KEY CONCEPTS
• New research shows a paramagnetic material can act in a thermoelectric manner. 
• Evaluation of lithium-doped manganese telluride showed that thermoelectric properties were found at temperatures above 300 K. 
• Paramagnon drag occurs in a more localized manner covering only two to three atoms in length over a short period of time.

Research is continuing to develop more effective ways for utilizing heat that is the by-product of inefficient electrical and mechanical processes. One prime example is the wasteful heat generated from a light bulb (see Figure 2).


Figure 2. Paramagnetic materials now can be used to harness wasteful heat generated from components such as a light bulb. (Figure courtesy of The Ohio State University.)

Thermoelectric materials have shown promise in converting heat into electricity by taking advantage of significant temperature differences between the heat produced by a specific device and ambient temperature. One parameter that researchers are working to improve is the figure of merit (ZT), which is a unitless figure directly related to the efficiency of a thermoelectric material.

In a previous TLT material, researchers used spin caloritronics to include the spin of electrons in polarized materials by adding this phenomenon to materials that have strong electrical conductivity but weak thermal conductivity (1). A theoretical study indicated that a transition metal oxide based on cobalt, nickel and zinc appears to exhibit superior thermoelectric properties. A potential transition metal oxide was found by substituting 25% zinc and 75% nickel for one of the three cobalt atoms.

The origin of thermoelectric materials is due to their ferromagnetic properties. Joseph Heremans, professor of mechanical and aerospace engineering and Ohio Eminent Scholar in Nanotechnology at The Ohio State University in Columbus, Ohio, says, “Magnetic moments created by unfilled energy levels in atomic orbitals are aligned to produce permanent magnetism. As the material is heated, magnetism declines and the electron spins fall out of alignment creating waves known as magnons.”

Magnons are able to interact with the electrons present in ferromagnetic materials when a temperature gradient is present in a material. Heremans says, “This interaction leads to the movement of electrons to the cold side of the material. As one side of a ferromagnetic material is heated, the cold side becomes more magnetic producing spin that leads to the electron flow generating electricity. Magnons are involved by carrying electrons to the cold side of the material in a process known as magnon drag. An analogy for this is the example of a mudslide. Particles of sand by themselves do not cause much of a problem when flowing down the side of a mountain. But when water is used to drag the sand, this creates a more significant effect leading to the movement of a greater flow of sand.”

Magnon drag makes a considerable contribution to the effectiveness of thermoelectric materials. Magnetic materials lose their effectiveness at high temperature becoming converted into paramagnets. Heat leads to the speeding up of atoms but enables them to spin in all different direction reducing their alignment and magnetism. This creates the phenomenon of paramagnetism. 

Heremans says, “An analogy for the conversion of magnetism to paramagnetism is the melting of water where the order present in the solid phase is lost as more randomness is present in the liquid phase.”

To date, there has been no indication that paramagnetic materials can act in a thermoelectric manner. New research has just determined that paramagnetic materials can help to improve the conversion of heat into electricity. 

Paramagnon drag
Heremans and his colleagues determined that a paramagnetic material can be designed to produce a thermoelectric figure of merit greater than one at temperatures higher than 900 K. He says, “We worked with lithium-doped manganese telluride, which is a well-known semiconductor that acts as a thermoelectric material that is known to generate a strong magnon drag effect. Our approach was to dope manganese telluride with specific concentrations of lithium and evaluate the thermopower produced as the temperature is increased.”

Heremans pointed out that no work had previously been done to evaluate paramagnetic materials. He says, “In the past, it was not believed that magnon drag could extent to paramagnets. When we worked with lithium-doped manganese telluride we initially found magnon drag at low temperatures. As the temperature increased to above 300 K and the lattice for this material melted, we found that magnons can survive in the paramagnetic regime. These particles we designated as paramagnons, and we found they produce a similar drag effect to what is seen in the ferromagnetic regime.”

Heremans believes that paramagnon drag occurs in a more localized manner that covers only two to three atoms in length for a short period of time. He uses the analogy of liquid water molecules that retain the memory of the solid before they melt. He says, “These liquid molecules are not completely random but tend to arrange themselves in smaller clusters than are found with ice.”

The researchers used neutron scattering experiments demonstrated the existence of paramagnons. Heremans says, “In the paramagnetic regime, we found that paramagnons contribute 80%-90% of the thermoelectric effect at high temperatures.”

Lithium doping is an essential aspect of this work. The researchers evaluated the performance of non-intentionally doped manganese telluride and found that the figure of merit is much lower than when lithium doping is carried out. 

This is the first study to determine that paramagnetic materials can contribute to the thermoelectric effect. Heremans indicated that the researchers will not be revisiting other semiconductors to determine their effectiveness in the paramagnetic regime. He says, “We will be evaluating the thermoelectric materials, lead and germanium manganese telluride, which should be effective in the paramagnetic regime at elevated temperatures.”

Additional information on this study can be found in a recent article (2) or by contacting Heremans at heremans.1@osu.edu. 

REFERENCES
1. Canter, N. (2019), “New thermoelectric material based on cobalt,” TLT, 75 (8), pp. 14-15.
2. Zheng, Y., Lu, T., Polash, M., Rasoulianboroujeni, M., Liu, N., Manley, M., Deng, Y., Sun, P., Chen, X., Hermann, R., Vashaee, D., Heremans, J. and Zhao, H. (2019), “Paramagnon drag in high thermoelectric figure of merit Li-doped MnTe,” Science Advances, 5 (9), eaat9461.