Showing papers by "Saburo Takahashi published in 2011"
TL;DR: In this paper, the inverse spin-Hall effect (ISHE) induced by the spin pumping has been investigated systematically in simple ferromagnetic/paramagnetic bilayer systems, and the spectral shape of the electromotive force is well reproduced using a simple Lorentz function, indicating that the force is due to the ISHE induced by spin pumping; extrinsic magnetogalvanic effects are eliminated in this measurement.
Abstract: 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 Ni81Fe19/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 to the ISHE induced by the spin pumping; extrinsic magnetogalvanic effects are eliminated in this measurement. The electromotive force varies systematically by changing the microwave power, magnetic-field angle, and film size, being consistent with the prediction based on the Landau–Lifshitz–Gilbert equation combined with the models of the ISHE and spin pump...
473 citations
TL;DR: Experimental evidence is shown that spin pumping enables spin injection free from this condition; room-temperature spin injection into GaAs from Ni(81)Fe(19) through an Ohmic contact is demonstrated through dynamical spin exchange, and this exchange can be controlled electrically by applying a bias voltage across a Ni( 81) Fe(19)/GaAs interface, enabling electric tuning of the spin-pumping efficiency.
Abstract: Injection of spin currents into solids is crucial for exploring spin physics and spintronics. There has been significant progress in recent years in spin injection into high-resistivity materials, for example, semiconductors and organic materials, which uses tunnel barriers to circumvent the impedance mismatch problem; the impedance mismatch between ferromagnetic metals and high-resistivity materials drastically limits the spin-injection efficiency. However, because of this problem, there is no route for spin injection into these materials through low-resistivity interfaces, that is, Ohmic contacts, even though this promises an easy and versatile pathway for spin injection without the need for growing high-quality tunnel barriers. Here we show experimental evidence that spin pumping enables spin injection free from this condition; room-temperature spin injection into GaAs from Ni(81)Fe(19) through an Ohmic contact is demonstrated through dynamical spin exchange. Furthermore, we demonstrate that this exchange can be controlled electrically by applying a bias voltage across a Ni(81)Fe(19)/GaAs interface, enabling electric tuning of the spin-pumping efficiency.
291 citations
TL;DR: In this article, a linear response theory of the spin Seebeck effect was proposed, i.e., a spin voltage generation from heat current flowing in a ferromagnet.
Abstract: We formulate a linear response theory of the spin Seebeck effect, i.e., a spin voltage generation from heat current flowing in a ferromagnet. Our approach focuses on the collective magnetic excitation of spins, i.e., magnons. We show that the linear-response formulation provides us with a qualitative as well as quantitative understanding of the spin Seebeck effect observed in a prototypical magnet, yttrium iron garnet.
238 citations
TL;DR: It is shown that lateral spin valves with low-resistivity NiFe/MgO/Ag junctions enable efficient spin injection with high applied current density, which leads to the spin-valve voltage increasing 100-fold, suggesting a route to faster and manipulable spin transport for the development of pure spin-current-based memory, logic and sensing devices.
Abstract: The non-local spin injection in lateral spin valves is strongly expected to be an effective method to generate a pure spin current for potential spintronic application. However, the spin-valve voltage, which determines the magnitude of the spin current flowing into an additional ferromagnetic wire, is typically of the order of 1 μV. Here we show that lateral spin valves with low-resistivity NiFe/MgO/Ag junctions enable efficient spin injection with high applied current density, which leads to the spin-valve voltage increasing 100-fold. Hanle effect measurements demonstrate a long-distance collective 2π spin precession along a 6-μm-long Ag wire. These results suggest a route to faster and manipulable spin transport for the development of pure spin-current-based memory, logic and sensing devices.
183 citations
Journal Article•
TL;DR: In this article, the spin Seebeck effect is greatly enhanced by nonequilibrium phonons that drag the low-lying spin excitations, and the authors further argue that this scenario gives a clue to understand the observation of the spin seebeck effect that is unaccompanied by a global spin current.
Abstract: We investigate both theoretically and experimentally a gigantic enhancement of the spin Seebeck effect in a prototypical magnet LaY$_2$Fe$_5$O$_{12}$ at low temperatures. Our theoretical analysis sheds light on the important role of phonons; the spin Seebeck effect is enormously enhanced by nonequilibrium phonons that drag the low-lying spin excitations. We further argue that this scenario gives a clue to understand the observation of the spin Seebeck effect that is unaccompanied by a global spin current, and predict that the substrate condition affects the observed signal.
106 citations
TL;DR: In this article, a numerical technique for calculating the spin Seebeck signal detected by the NM probe, which converts spin current to charge current by the inverse spin Hall effect, was developed.
Abstract: Thermally driven spin-wave spin current in a ferromagnetic material (FM) and the resulting electric signal in a normal metal (NM) probe placed on the FM are theoretically investigated by taking into account the fluctuation-dissipation theorem for thermally fluctuating spin at the interface of an FM-NM junction. We develop a numerical technique for calculating the spin Seebeck signal detected by the NM probe, which converts spin current to charge current by the inverse spin Hall effect. The spin current is induced in the NM probe via an exchange interaction when the FM senses the temperature gradient. Numerical simulation clarifies the role of the sample boundary in the spatial distribution of spin current in both FM and NM.
