Tolerance of spin-Seebeck thermoelectricity against swift heavy-ion irradiation
S. Okayasu1), K. Harii2), J. Ieda1), M. Ono1), M. Ishida3), M. Kobata1), K. Yoshii1), T. Fukuda1)
and E. Saitoh4)
1): Spin Gr., JAEA 2): QST 3): NEC 4): Univ. of Tokyo

Thermoelectric (TE) elements, which transduce heat toelectricity, attract growing attention since it exhibits a promising
pathway to address demands for energy harvesting technologies
that would reutilize waste heat energy at factories and cars. The
potential of the TE devices in combination with radioisotope
elements may be beyond such commoditized uses to more harshenvironments as the isotope batteries for spacecraft in deepspace. The elemental structure of the TE device is a junction
consisting of two different semiconducting, or metallic materials,
a thermocouple. Applying temperature gradient on the TE
element, electrical voltage is generated on the junction. The TE
device is a series of junctions and these junctions can beaffected by high energy radiation. Therefore, the options of theradioactive isotopes for a nuclear battery are limited to somespecial nuclear species which emit an alpha ray only due to theshielding problem.
After recent discovery of the spin- driven thermoelectric(STE) generation, the notable advantage of the STE generation
stems from the availability of arranging heat and charge

The SSE voltage in Fig. 1-(c) completely disappears above thedose of Φ = 1×1012 ions/cm2, where the entire sample surface iscovered by the columnar defects, and the limitation of heavy ion
irradiation on the SSE devices is obtained. Using the limitation,
we evaluated lifetime of the SSE devices on the surface of aspent nuclear fuel (SNF). The fission fragments with ~100 MeV
energies can escape from a vitrified waste surface, and the SSE
devices may be affected by the fragments. According to themeasured dose of neutrons at the surface of the SNF ~1.5×1010
n/s, we estimate the maximum flux of the fission fragments ~108 ions/cm2/y. This crude estimate assures that the STE devices
work for more than 100 years around SNFs without degradation.
Tolerance of thermoelectric devices based on SSE against
swift heavy ion irradiation has been investigated [3]. It becomes
clear that the spin thermoelectricity devices may work morethan hundreds of years with radioactive thermal sources. The
present study demonstrates that SSE-based devices areapplicable to TE generation even in harsh environments for along time period.

conduction paths in the direction orthogonal to each path,allowing simple and flexible device structures, low-cost
fabrication processes, and the unique scaling characteristics ofoutput signals to device dimensions [1][2]. Furthermore, theSTE devices are expected to be tolerant against radiations, andtheir application fields expand to aerospace usage as magnetic
memories. If we combine the STE devices to the nuclear
batteries, it is possible to develop the next-generation nuclear
battery. The long-term performance of the STE device, however,under radioactive conditions is unconfirmed. We contrived anew method to avoid the weakness in the TE junctions via theradiations by replacement to STE devices. In this study, weinvestigated the performance of STE devices under heavy-ion
beam irradiation with varying the dose level.
The STE sample for this study consists of metallic (Pt) andmagnetic insulator (Y3Fe5O12, YIG) films formed on a substrate(Gd3Ga5O12, GGG) based on spin Seebeck effect (SSE). Swiftheavy ion irradiation on samples was performed at the Tandem
accelerator in JAEA Tokai Lab with 320 MeV gold ions at room
temperature. As is well known, when a high energy heavy ion
passes through the samples, high-density electric excitations
occur in semi-conducting or insulating materials, and nanoscale
amorphous columnar defects are formed along the ion tracks.
Since amorphous YIG becomes paramagnetic and does notcontribute to the spin transport, the irradiated regions become
ineffective for thermoelectric energy conversion via the SSE.
The SSE signal becomes smaller with increase the irradiationdose (Fig. 1-(a)). The linearity of the SSE voltages with ΔT ismaintained for all as-irradiated samples suggesting that thedecrease of the SSE voltage is a result of an increase of thedamaged area of the sample (Fig. 1-(b)).

Fig.1-(a) Measured SSE voltage for the ion-irradiated samples

with the fluence, Φ =0, 1x1010, 1x1011,1x1012, and1.4x1012 ions/cm2 (from top to bottom). The output
signals as a function of applied magnetic fields. The
temperature difference ΔT between Pt and the substrate
is fixed as 8 K.
-(b) The ΔT dependence of the SSE voltages with thedifferent as-irradiated samples.
-(c) The fluence dependence of the SSE voltages and thecalculated coverage ratio of the sample surface by theion tracks (solid curve).
References[1] K.-I. Uchida et al., Nature 455, 778–781 (2008)[2] K.-I. Uchida et al., Nature Mater. 9, 894 (2010)[3] S. Okayasu et al., J.of App. Phy.,128,083902(2010).