Spin-Seebeck Effect Causing New Magnetic Semiconductor To Become Energy Harvester

A gallium manganese arsenide semiconductor may be the material to revolutionize how electronic circuits power and recycle their waste heat, through a research of Ohio State University scientists, led by Joseph Heremans and Roberto Myers, both from OSU. They relate about the discovery of an effect converting heat into a quantum mechanical phenomenon, known as spin, in the above-mentioned semiconductor.

The two researchers tried to combine spintronics (electronics that use the spin of electrons to read/write data) with thermoelectricity, which transforms heat into electricity through the Seebeck effect. The spin-Seebeck effect (the conversion of heat to spin polarization) had been observed in action in 2008, but only on a metal rod, not on a semiconductor. They called their new heat-to-electron spin discovery “thermo-spintronics.”

“Spintronics is considered as a possible basis for new computers in part because the technology is claimed to produce no heat. Our measurements shed light on the thermodynamics of spintronics, and may help address the validity of this claim,” Heremans said.

“All of the computers we have now could actually run much faster than they do, but they’re not allowed to — because if they did, they would fail after a short time,” Myers said. “So a huge amount of money in the semiconductor industry is put toward thermal management.”

Gallium arsenide is usually found in electronics such as PDAs and cell phones, but the addition of manganese makes the semiconductor have magnetic properties. It is for the first time that the spin-Seebeck effect has been independently verified in a semiconductor.

How the scientists performed their tests

The spin of the charges in the gallium-manganese-arsenide semiconductor line up parallel to the orientation of the material’s overall magnetic field. When the researchers tried to detect the spin of the electrons, they had been actually measuring whether the electrons in any particular area of the material were oriented as “spin-up” or “spin-down.”

They heated one side of the single-crystal semiconductor film and measured the spin orientations on the hot and cool side, observing the spin-up or spin-downs. Then, the researchers found out that two pieces of material do not need to be physically connected for the effect to propagate from one to another. They separated two pieces of material electrically with the help of a pile, and applied heat to one side. The effect persisted despite the material not having electric contact.

“We figured that each piece would have its own distribution of spin-up and spin-down electrons,” said Myers. “Instead, one side of the first piece was spin up, and the far side of the second piece was spin down. The effect somehow crossed the gap.”

“The original spin-Seebeck detection by the Tohoku group baffled all theoreticians,” Heremans added. “In this study, we’ve independently confirmed those measurements on a completely different material. We’ve proven we can get the same results as the Tohoku group, even when we take the measurements on a sample that’s been separated into two pieces, so that electrons couldn’t possibly pass between them.”

It’s nice that although the phenomenon driving the spin-Seebeck effect is still a mystery, the researchers find uses of it through various experiments.