Showing papers by "Saburo Takahashi published in 2010"
TL;DR: It is shown that a spin wave in an insulator can be generated and detected using spin-Hall effects, which enable the direct conversion of an electric signal into aspin wave, and its subsequent transmission through (and recovery from) an insulators over macroscopic distances.
Abstract: An insulator does not conduct electricity, and so cannot in general be used to transmit an electrical signal. However, the electrons within an insulator possess spin as well as charge, so it is possible for them to transmit a signal in the form of a spin wave. Kajiwara et al. have now developed a hybrid metal–insulator–metal structure in which an electrical signal in one metal layer is directly converted to a spin wave in the insulating layer. This wave is then transmitted to the second metal layer, where the signal can be directly recovered as an electrical voltage. The observation of voltage transmission in an insulator raises the prospect of insulator-based spintronics and other novel forms of signal delivery. An insulator does not conduct electricity, and so cannot in general be used to transmit an electrical signal. But an insulator's electrons possess spin in addition to charge, and so can transmit a signal in the form of a spin wave. Here a hybrid metal–insulator–metal structure is reported, in which an electrical signal in one metal layer is directly converted to a spin wave in the insulating layer; this wave is then transmitted to the second metal layer, where the signal can be directly recovered as an electrical voltage. The energy bandgap of an insulator is large enough to prevent electron excitation and electrical conduction1. But in addition to charge, an electron also has spin2, and the collective motion of spin can propagate—and so transfer a signal—in some insulators3. This motion is called a spin wave and is usually excited using magnetic fields. Here we show that a spin wave in an insulator can be generated and detected using spin-Hall effects, which enable the direct conversion of an electric signal into a spin wave, and its subsequent transmission through (and recovery from) an insulator over macroscopic distances. First, we show evidence for the transfer of spin angular momentum between an insulator magnet Y3Fe5O12 and a platinum film. This transfer allows direct conversion of an electric current in the platinum film to a spin wave in the Y3Fe5O12 via spin-Hall effects4,5,6,7,8,9,10,11. Second, making use of the transfer in a Pt/Y3Fe5O12/Pt system, we demonstrate that an electric current in one metal film induces voltage in the other, far distant, metal film. Specifically, the applied electric current is converted into spin angular momentum owing to the spin-Hall effect7,8,10,11 in the first platinum film; the angular momentum is then carried by a spin wave in the insulating Y3Fe5O12 layer; at the distant platinum film, the spin angular momentum of the spin wave is converted back to an electric voltage. This effect can be switched on and off using a magnetic field. Weak spin damping3 in Y3Fe5O12 is responsible for its transparency for the transmission of spin angular momentum. This hybrid electrical transmission method potentially offers a means of innovative signal delivery in electrical circuits and devices.
1,391 citations
TL;DR: In this paper, it was shown that the magnetic insulator LaY(2)Fe(5)O(12) can convert a heat flow into a spin voltage, which can then be converted into an electric voltage as a result of the inverse spin Hall effect.
Abstract: Thermoelectric generation is an essential function in future energy-saving technologies. However, it has so far been an exclusive feature of electric conductors, a situation which limits its application; conduction electrons are often problematic in the thermal design of devices. Here we report electric voltage generation from heat flowing in an insulator. We reveal that, despite the absence of conduction electrons, the magnetic insulator LaY(2)Fe(5)O(12) can convert a heat flow into a spin voltage. Attached Pt films can then transform this spin voltage into an electric voltage as a result of the inverse spin Hall effect. The experimental results require us to introduce a thermally activated interface spin exchange between LaY(2)Fe(5)O(12) and Pt. Our findings extend the range of potential materials for thermoelectric applications and provide a crucial piece of information for understanding the physics of the spin Seebeck effect.
1,011 citations
TL;DR: In this article, the spin Seebeck effect is greatly enhanced by nonequilibrium phonons that drag the low-lying spin excitations, and they further argue that this scenario gives a clue to understand the observation of spin seebeck effect that is unaccompanied by a global spin current, and predict that the substrate condition affects the observed signal.
Abstract: We investigate both theoretically and experimentally a gigantic enhancement of the spin Seebeck effect in a prototypical magnet LaY2Fe5O12 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.
143 citations
TL;DR: It is reported that spin-polarized quasiparticles in superconducting aluminium layers have surprisingly long spin lifetimes, nearly a million times longer than in their normal state.
Abstract: There has been an intense search in recent years for long-lived spin-polarized carriers for spintronic and quantum-computing devices. Here we report that spin-polarized quasiparticles in superconducting aluminium layers have surprisingly long spin lifetimes, nearly a million times longer than in their normal state. The lifetime is determined from the suppression of the aluminium's superconductivity resulting from the accumulation of spin-polarized carriers in the aluminium layer using tunnel spin injectors. A Hanle effect, observed in the presence of small in-plane orthogonal fields, is shown to be quantitatively consistent with the presence of long-lived spin-polarized quasiparticles. Our experiments show that the superconducting state can be significantly modified by small electric currents, much smaller than the critical current, which is potentially useful for devices involving superconducting qubits.
92 citations
TL;DR: It is revealed that, despite the absence of conduction electrons, the magnetic insulator LaY(2)Fe(5)O(12) can convert a heat flow into a spin voltage, which can be converted into an electric voltage as a result of the inverse spin Hall effect.
