Magnon Drag and Spin Entropy in Antiferromagnetic Thermoelectrics
While the progress in traditional thermoelectric materials research based on the engineering of the charge carriers and phonons characteristics is reaching a plateau, adding spin and spin wave to the picture offers new degrees of freedom for developing new materials with unprecedented thermoelectric properties. The effect of spin on thermoelectric properties is a non-trivial topic with many aspects. In particular, spin entropy in hopping systems, dilute Kondo systems due to the resonant interaction of the magnetic impurities with free electrons, and magnon electron drag in magnetically ordered systems have all resulted in enhancement of the thermopower. Another significant effect, although not yet theoretically confirmed, is the paramagnon drag. Paramagnon drag has been experimentally observed in many paramagnetic spin fluctuation systems such as (Sc,Lu,Y)Co2  or UAl2  near their spin fluctuation temperatures. MnTe, a semiconducting A-type antiferromagnet, shows a robust magnon-electron drag near and below the Neel temperature. Interestingly, as we will show, this rather large enhancement of the thermopower did not diminish above the Neel temperature (~305K) and remained consistently high up to ~1000K (our highest measurement temperature). We will discuss several possible explanations of the observed large thermopower above the ordering temperature. Although spin entropy, magnon drag, and paramagnon drag have already shown zT enhancement in several materials, this is the first time that such effects lead to zT>1. A better understanding of the thermopower in the paramagnetic phase of MnTe can guide the development of higher performance thermoelectric materials which are not limited to the trade-off between the electrical conductivity and the thermopower.