Akihiro Kirihara

Thermoelectric Generation Based on Spin Seebeck Effects

The spin Seebeck effect (SSE) refers to the generation of a spin current as a result of a temperature gradient in magnetic materials including insulators. The SSE is applicable to thermoelectric generation because the thermally generated spin current can be converted into a charge current via spin-orbit interaction in conductive materials adjacent to the magnets. The insulator-based SSE device exhibits unconventional characteristics potentially useful for thermoelectric applications, such as simple structure, device-design flexibility, and convenient scaling capability. In this article, we review recent studies on the SSE from the viewpoint of thermoelectric applications. Firstly, we introduce the thermoelectric generation process and measurement configuration of the SSE, followed by showing fundamental characteristics of the SSE device. Secondly, a theory of the thermoelectric conversion efficiency of the SSE device is presented, which clarifies the difference between the SSE and conventional thermoelectric effects and the efficiency limit of the SSE device. Finally, we show preliminary demonstrations of the SSE in various device structures for future thermoelectric applications and discuss prospects of the SSE-based thermoelectric technologies.

Role of bulk-magnon transport in the temporal evolution of the longitudinal spin-Seebeck effect

M. Agrawal, 2, ∗ V. I. Vasyuchka, A. A. Serga, A. Kirihara, 3 P. Pirro, T. Langner, M. B. Jungfleisch, A. V. Chumak, E. Th. Papaioannou, and B. Hillebrands Fachbereich Physik and Landesforschungszentrum OPTIMAS, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany Graduate School Materials Science in Mainz, Gottlieb-Daimer-Strasse 47, 67663 Kaiserslautern, Germany Smart Energy Research Laboratories, NEC Corporation, Tsukuba 305-8501, Japan (Dated: May 11, 2014)

Flexible heat-flow sensing sheets based on the longitudinal spin Seebeck effect using one-dimensional spin-current conducting films

Heat-flow sensing is expected to be an important technological component of smart thermal management in the future. Conventionally, the thermoelectric (TE) conversion technique, which is based on the Seebeck effect, has been used to measure a heat flow by converting the flow into electric voltage. However, for ubiquitous heat-flow visualization, thin and flexible sensors with extremely low thermal resistance are highly desired. Recently, another type of TE effect, the longitudinal spin Seebeck effect (LSSE), has aroused great interest because the LSSE potentially offers favourable features for TE applications such as simple thin-film device structures. Here we demonstrate an LSSE-based flexible TE sheet that is especially suitable for a heat-flow sensing application. This TE sheet contained a Ni0.2Zn0.3Fe2.5O4 film which was formed on a flexible plastic sheet using a spray-coating method known as “ferrite plating”. The experimental results suggest that the ferrite-plated film, which has a columnar crystal structure aligned perpendicular to the film plane, functions as a unique one-dimensional spin-current conductor suitable for bendable LSSE-based sensors. This newly developed thin TE sheet may be attached to differently shaped heat sources without obstructing an innate heat flux, paving the way to versatile heat-flow measurements and management.

Local Spin-Seebeck Effect Enabling Two-Dimensional Position Sensing

The spin-Seebeck effect (SSE) in magnetic insulators is shown to be applicable to two-dimensional (2D) position sensing using an Y3Fe5O12 (YIG) slab covered with a Pt-film mesh. When a part of the YIG-slab/Pt-mesh sample was heated, the position of the heated part of the sample was found to be known from the measured SSE signals in the Pt mesh. Since the SSE-based position-sensing method allows commonly-used insulators to produce 2D position information, it may be useful for constructing versatile thermally-driven user-interface devices and image-information sensors.

Damping of parametrically excited magnons in the presence of the longitudinal spin Seebeck effect

The impact of the longitudinal spin Seebeck effect (LSSE) on the magnon damping in magnetic-insulator/nonmagnetic-metal bilayers was recently discussed in several reports. However, results of those experiments can be blurred by multimode excitation within the measured linewidth. In order to avoid possible intermodal interference, we investigated the damping of a single magnon group in a platinum covered Yttrium Iron Garnet (YIG) film by measurement of the threshold of its parametric excitation. Both dipolar and exchange spin-wave branches were probed. It turned out that the LSSE-related modification of spin-wave damping in a micrometer-thick YIG film is too weak to be observed in the entire range of experimentally accessible wavevectors. At the same time, the change in the mean temperature of the YIG layer, which can appear by applying a temperature gradient, strongly modifies the damping value.

Spin-Seebeck thermoelectric converter

Thermoelectric conversion (TEC) technologies, which convert heat into electricity, have received a great attention, because they are expected to be a powerful approach to utilize wasted thermal energy. Here we present novel thermoelectric converters based on the spin Seebeck effect (SSE), and show their scaling law which is largely different from that of conventional TEC devices. We experimentally demonstrate that the TEC output signals straightforwardly increase with the size of the converters. This scaling law enables us to implement simple-structured thermoelectric converters by using productive film-coating methods. Such coating-based TEC techniques may pave the way for a wide range of applications using a variety of heat sources.

