J. Ieda

Observation of the spin Seebeck effect

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.

Universality classes for domain wall motion in the ferromagnetic semiconductor (Ga,Mn)As

Magnetic domain wall motion induced by magnetic fields and spin-polarized electrical currents is experimentally well established. A full understanding of the underlying mechanisms, however, remains elusive. For the ferromagnetic semiconductor (Ga,Mn)As, we have measured and compared such motions in the thermally activated subthreshold, or “creep,” regime, where the velocity obeys an Arrhenius scaling law. Within this law, the clearly different exponents of the current and field reflect different universality classes, showing that the drive mechanisms are fundamentally different.

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

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.

Matter-Wave Solitons in an F=1 Spinor Bose–Einstein Condensate

Following our previous work [J. Ieda, T. Miyakawa and M. Wadati: cond-mat/0404569] on a novel integrable model describing soliton dynamics of an F =1 spinor Bose–Einstein condensate, we discuss in detail the properties of the multi-component system with spin-exchange interactions. The exact multiple bright soliton solutions are obtained for the system where the mean-field interaction is attractive ( c 0 < 0) and the spin-exchange interaction is ferromagnetic ( c 2 < 0). A complete classification of the one-soliton solution with respect to the spin states and an explicit formula of the two-soliton solution are presented. For solitons in polar state, there exists a variety of different shaped solutions including twin peaks. We show that a “singlet pair” density can be used to distinguish those energetically degenerate solitons. We also analyze collisional effects between solitons in the same or different spin state(s) by computing the asymptotic forms of their initial and final states. The result reveals th...

Magnetic memory and current amplification devices using moving domain walls

A moving magnetic domain wall produces an electromotive force (emf). It is therefore possible to read the state of a magnetic memory device via the emf it produces when subjected to an interrogation pulse. It is also possible to amplify currents in pulse circuits, opening up the possibility of all magnetic logic circuits.

Inverse scattering method for square matrix nonlinear Schrödinger equation under nonvanishing boundary conditions

Matrix generalization of the inverse scattering method is developed to solve the multicomponent nonlinear Schrodinger equation with nonvanishing boundary conditions. It is shown that the initial value problem can be solved exactly. The multi-soliton solution is obtained from the Gel’fand-Levitan-Marchenko [Amer. Math. Soc. Transl. 1, 253 (1955)] equation.

Multicomponent Bright Solitons in F ¼ 2 Spinor Bose-Einstein Condensates

We study soliton solutions for the Gross--Pitaevskii equation of the spinor Bose--Einstein condensates with hyperfine spin F=2 in one-dimension. Analyses are made in two ways: by assuming single-mode amplitudes and by generalizing Hirotas direct method for multi-components. We obtain one-solitons of single-peak type in the ferromagnetic, polar and cyclic states, respectively. Moreover, twin-peak type solitons both in the ferromagnetic and the polar state are found.

Phenomenological analysis for spin-Seebeck effect in metallic magnets

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.

Exact soliton solutions of spinor bose-einstein condensates

We study matter-wave solitons in Bose-Einstein condensates of ultracold gaseous atoms with spin degrees of freedom and present a class of exact solutions based on the inverse scattering method. The one-soliton solutions are classified with respect to the spin states. We analyze collisional effects between solitons in the same or different spin state(s), which reveals a very interesting possibility: we can manipulate the spin dynamics by controlling the parameters of colliding solitons.

Bose–Einstein Condensation of Ideal Bose Gases

Bose–Einstein condensation (BEC) is studied for ideal boson gases with a wide class of the dispersion relations. A criterion of the BEC, the transition temperature and a fraction of the condensate are calculated under the appropriate thermodynamic limits. The correspondence between the dispersion relation (spectrum) and the trap potential is shown. This gives the criterion for the trap shape and the dimensionality of the system.