Showing papers by "Saburo Takahashi published in 2009"

Observation of the Spin Seebeck Effect.

Abstract: The generation of electric voltage by placing a conductor in a temperature gradient is called the Seebeck effect. Its efficiency is represented by the Seebeck coefficient, S, which is defined as the ratio of the generated electric voltage to the temperature difference, and is determined by the scattering rate and the density of the conduction electrons. The effect can be exploited, for example, in thermal electric-power generators and for temperature sensing, by connecting two conductors with different Seebeck coefficients, a device called a thermocouple. Here we report the observation of the thermal generation of driving power, or voltage, for electron spin: the spin Seebeck effect. Using a recently developed spin-detection technique that involves the spin Hall effect, we measure the spin voltage generated from a temperature gradient in a metallic magnet. This thermally induced spin voltage persists even at distances far from the sample ends, and spins can be extracted from every position on the magnet simply by attaching a metal. The spin Seebeck effect observed here is directly applicable to the production of spin-voltage generators, which are crucial for driving spintronic devices. The spin Seebeck effect allows us to pass a pure spin current, a flow of electron spins without electric currents, over a long distance. These innovative capabilities will invigorate spintronics research.

Electric detection of spin wave resonance using inverse spin-Hall effect

Abstract: Spin wave resonance in Ni81Fe19/Pt thin wire arrays has been investigated using the inverse spin-Hall effect (ISHE). The spin wave in the Ni81Fe19 layer drives spin pumping, generation of spin currents from magnetization precession, and the pumped spin current is converted into a charge current by ISHE in the Pt layer. We found an electromotive force transverse to the spatial and the spin-polarization directions of the spin current. The experimental results indicate that the amplitude of the electromotive force is proportional to the spin wave resonance absorption intensity, enabling the electric measurement of spin wave resonance in nanostructured magnetic systems.

Phenomenological analysis for spin-Seebeck effect in metallic magnets

Abstract: The two-band spin diffusion model has been extended to nonequilibrium systems to investigate the recently discovered spin-Seebeck effect in a ferromagnetic metal. A calculation using this model well reproduces the experimental results for a Ni81Fe19 film; the gradient of electrochemical potential is different between up- and down-spin bands affected by a temperature difference between the ends of the film.

Spin transfer torque in magnetic tunnel junctions with synthetic ferrimagnetic layers

Abstract: Based on the spin-polarized free-electron model, spin and charge transports are analyzed in the magnetic tunnel junctions with the synthetic ferrimagnetic layers in the ballistic regime, and the spin-transfer torque is derived In the realistic junctions, the spin torque exerted on the magnetizations of two ferromagnetic layers in the synthetic ferrimagnetic layer shows a trend to rotate the same direction It is suggested that, through the antiferromagnetic interlayer coupling in the synthetic ferrimagnetic layer, this trend induces the cooperative reversal of magnetizations in two ferromagnetic layers, and expected that this cooperative rotation reduces the critical current for the magnetization reversal in the synthetic ferrimagnetic layer

Spin transfer torque in magnetic tunnel junctions with synthetic ferrimagnetic layers

Abstract: Based on the spin-polarized free-electron model, spin and charge transports are analyzed in the magnetic tunnel junctions with the synthetic ferrimagnetic layers in the ballistic regime, and the spin-transfer torque is derived. In the realistic junctions, the spin torque exerted on the magnetizations of two ferromagnetic layers in the synthetic ferrimagnetic layer shows a trend to rotate the same direction. It is suggested that, through the antiferromagnetic interlayer coupling in the synthetic ferrimagnetic layer, this trend induces the cooperative reversal of magnetizations in two ferromagnetic layers, and expected that this cooperative rotation reduces the critical current for the magnetization reversal in the synthetic ferrimagnetic layer.

