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.
Highlights
Transition metal oxides exhibit a variety of fascinating and useful magnetic, electronic, and optical properties
We reported the synthesis of epitaxial films of NCO on spinel MgAl2O4 substrate at growth temperatures varying from 200◦C to 600◦C using pulsed laser deposition.[14]
The lattice constant along the c axis calculated from the vertical position of the (226) peak in reciprocal space mapping (RSM) scans for the NCO500 films is 8.50 Å, which is comparatively larger than 8.17 Å of the NCO300 film and than the bulk value (8.116 Å)
Summary
Transition metal oxides exhibit a variety of fascinating and useful magnetic, electronic, and optical properties. The bandlike conduction in NCO500 happens with a much lower carrier density, which we attribute to the different cation distributions in the two films: the more perfect inverse spinel structure in the NCO500 film provides a much lower level of carrier doping. In both NCO300 and NCO500 films, we find a crossover from coherent to incoherent transport below ∼70 K, which indicates the tendency of carriers toward localization. We suggest that this tendency results from disorder-induced localization effects,[19] reminiscent of the localization tendencies found in magnetoresistive manganites[20,21,22,23,24] and in Zn-doped cuprates.[25]
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