Showing papers by "Saburo Takahashi published in 2018"
TL;DR: In this paper, the spin Hall magnetoresistance (SMR) of Pt/Co bilayers was studied and the authors found that the SMR increases with increasing Co thickness, indicating that magnon-related effects in the Co layer play little role.
Abstract: We have studied the spin Hall magnetoresistance (SMR), the magnetoresistance within the plane transverse to the current flow, of Pt/Co bilayers. We find that the SMR increases with increasing Co thickness: the effective spin Hall angle for bilayers with thick Co exceeds the reported values of Pt when a conventional drift-diffusion model is used. An extended model including spin transport within the Co layer cannot account for the large SMR. To identify its origin, contributions from other sources are studied. For most bilayers, the SMR increases with decreasing temperature and increasing magnetic field, indicating that magnon-related effects in the Co layer play little role. Without the Pt layer, we do not observe the large SMR found for the Pt/Co bilayers with thick Co. Implementing the effect of the so-called interface magnetoresistance and the textured induced anisotropic scattering cannot account for the Co thickness dependent SMR. Since the large SMR is present for W/Co but its magnitude reduces in W/CoFeB, we infer that its origin is associated with a particular property of Co.
40 citations
TL;DR: In this paper, the spin current propagation in paramagnetic GGG over several microns was investigated and it was shown that exchange coupling is not required for efficient spin transport, which challenges conventional models and provides new material designs for spintronic devices.
Abstract: The discovery of new materials that efficiently transmit spin currents has been important for spintronics and material science. The electric insulator $\mathrm{Gd}_3\mathrm{Ga}_5\mathrm{O}_{12}$ (GGG) is a superior substrate for growing magnetic films, but has never been considered as a conduit for spin currents. Here we report spin current propagation in paramagnetic GGG over several microns. Surprisingly, the spin transport persists up to temperatures of 100 K $\gg$ $T_{\mathrm{g}} = 180$ mK, GGG's magnetic glass-like transition temperature. At 5 K we find a spin diffusion length ${\lambda_{\mathrm{GGG}}} = 1.8 \pm 0.2 {\mu}$m and a spin conductivity ${\sigma}_{\mathrm{GGG}} = (7.3 \pm 0.3) \times10^4$ $\mathrm{Sm}^{-1}$ that is larger than that of the record quality magnet $\mathrm{Y}_3\mathrm{Fe}_5\mathrm{O}_{12}$ (YIG). We conclude that exchange coupling is not required for efficient spin transport, which challenges conventional models and provides new material-design strategies for spintronic devices.
39 citations
TL;DR: In this paper, the spin Hall magnetoresistance (SMR) of Pt/Co bilayers was studied and the authors found that the SMR increases with increasing Co thickness, indicating that magnon-related effects in the Co layer play little role.
Abstract: We have studied the spin Hall magnetoresistance (SMR), the magnetoresistance within the plane transverse to the current flow, of Pt/Co bilayers. We find that the SMR increases with increasing Co thickness: the effective spin Hall angle for bilayers with thick Co exceeds the reported values of Pt when a conventional drift-diffusion model is used. An extended model including spin transport within the Co layer cannot account for the large SMR. To identify its origin, contributions from other sources are studied. For most bilayers, the SMR increases with decreasing temperature and increasing magnetic field, indicating that magnon-related effects in the Co layer play little role. Without the Pt layer, we do not observe the large SMR found for the Pt/Co bilayers with thick Co. Implementing the effect of the so-called interface magnetoresistance and the textured induced anisotropic scattering cannot account for the Co thickness dependent SMR. Since the large SMR is present for W/Co but its magnitude reduces in W/CoFeB, we infer its origin is associated with a particular property of Co.
34 citations
TL;DR: The electric power generation from environmental fluctuations by using superconducting vortex strings in MoGe/YIG bilayer system is reported, which spontaneously generates electricity by rectifying environmental fluctuations without using atomic asymmetry.