Abstract: Thermoelectric generation is an essential function of future energy-saving technologies. However, this generation has been an exclusive feature of electric conductors, a situation which inflicts a heavy toll on its application; a conduction electron often becomes a nuisance in thermal design of devices. Here we report electric-voltage generation from heat flowing in an insulator. We reveal that, despite the absence of conduction electrons, a magnetic insulator LaY2Fe5O12 converts a heat flow into spin voltage. Attached Pt films transform this spin voltage into electric voltage by the inverse spin Hall effect. The experimental results require us to introduce thermally activated interface spin exchange between LaY2Fe5O12 and Pt. Our findings extend the range of potential materials for thermoelectric applications and provide a crucial piece of information for understanding the physics of the spin Seebeck effect.
84 citations
TL;DR: In this paper, the inverse spin-Hall effect (ISHE) was used to detect the spin-Seebeck effect (SSE) in a ferromagnetic Ni 81 Fe 19 film at room temperature.
67 citations
01 Jan 2010
TL;DR: In this article, the spin current across the interface between a normal metal and an insulating ferromagnet in a junction was studied, which is proportional to spin accumulation as well as the population of magnons.
Abstract: We study the spin current across the interface between a normal metal and an insulating ferromagnet in a junction. At the interface, conduction electrons in the normal metal interact with localized moments of the ferromagnet via the sd-type exchange interaction. In the presence of spin accumulation in the normal metal, the spin accumulation decays by spin-flip scattering of conduction electrons at the interface, thereby emitting magnons through the exchange interaction at the interface. Using the linear response theory, we obtain the spin current through the interface, which is proportional to spin accumulation as well as the population of magnons.
58 citations
TL;DR: In this article, a semiclassical theory of spin diffusion in a ferromagnetic metal subject to a temperature gradient is presented, where spin-flip scattering can generate pure thermal spin currents by shortcircuiting spin channels while suppressing spin accumulations.
18 citations
TL;DR: In this paper, 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.
13 citations
TL;DR: In this paper, the lifetime of spin-polarized quasi-particles in superconducting aluminum layers has been determined from the suppression of the aluminum's superconductivity resulting from the accumulation of spin polarized carriers in the aluminum layer using tunnel spin injectors.
Abstract: There has been an intense search in recent years for long-lived spin-polarized carriers for spintronic and quantum-computing devices. Here we report that spin polarized quasi-particles in superconducting aluminum layers have surprisingly long spin-lifetimes, nearly a million times longer than in their normal state. The lifetime is determined from the suppression of the aluminum's superconductivity resulting from the accumulation of spin polarized carriers in the aluminum layer using tunnel spin injectors. A Hanle effect, observed in the presence of small in-plane orthogonal fields, is shown to be quantitatively consistent with the presence of long-lived spin polarized quasi-particles. Our experiments show that the superconducting state can be significantly modified by small electric currents, much smaller than the critical current, which is potentially useful for devices involving superconducting qubits.
11 citations
TL;DR: In this paper, a superconductor/ferromagnetic insulator/superconductor (S/FI/S) junction was considered, and it was found that the currentvoltage (I-V) characteristic shows two resonant peaks.
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. 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.
TL;DR: In this article, the effect of the interlayer exchange coupling of synthetic ferrimagnetic (SyF)-free layer on the current induced magnetization switching (CIMS) in magnetic tunnel junctions (MTJs), in the thermal activation regime, was examined.
Abstract: In order to study the effect of the interlayer exchange coupling of synthetic ferrimagnetic (SyF)-free layer on the current induced magnetization switching (CIMS) in magnetic tunnel junctions (MTJs), in the thermal activation regime, we examine the magnetization dynamics of MTJs with the SyF-free layer based on a macrospin model, combining with the STT analysis. Compared with the MTJ with a single layer, the reduction of the critical current (Jc) is observed in the MTJ with the SyF-free layer. The considerable reduction of Jc is also observed in MTJs with the ferromagnetically coupled-free layer over that with the antiferromagnetically coupled free layer. In connection with CIMS in each SyF-free layer of the MTJ, we found two different types of CIMS dependent on Jex, that is, decoupled CIMS and coupled CIMS.
TL;DR: In this article, the effect of spin dynamics on the phase dynamics in a ferromagnetic Josephson junction, which is composed of two superconductors separated by a magnet, was studied.
Abstract: We study the effect of the spin dynamics on the phase dynamics in a ferromagnetic Josephson junction, which is composed of two superconductors separated by a ferromagnet. A new phenomenological model for the phase variable is proposed by including the spin dynamics excited by a ferromagnetic resonance (FMR) in the gauge invariant phase of s -wave SC’s. We find that the I – V characteristics show step structures due to the FMR. The step amplitude induced by FMR in the I – V characteristic has a strong dependence on the geometry of the cross section in the junction.
01 Jan 2010
TL;DR: In this article, a spin and charge transport in magnetic tunnel junctions with synthetic ferrimagnetic layers in the ballistic regime was analyzed and the spin transfer torque was derived based on a spin-polarized free-electron model.
Abstract: Based on a spin-polarized free-electron model, spin and charge transports are analyzed in magnetic tunnel junctions with synthetic ferrimagnetic layers in the ballistic regime, and the spin transfer torque is derived. We characterize the synthetic ferrimagnetic free layer by extending an arbitrary direction of magnetizations of the two free layers forming the synthetic ferrimagnetic free layer. The synthetic ferrimagnetic configuration exerts the approximately optimum torque for small magnetization angle of the first layer relative to that of the pinned layer. For approximately anti-parallel magnetization of the first layer to that of the pinned layer, the parallel magnetization of two magnetic layers is favorable for magnetization reversal rather than the synthetic ferrimagnetic configuration.