Corrections to “Thermoelectric Generation Based on Spin Seebeck Effects” [DOI: 10.1109/JPROC.2016.2535167]

Corrections were made to information presented in Figure 2 from the paper, “Thermoelectric generation based on spin Seebeck effects,” Uchida, F. et al, Proc. IEEE, 2016, DOI: 10.1109/JPROC.2016.2535167.

Gamma radiation resistance of spin Seebeck devices

Thermoelectric devices based on the spin Seebeck effect (SSE) were irradiated with gamma (γ) rays with the total dose of around 3 × 105 Gy in order to investigate the γ-radiation resistance of the devices. To demonstrate this, Pt/Ni0.2Zn0.3Fe2.5O4/Glass and Pt/Bi0.1Y2.9Fe5O12/Gd3Ga5O12 SSE devices were used. We confirmed that the thermoelectric, magnetic, and structural properties of the SSE devices are not affected by the γ-ray irradiation. This result demonstrates that SSE devices are applicable to thermoelectric generation even in high radiation environments.

Optical spectroscopy of charge-tunable quantum dots emitting at 1.2 μm

In this study, we investigated the optical characteristics of single charge-tunable InAs QDs emitting at 1.2 μm, whose size is about twice those discussed in the previous reports. For probing electronic shell structures, large QDs are favorable in terms of deep confinement potentials and weak Coulomb interaction.

Time-resolved photoluminescence measurement of exciton and biexciton in an InAs/GaAs single quantum dot

Dynamical behaviors of the exciton and biexciton luminescence at the wavelength of 1.18 μm from an InAs/GaAs single quantum dot are studied at 4.3 K. The rate equation analysis on the luminescence decays shows that the biexciton lifetime is longer than the exciton lifetime. The estimated lifetimes reflect molecular nature of the biexciton in the single quantum dot. Single photon sources working at the communication wavelength are quite desired for the practical applications of quantum information processing typically represented by quantum key distribution. In such a situation, the spectroscopic studies on single quantum dots having the resonant wavelength longer than 1 μm have recently attracted much attention [1,2]. Apart from these practical interests, InAs self-assembled quantum dots having the longer resonant wavelength will show interesting properties, such as large binding energy of biexciton, large energy separation between the confinement levels, due to strong confinement effects [3]. In this presentation, we report on the time-dependence of the luminescence peaks attributed to the exciton and biexciton in an InAs/GaAs single quantum dot at the wavelength of 1.18 μm by using time-correlated photon counting technique. We analyzed the time dependence of the luminescence decays of the exciton and biexciton by rate equations describing the cascade process from the biexciton to exciton after the biexciton radiative recombination. As a result of the rate equation analysis, we found that the biexciton radiative lifetime, τB, is longer than the exciton lifetime, τX. The ratio τX/τB is expected to be two when the constituent excitons of a biexciton are independent. The deviation of the ratio from two for the excitons in low dimensional structures was theoretically predicted by Citrin and Takagahara as a result of the quantum confinement and dielectric confinement [4,5]. Bacher et al. reported the ratio, τX/τB, close to one by the time-resolved measurement on the exciton and biexciton luminescence in single CdSe/ZnSe quantum dots [6]. Santori et al. reported the ratio τX/τB ~ 1.5 with InAs/GaAs single quantum dots whose resonant wavelength was about 0.88 μm [7]. Our estimated ratio τX/τB even less than one is explained with the quantum confinement effects dependent on the quantum dot dimension. We used the InAs quantum dots grown on (001) GaAs surface by molecular beam epitaxy at 590 °C. The average diameter and height of the quantum dots were estimated to be 49 nm and 13 nm respectively by using an atomic force microscope. The dot density was about 10 dots/cm. In order to reduce the number of the quantum dots in the focus area of a microscope, we fabricated micro apertures on the 100-nm-thick Au layer on the sample surfaces by using electron beam lithography and lift-off technique. The sample was cooled down to 4.3 K in a conduction type microscope cryostat mounted on a piezo-actuated translation stage. Figure 1(a) shows luminescence spectra observed from a 0.6-μm-diameter aperture under band-to-band excitation using a He-Ne laser. At weak excitation, shown in the lowest trace in Fig. 1(a), a single peak labeled X was observed at 1.0483 eV. Another peak labeled B appeared at 1.0471 eV when the excitation intensity was increased. The integrated intensities of these peaks were plotted as a function of the excitation laser intensity in Fig. 1(b). At low excitation power, the intensity of the peak X is proportional to the excitation laser intensity, while that of the peak B is proportional to the square of the excitation laser intensity. According to this excitation intensity dependence, the peak X and B are attributed to the exciton and biexciton recombination respectively. The binding energy of the biexciton was estimated to be 1.2 meV. The time-dependences of these luminescence JTuH3-4