A study on enhancement in thermoelectric performance of magnetic materials

Abstract

This dissertation is dedicated to successful examples of development of multiple strategies for high thermoelectric performances in the Seebeck and spin Seebeck effects, which are the representative longitudinal and transverse thermoelectric effects, respectively. By virtue of “spin thermoelectrics,” which enables heat-to-electricity conversion by intervention of spin-degree-of freedom, the greatly improved thermoelectric performances in different types of magnetic materials are explored. In the first part, an unprecedented pathway to Seebeck current amplification in a magnetic-impurity-doped half-metal is proposed thanks to a “spin polarization relaxation” mechanism. By using higher-manganese silicide doped with magnetic elements of Fe and Co as an experimental platform, it is shown that the alteration of magnetic structures by magnetic-impurity-doping and accompanying modification of spin-dependent band structures are the keys to understanding the advanced thermoelectric properties. Spin-polarized density functional theory calculations disclose that antiferromagnetic couplings between Fe/Co dopants and Mn ions give rise to magnetization-induced band shifts and finally intensifies the Seebeck currents based on two-spin-channel transport. In the second part, the remarkable effects of interface properties of normal metal/ferromagnet structure on the spin Seebeck effect are explored in polycrystalline Pt/NiFe2O4 system. With different interface conditions of Pt/NiFe2O4 samples, the spin Seebeck signals are shown to be strongly affected by mesoscale surface defects (cracks, pores, and grain grooves) and surface roughness. By deliberately controlling such interface morphologies with the variations of polishing force and post-annealing temperature, a significantly large spin Seebeck coefficient is obtained, of which value has never been reported in bulk polycrystalline magnetic materials thus far. In the third part, the importance of bulk properties of normal metal/ferromagnet structure on the spin Seebeck performance is also emphasized. In particular, both optimization of bulk properties of NiFe2O4 and interface properties of Pt/NiFe2O4 results in a high spin Seebeck conversion efficiency with the demonstration of a “phonon-glass magnon-crystal.” The phase separation of NiFe2O4 via a simple heat treatment forms a distinctive hierarchical microstructure of nano-sized NiFe2O4 embedded in micro-sized NiO precipitates: this intriguing structure selectively scatters phonons while barely affecting magnons, which leads to robust magnon currents responsible for the improved spin Seebeck conversion efficiency. Our studies presented in this thesis provide promising ways to increase thermoelectric performances in the multiple thermoelectric effects with intervention of spin thermoelectrics. Such design and development of highly efficient magnetism-mediated thermoelectric materials are expected to provide useful insight into artificially structuring of spin-thermoelectric materials and ensure more intensive research aiming for their device applications.