Paramagnetic spins drive electrons, produce electricity from heat

An international team of researchers has observed that local thermal perturbations of spin in a solid can convert heat into energy even in a paramagnetic material, while spins are not expected to correlate long enough. This effect, called by the paramagnon resistance thermopower researchers, converts a temperature difference into an electrical voltage. This discovery could allow more efficient recovery of thermal energy, for example by converting the heat released by cars into electrical energy to improve energy efficiency, or by feeding intelligent clothing through body heat.

The research team includes scientists from North Carolina State University, the ORNL (Oak Ridge National Laboratory) Department of Energy, the Chinese Academy of Sciences and Ohio State University.

In solids with magnetic ions (eg, manganese), thermal perturbations of the spins can be aligned with each other (ferromagnetic or antiferromagnetic) or not (paramagnetic). However, spins are not completely random in paramagnetics: they form short – lived, short – range, locally ordered structures – paramagnons – that only exist for a millionth of a millionth of a second and do not occur at all. extend only on two to four atoms. In a new article describing the work, the researchers showed that despite these disadvantages, even the paramagnons can evolve in a temperature difference and propel free electrons with them, thus creating a thermopower with paramagnetic drag.

In a proof of concept, the team observed that the paramagnon drag in manganese telluride (MnTe) extends to very high temperatures and generates a much higher thermoelectric power than can be expected. generate only electrons charges.

The research team tested the paramagnetic drag thermopower concept by heating lithium-doped MnTe at approximately 250 degrees Celsius higher than its Néel temperature (34 degrees Celsius) – the temperature at which the spins in the material lose their order magnetic long-range and the material. becomes paramagnetic.

"Above Néel's temperature, one would expect the thermal power generated by the spin waves to decrease," said Daryoosh Vashaee, professor of electrical engineering and computer science and materials science at NC State, and co – Corresponding article describing the work. "However, we did not see the expected decline and we wanted to know why."

At ORNL, the team used neutron spectroscopy at the source of spallation neutrons to determine what was happening in the material. "We observed that even though there were no sustained spin waves, localized ion groups would correlate their spins long enough to produce visible magnetic fluctuations," he said. says Raphael Hermann, materials scientist at ORNL and corresponding co-author of the article. The team has shown that the life span of these spin waves – about 30 femtoseconds – is long enough to allow drag entrainment of electron charges, which takes only about a femtosecond, a quadrillion of second. "Short-duration spin waves, therefore, could propel the charges and create enough thermal power to prevent the predicted fall," Hermann explains.

"Before this work, it was thought that magnetic drag could exist only in magnetic materials, not in paramagnetic ones," explains Joseph Heremans, professor of mechanical and aerospace engineering at Ohio State University and co-correspondent of the # 39; author. "Because the best thermoelectric materials are semiconductors and we do not know any ferromagnetic semiconductors at room temperature or higher, we had never thought that magnon drag could increase thermoelectric efficiency in This new discovery changes completely, we can now study paramagnetic semiconductors, there are many. "

"When we observed the sudden increase in the Seebeck coefficient below and near Neel's temperature, and that excess value was extended to high temperatures, we suspected that something basically related to the rotation had to be involved, "says Huaizhou Zhao, a professor at the Chinese Academy of Sciences. in Beijing and corresponding co-author of the article. "So we formed a research team with complementary expertise that laid the foundation for this discovery."

"The towers allow a new paradigm in thermoelectricity by easing the fundamental tradeoffs imposed by Pauli's exclusion from electrons," said Vashaee. "Just as during the discovery of the spin-Seebeck effect, which led to the new spincaloritronic zone, where the angular momentum of the spin is transferred to the electrons, both the spin waves (magnons) and local thermal fluctuations of magnetization in the paramagnetic state (ie paramagnons) can transfer their linear pulse to the electrons and generate thermopower. "

The search appears in Progress of science.