  Quantized spin-wave known as magnon, a bosonic quasiparticle, can drag
electrons or holes via sd exchange interaction and boost the thermopower over
the conventional diffusive thermopower. P-type magnon-drag thermopower has been
observed in both ferromagnetic and antiferromagnetic metallic and degenerate
semiconductors. However, it has been less reported for intrinsic or n-type
magnetic semiconductors; therefore, the impact of magnon-bipolar carrier drag
on thermopower has remained unexplored. Here, a theoretical model for
magnon-bipolar carrier drag thermopower is derived based on the magnon-carrier
interaction lifetimes. The model predicts that the bipolar carrier drag
thermopower becomes independent of both the carrier and magnon relaxation
times. A proof of concept experiment is presented that confirms this
prediction. We also report the observation of magnon-carrier drag thermopower
in n-type and intrinsic ferromagnetic semiconductors experimentally. The p-type
antiferromagnetic MnTe is doped with different amounts of Cr to produce
non-degenerate and n-type semiconductors of various carrier concentrations. Cr
dopants have a donor nature and create ferromagnetic-antiferromagnetic clusters
due to the Cr3+ oxidation state. Heat capacity measurements confirm the
presence of magnons in Cr-doped MnTe. It is shown that the magnon-drag
thermopower is significantly reduced for 3%-5% Cr-doped samples due to bipolar
drag effects and becomes negative for 14% and 20% Cr-doped MnTe due to dominant
magnon-electron drag thermopower.
