Magnon-bipolar carrier drag thermopower in antiferromagnetic/ferromagnetic semiconductors: theoretical formulation and experimental evidence

TL;DR

This paper develops a theoretical model for magnon-bipolar carrier drag thermopower in magnetic semiconductors, predicts its independence from relaxation times, and provides experimental evidence of this phenomenon in doped MnTe.

Contribution

The paper introduces a novel theoretical formulation for magnon-bipolar carrier drag thermopower and experimentally verifies its predictions in various doped magnetic semiconductors.

Findings

Magnon-bipolar drag thermopower is independent of carrier and magnon relaxation times.

Experimental observation of magnon-carrier drag thermopower in n-type and intrinsic ferromagnetic semiconductors.

Cr doping alters the sign and magnitude of thermopower due to magnon-electron interactions.

Abstract

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.