Utilization of "nuclear spin," the rotation motion of atomic nuclei: ramifications for thermal power generation

A research group comprised of Assistant Professor Takashi Kikkawa and Professor Eiji Saitoh of the University of Tokyo School of Engineering, in collaboration with Associate Professor Yuki Shiomi from the Graduate School of Arts and Sciences, University of Tokyo; Academic Researcher Saburo Takahashi from the Advanced Institute for Materials Research, Tohoku University; and Assistant Professor Koichi Oyanagi from the Graduate School of Science and Engineering and the Department of Physical Science and Materials Engineering, Iwate University successfully demonstrated novel thermal power generation using nuclear spin, which occurs as a result of the rotational motion of atomic nuclei.

The phenomenon in which the temperature difference in the environment produces electricity is referred to as the thermoelectric conversion phenomenon. This phenomenon has been investigated globally over 200 years since the discovery of the Seebeck effect in 1821. Through it's utilization, thermal power generation that creates electrical energy from exhaust heat can be realized, and is attracting attention as a basic element of next-generation clean energy technology.

The research group demonstrated a novel thermoelectric conversion phenomenon using nuclear spin, which is the property of the rotation of atomic nuclei in matter. Nuclear spins undergo thermal fluctuation with extremely small energy compared with electrons, even in the ultra-low temperature region near absolute zero, where the movement of electrons completely stops. The research group succeeded in converting this thermal fluctuation into electric power using the spintronics technology.

In their demonstration, the group focused on manganese carbonate (MnCO₃). MnCO₃ is a magnet material composed of ⁵⁵Mn nuclei with large nuclear spins (I = 5/2), and it is known to have a very strong interaction between nuclear spins and electron spins. This interaction not only increases the nuclear spin polarization rate, but also makes the spin direction controllable by an external magnetic field.

For the experiment, a sample in which platinum (Pt) was deposited on MnCO₃ was used. When a temperature difference is induced to this junction structure, the thermal fluctuation of the spin of the ⁵⁵Mn nucleus of MnCO₃ produces a spin current, which is the flow of the spin properties (magnetic flow), at the MnCO₃ / Pt interface. The spin current created in this manner can be detected as a voltage by a relativistic effect, referred to as the inverse spin Hall effect in Pt.

The research group found that the strength of the voltage signal increased at ultra-low temperatures of 0.1 K (-273.05 ℃), and the signal was not suppressed even in a strong magnetic field region (14 Tesla). According to Assistant Professor Kikkawa, "Nuclear spin has been mainly used as a tool for analysis, such as in magnetic resonance imaging (MRI) technology used in the medical field. With our achievement here, we have gained a new perspective that nuclear spin has the function of generating electricity and electric current at low temperatures, which can be utilized in thermal energy based technologies. We are expecting to develop new thermal engineering, which was difficult in conventional electronic technology. This is important in quantum science and technology, such as cooling technology and thermal control to stabilize the ultra-low temperature environment, and it can be realized by further investigating the phenomenon discovered in this study."

■ Seebeck effect: When two different metals are connected and a temperature difference is applied to both contacts, a voltage is generated between the metals and an electric current flows. Discovered by German scientist, Thomas Johann Seebeck in 1821.

■ Inverse spin Hall effect: A phenomenon in which a voltage is generated in the direction perpendicular to the direction in which the spin current flows. The interaction between the spin and orbit of an electron causes the upward-spin and downward-spin electrons to be scattered in opposite directions.