Abstract: A rectenna, standing for a rectifying antenna, is an apparatus which generates d.c. electricity from electric fluctuations. It is expected to realize wireless power transmission as well as energy harvesting from environmental radio waves. To realize such rectification, devices that are made up of internal atomic asymmetry such as an asymmetric junction have been necessary so far. Here we report a material that spontaneously generates electricity by rectifying environmental fluctuations without using atomic asymmetry. The sample is a common superconductor without lowered crystalline symmetry, but, just by putting it in an asymmetric magnetic environment, it turns into a rectifier and starts generating electricity. Superconducting vortex strings only annihilate and nucleate at surfaces, and this allows the bulk electrons to feel surface fluctuations in an asymmetric environment: a vortex rectenna. The rectification and generation can be switched on and off with only a slight change in temperature or external magnetic fields.
33 citations
TL;DR: (Fe, FeCo)-(Al-,Y-fluoride) nanogranular films exhibiting giant Faraday effect, 40 times larger than Bi-YIG are introduced.
Abstract: Magneto-optical Faraday effect is widely applied in optical devices and is indispensable for optical communications and advanced information technology. However, the bismuth garnet Bi-YIG is only the Faraday material since 1972. Here we introduce (Fe, FeCo)-(Al-,Y-fluoride) nanogranular films exhibiting giant Faraday effect, 40 times larger than Bi-YIG. These films have a nanocomposite structure, in which nanometer-sized Fe, FeCo ferromagnetic granules are dispersed in a Al,Y-fluoride matrix.
21 citations
TL;DR: In this article, the inverse spin Hall effects induced by spin injection from a ferrimagnetic insulator Y3Fe5O12 into a superconductor NbN using longitudinal spin Seebeck effects were measured.
Abstract: Coherence peak effects in a superconductor induced by a thermal spin current are reported. We measured inverse spin Hall effects induced by spin injection from a ferrimagnetic insulator Y3Fe5O12 into a superconductor NbN using longitudinal spin Seebeck effects. In the vicinity of the superconducting transition temperature of the NbN, a large enhancement of the spin Seebeck voltage is observed, whose sign is opposite to that for the vortex Nernst effect, but is consistent with a calculation for a coherence peak effect in the superconductor NbN.
18 citations
TL;DR: In this paper, the inverse spin Hall effects induced by spin injection from a ferrimagnetic insulator into a superconductor NbN using longitudinal spin Seebeck effects were measured.
Abstract: Coherence peak effects in a superconductor induced by a thermal spin current are reported. We measured inverse spin Hall effects induced by spin injection from a ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ into a superconductor NbN using longitudinal spin Seebeck effects. In the vicinity of the superconducting transition temperature of the NbN, a large enhancement of the spin Seebeck voltage is observed, whose sign is opposite to that for the vortex Nernst effect, but is consistent with a calculation for a coherence peak effect in the superconductor NbN.
12 citations
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TL;DR: In this article, the authors report an exceptionally large interfacial charge-to-spin conversion efficiency (i.e., the inverse Edelstein length Lamda_IEE of a few nanometers) in epitaxial (epi-) Pt/Co bilayers from a REE-induced magnetoresistance analysis using a new drift-diffusion model that incorporates both REE and bulk spin Hall effect.
Abstract: The Rashba-Edelstein effect (REE), which characterizes the generation of a transverse spin polarization at interfaces with a longitudinal external electrical field, provides new opportunities to the efficient spin current generation and the manipulation of spin orbit torques (SOTs) in magnetic hetero-structures. Here in this work, we report an exceptionally large interfacial charge-to-spin conversion efficiency (i.e. the inverse Edelstein length Lamda_IEE of a few nanometers) in epitaxial (epi-) Pt/Co bilayers from a REE-induced magnetoresistance analysis using a new drift-diffusion model that incorporates both REE and bulk spin Hall effect. From the spin torque ferromagnetic resonance analysis, a tenfold enhancement in the field-like (FL) SOT efficiency is demonstrated compared to polycrystalline samples, showing quantitative consistency with the large Lamda_IEE. Additionally, a twofold increase in the damping-like (DL) SOT efficiency is observed in epi-samples, with Pt films having lower resistivity. Our study demonstrates that both DL- and FL-SOTs can be significantly modulated by the Rashba SO coupling with hetero-interface modification, providing new perspectives to the engineering of SOTs in spintronic devices.
5 citations