Effect of growth temperature on the terahertz-frequency conductivity of the epitaxial transparent conducting spinel NiCo2O4 films

Punam Silwal¹, Chan La-o-vorakiat², Elbert E. M. Chia², Dae Ho Kim³, and Diyar Talbayev¹

¹Department of Physics and Engineering Physics, Tulane University, 6400 Freret Street, New Orleans, LA 70118, USA
²Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
³Corporate R&D institute, Samsung Electro-Mechanics, 150 Maeyeong-ro, Suwon, Gyunggido 443-743, Korea 

Abstract

We have measured the terahertz-frequency optical conductivity of the epitaxial inverse spinel NiCo₂O₄ films grown at different temperatures. The low-temperature-grown film exhibits a metallic behavior with ferrimagnetic ordering, while the high-temperature-grown film shows greatly suppressed magnetization and insulating behavior. Both films exhibit band-like coherent conduction at intermediate temperatures, albeit with very different carrier densities consistent with the proposed models of cation valencies in this mixed-valence material. Both films also display a crossover to incoherent transport at low temperatures, indicating a disorder-induced tendency toward localization.

The real (ε₁) and imaginary (ε₂) parts of the measured dielectric function of the NCO300 film. 

Two different spectrometers were used for this study: a home-built spectrometer THz-TDS, and a commercial spectrometer mede by Teraview. TeraView's TPS Spectra 3000 was used to perform measurements.