47 citations
TL;DR: In this article, the supercurrent resulting from coupling of the Josephson phase and the spin wave excited by microwave radiation in a ferromagnetic Josephson junction was studied, in which two superconductors are separated by a Ferromagnet.
Abstract: We study the supercurrent resulting from coupling of the Josephson phase and the spin wave excited by microwave radiation in a ferromagnetic Josephson junction, in which two superconductors are separated by a ferromagnet. To explore how the spin-wave excitation affects the current–voltage curve, the resistively shunted junction model, which is an equation of motion for the Josephson phase, is extended by considering the gauge invariance including magnetization. When the magnetization is driven by the microwave adjusted to the ferromagnetic resonance frequency, the dc supercurrent is induced in the junction and the current–voltage curve shows step structures as a function of applied voltage. The position of each step in voltage is proportional to the microwave frequency multiplied by an even number. This means that the even number of magnons is necessary for the singlet Cooper pair to go through the ferromagnetic layer. The magnitudes of step height can be controlled by tuning the shape of the interface. Our results present a new route to observe the spin-wave excitation by the Josephson effect.
18 citations
TL;DR: In this article, a superconductor/ferromagnetic insulator/superconductor (S/FI/S) junction with spin-wave coupling was investigated, and it was found that the currentvoltage (I-V ) characteristic shows two resonant peaks for each mode of the EM field.
Abstract: Coupling of Josephson-phase and spin-waves is theoretically studied in a superconductor/ferromagnetic insulator/superconductor (S/FI/S) junction. Electromagnetic (EM) field inside the junction and the Josephson current coupled with spin-waves in FI are calculated by combining Maxwell and Landau–Lifshitz–Gilbert equations. In the S/FI/S junction, it is found that the current–voltage ( I – V ) characteristic shows two resonant peaks for each mode of the EM field. Voltages at the resonant peaks are obtained as a function of the normal modes of EM field, which indicates a composite excitation of the EM field and spin-waves in the S/FI/S junction. We also examine another type of junction, in which a nonmagnetic insulator (I) is located at one of interfaces between S and FI. In such a S/I/FI/S junction, three resonant peaks appear in the I – V curve, since the Josephson-phase couples to the EM field in the I layer.
10 citations
TL;DR: In this article, the spin valve voltage was increased to 100fold by using low resistive NiFe/MgO/Ag junctions, and Hanle effect measurements demonstrated a long-distance collective 2-pi spin precession along a 6 µm long Ag wire.
Abstract: The nonlocal spin injection in lateral spin valves is highly expected to be an effective method to generate a pure spin current for potential spintronic application. However, the spin valve voltage, which decides the magnitude of the spin current flowing into an additional ferromagnetic wire, is typically of the order of 1 {\mu}V. Here we show that lateral spin valves with low resistive NiFe/MgO/Ag junctions enable the efficient spin injection with high applied current density, which leads to the spin valve voltage increased hundredfold. Hanle effect measurements demonstrate a long-distance collective 2-pi spin precession along a 6 {\mu}m long Ag wire. These results suggest a route to faster and manipulable spin transport for the development of pure spin current based memory, logic and sensing devices.
8 citations
TL;DR: In this paper, a spin current carried by spin waves was found to exchange each other at a Pt/Y3Fe5O12 interface, which can propagate both in insulators and metals.
Abstract: Conduction electron can carry a flow spin angular momentum, a spin current. This paper describes another type of spin current: a spin current carried by spin waves. Some types of spin waves can carry a spin current, which can propagate both in insulators and metals. These two types of spin currents were found to exchange each other at a Pt/Y3Fe5O12 interface.
8 citations
TL;DR: In this paper, the spin transfer torque exerted on the magnetizations of ferromagnetic bilayers in the SyF free layer tends to rotate the magnetization in the same direction as a combined motion of the Syf free layer regardless of the bias direction.
Abstract: Based on the Keldysh Green’s function method within the tight-binding model, the spin transfer torque is analyzed in a magnetic tunnel junction with a synthetic ferrimagnetic (SyF) free layer in the ballistic regime. The spin transfer torque exerted on the magnetizations of ferromagnetic bilayers in the SyF free layer tends to rotate the magnetizations in the same direction as a combined motion of the SyF free layer regardless of the bias direction. This rotation causes a reduction of the critical current for the magnetization switching of the SyF free layer which is consistent with experimental observations.
Posted Content•
04 Mar 2011
TL;DR: In this article, the spin valve voltage was increased by a hundredfold Hanle effect measurements, leading to a long-distance collective 2-pi spin precession along a 6 µm long Ag wire.
Abstract: The nonlocal spin injection in lateral spin valves is highly expected to be an effective method to generate a pure spin current for potential spintronic application However, the spin valve voltage, which decides the magnitude of the spin current flowing into an additional ferromagnetic wire, is typically of the order of 1 {\mu}V Here we show that lateral spin valves with low resistive NiFe/MgO/Ag junctions enable the efficient spin injection with high applied current density, which leads to the spin valve voltage increased hundredfold Hanle effect measurements demonstrate a long-distance collective 2-pi spin precession along a 6 {\mu}m long Ag wire These results suggest a route to faster and manipulable spin transport for the development of pure spin current based memory, logic and sensing devices