Sign reversal of ac Josephson current in a ferromagnetic Josephson junction

Abstract: The ac Josephson effect in a ferromagnetic Josephson junction, which is composed of two superconductors separated by a ferromagnetic metal (FM), is studied by a tunneling Hamiltonian and Green's function method. We obtain two types of superconducting phase dependent current, i.e., Josephson current and quasiparticle-pair-interference current (QPIC). These currents change their signs with thickness of the FM layer due to the 0-$\pi$ transition characteristic to the ferromagnetic Josephson junction. As a function of applied voltage, the Josephson critical current shows a logarithmic divergence called the Riedel peak at the gap voltage, while the QPIC shows a discontinuous jump. The Riedel peak reverses due to the 0-$\pi$ transition and disappears near the 0-$\pi$ transition point. The discontinuous jump in the QPIC also represents similar behaviors to the Riedel peak. These results are in contrast to the conventional ones.

Landau–Lifshitz–Gilbert study of the effect of pulse width on spin-transfer torque magnetization switching

Abstract: We studied the effect of a current pulse width on current-induced magnetization switching in magnetic tunnel junctions based on a macrospin model of the free layer. We performed finite temperature Langevin simulations of the Landau–Lifshitz–Gilbert–Slonczewski equation with an additional spin-torque term. By evaluating the switching current density, we obtained the diagram in the plane of the critical current density and the pulse width at 300 K. As the pulse width increased, we observed an adiabatic regime in the shorter pulse widths, an intermediate crossover regime, and a thermally activated regime in long pulse widths. We found that the easy-plane anisotropy field shifts the crossover pulse width to the lower pulse width, suggesting that the reversed region is enhanced by controlling the device shape. Our results are consistent with those of recent experiments over the pulse widths ranging from 10−1 to 105 ns.

Sign Reversal of AC Josephson Current in a Ferromagnetic Josephson Junction

Abstract: The ac Josephson effect in a ferromagnetic Josephson junction, which is composed of two superconductors separated by a ferromagnetic metal (FM), is studied by a tunneling Hamiltonian and Green's function method. We obtain two types of superconducting phase dependent currents, i.e., Josephson current and quasiparticle-pair-interference current (QPIC). These currents change their signs with thickness of the FM layer due to the 0–π transition characteristic to the ferromagnetic Josephson junction. As a function of applied voltage, the Josephson critical current shows a logarithmic divergence called the Riedel peak at the gap voltage, while the QPIC shows a discontinuous jump. The Riedel peak reverses due to the 0–π transition and disappears near the 0–π transition point. The discontinuous jump in the QPIC also represents similar behaviors to the Riedel peak. These results are in contrast to the conventional ones.

Phase and spin dynamics in a superconductor/ferromagnet/superconductor junction

Abstract: We study a phase dynamics induced by a spin dynamics in a ferromagnetic Josephson junction, in which two superconductors (SC's) are separated by a ferromagnetic layer A new phenomenological model for the phase variable is proposed by including the spin dynamics excited by the ferromagnetic resonance (FMR) in the gauge invariant phase of s-wave SC's We found that the current-voltage characteristics show step structures by tuning the microwave frequency to FMR The result originats from the coupling between the spin and the phase dynamics, and provides a new route to observe the spin wave excitation by using the Josephson effect

Quantum Monte Carlo Study of Anderson Magnetic Impurities in Semiconductors

Abstract: We use quantum Monte Carlo simulations to study the electronic properties of Anderson magnetic impurities in a semiconductor host. We find that in a semiconductor the magnetic impurities exhibit ferromagnetic correlations, which can have a much longer range than in a metallic host. In particular, the range is longest when the Fermi level is located between the top of the valence band and the impurity bound state. We study the dependence of the ferromagnetic correlations on the parameters of the Anderson model, and the dimensionality and band structure of the host material. Using the tight-binding approximation for calculating the host band structure and the impurity–host hybridization, we obtain an impurity bound state, which is located at ≈100meV above the top of the valence band, which is in agreement with the transport measurements on GaAs with dilute Mn impurities.