Paramagnon drag in high thermoelectric figure of merit Li-doped MnTe

Local thermal magnetization fluctuations in Li-doped MnTe are found to increase its thermopower a strongly at temperatures up to 900 K. Below the Neel temperature (T-N similar to 307 K), MnTe is antiferromagnetic, and magnon drag contributes alpha(md) to the thermopower, which scales as similar to T-3. Magnon drag persists into the paramagnetic state up to >3 x T-N because of long-lived, short-range antiferromagnet-like fluctuations (paramagnons) shown by neutron spectroscopy to exist in the paramagnetic state. The paramagnon lifetime is longer than the charge carrier-magnon interaction time; its spin- spin spatial correlation length is larger than the free-carrier effective Bohr radius and de Broglie wavelength. Thus, to itinerant carriers, paramagnons look like magnons and give a paramagnon-drag thermopower. This contribution results in an optimally doped material having a thermoelectric figure of merit ZT > 1 at T > similar to 900 K, the first material with a technologically meaningful thermoelectric energy conversion efficiency from a spin-caloritronic effect.