrinsic spin Hall effect in the presence of lattice expansion

Soya N., Hayashi H., Harumoto T., Gao T., Haku S., Ando K.

Physical Review B （Physical Review B） 103 （ 17 ） 2021年05月

ISSN 24699950

　概要を見る

We report the robustness of the intrinsic spin Hall effect in Pt against a change in the lattice constant. We measure spin-Torque ferromagnetic resonance for Pt/Ni81Fe19 bilayers, where the lattice constant and carrier scattering time of the Pt layer are varied by incorporating the nitrogen atoms as interstitial impurities. We find that the spin Hall angle is enhanced by a factor of 3 as the resistivity of the Pt layer is increased by the nitrogen incorporation. The variation of the spin Hall conductivity is consistent with the prediction of the crossover of the intrinsic spin Hall effect from the moderately dirty regime to the dirty-metal regime. This result demonstrates the variation of the intrinsic spin Hall conductivity in Pt is dominated by the change in the scattering time, rather than the change in the band structure, under a few percent change in the lattice constant.
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Spin-orbit torques originating from the bulk and interface in Pt-based structures

Hayashi H., Musha A., Sakimura H., Ando K.

Physical Review Research （Physical Review Research） 3 （ 1 ） 2021年01月

ISSN 26431564

　概要を見る

We investigate spin-orbit torques in prototypical Pt-based spintronic devices. We find that, in Pt/Ni and Pt/Fe bilayers, the dampinglike torque efficiency depends on the thickness of the Pt layer. We also find that the dampinglike torque efficiency is almost identical in the Pt/Ni and Pt/Fe bilayers despite the stronger spin memory loss at the Pt/Fe interface. These results suggest that although the dominant source of the dampinglike torque is the bulk spin Hall effect in the Pt layer, a sizable dampinglike torque is generated by the interface in the Pt/Fe bilayer due to the stronger interfacial spin-orbit coupling. In contrast to the dampinglike torque, whose magnitude and sign are almost identical in the Pt/Ni and Pt/Fe bilayers, the fieldlike torque strongly depends on the choice of the ferromagnetic layer. The sign of the fieldlike torque originating from the bulk spin Hall effect in the Pt layer is opposite between the Pt/Ni and Pt/Fe bilayers, which can be attributed to the opposite sign of the imaginary part of the spin-mixing conductance. These results demonstrate that the spin-orbit torques are quite sensitive to the electronic structure of the ferromagnet layer.
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