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Magnon Drag in Paramagnetic Materials Could Boost Thermoelectric Efficiency

An international research team has found that local thermal perturbations of spins in a solid have the ability to transform heat to energy even in a paramagnetic material, where spins were not considered to correlate sufficiently long to do so.

Magnon and paramagnon drag: Magnons, waves created by spins of individual atoms (grey cones) drag electrons (green dots) along a thermal gradient to create thermopower. In the paramagnetic state, local thermal fluctuations form small magnon packets that can similarly drag electrons. Classical paramagnets do not produce drag. (Image credit: Renee Ripley, Ohio State University)

This effect, termed “paramagnon drag thermopower” by the researchers, transforms a temperature difference into an electrical voltage. This finding could lead to more efficient harvesting of thermal energy—for instance, converting exhaust heat from a car into electric power to improve fuel efficiency, or powering smart clothing using body heat.

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

In solids that have magnetic ions (for example, manganese), thermal perturbations of spins either align with one another (ferromagnets or antiferromagnets) or do not align (paramagnets). But in paramagnets, spins are not completely random: they form locally ordered, short-lived, short-range structures called paramagnons—which remain only for a millionth of a billionth of a second and extend over just two to four atoms.

In a new article reporting on the study, scientists demonstrate that in spite of these defects, even paramagnons have the ability to move in a temperature difference and thrust free electrons along with them, thus generating paramagnon drag thermopower.

In a proof-of-concept discovery, the researchers noticed that in the case of manganese telluride (MnTe), paramagnon drag extends to extremely high temperatures and produces thermopower that is considerably stronger than what electron charges alone can produce.

The researchers tested the paramagnon drag thermopower concept by heating lithium-doped MnTe to about 250 °C above its Néel temperature (34 °C). Néel temperature is the temperature at which the material’s spins lose their long-range magnetic order and the material turns paramagnetic.

Above the Néel temperature, one would expect the thermopower being generated by the spin waves to drop off,” stated Daryoosh Vashaee, professor of electrical and computer engineering and materials science at NC State and co-corresponding author of the article describing the study. “However, we didn’t see the expected drop off, and we wanted to find out why.”

At ORNL, the researchers used neutron spectroscopy at the Spallation Neutron Source to identify what was taking place inside the material.

We observed that even though there were no sustained spin waves, localized clusters of ions would correlate their spins long enough to produce visible magnetic fluctuations.

Raphael Hermann, Study Co-Corresponding Author and Materials Scientist, ORNL

The researchers demonstrated that the lifespan of these spin waves, which is about 30 fs, was sufficiently long to allow the dragging of electron charges that needs only about 1 fs, or one quadrillionth of 1 second. “The short-lived spin waves, therefore, could propel the charges and create enough thermopower to prevent the predicted drop off,” stated Hermann.

Before this work, it was believed that magnon drag could exist only in magnetically ordered materials, not in paramagnets,” stated Joseph Heremans, professor of mechanical and aerospace engineering at the Ohio State University and co-corresponding author of the paper.

Heremans continued, “Because the best thermoelectric materials are semiconductors, and because we know of no ferromagnetic semiconductor at room temperature or above, we never thought before that magnon drag could boost the thermoelectric efficiency in practical applications. This new finding changes that completely; we can now investigate paramagnetic semiconductors, of which there are a lot.”

When we observed the sudden rise of Seebeck coefficient below and near the Néel temperature, and this excess value extended to high temperatures, we suspected something fundamentally related to spins must be involved

Huaizhou Zhao, professor at the Chinese Academy of Science in Beijing, and co-corresponding author of the paper.

Zhao continued: “So we formed a research team with complementary expertise which laid the groundwork for this discovery.”

Spins enable a new paradigm in thermoelectricity by alleviating the fundamental tradeoffs imposed by Pauli exclusion on electrons.” stated Vashaee.

Just as in the discovery of the spin-Seebeck effect, which led to the new area of spincaloritronics, where the spin angular momentum is transferred to the electrons, both the spin waves (i.e., magnons) and the local thermal fluctuations of magnetization in the paramagnetic state (i.e., paramagnons) can transfer their linear momentum to electrons and generate thermopower.

Daryoosh Vashaee, Professor of Electrical and Computer Engineering and Materials Science, NC State

The study has been reported in Science Advances and has been funded by the National Science Foundation, the Air Force Office of Scientific Research, and the U.S. Department of Energy Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Graduate students and co-first authors Yuanhua Zheng of the Ohio State University, Tianqi Lu of the Chinese Academy of Sciences, and Mobarak H. Polash of NC State contributed equally to the study. The Spallation Neutron Source at ORNL is a DOE Office of Science User Facility.


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