Origin of Ferri-metallic behavior in NiCo₂O₄
Yugandhar Bitla1*, Y. Y. Chin2, C. H. Lin2, J. C. Lin3, S. C. Liao4, H. J. Liu1, C. H. Lai4, H. J. Lin2, C. T. Chen2, Q. He6, R. Liu7, Y.M. Zhu7, Q. Zhan7, Y. H. Chu1,3
1Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan
2National Synchrotron Radiation Research Center, Hsinchu, Taiwan
3Institute of Physics, Academia Sinica, Taipei, Taiwan
4Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan
5Key Laboratory of Polar Materials and Devices, East China Normal University, Shanghai, China
6Department of Physics, Durham University, Durham, United Kingdom
7Department of Material Physics and Chemistry, University of Science and Technology Beijing, Beijing, China
* presenting author:Yugandhar Bitla, email:yugibitla@gmail.com
The present work investigates the structural, electrical and magnetic properties of epitaxially grown NiCo₂O₄(NCO) thin films on a spinel substrate. It is observed that the thin film grown at 300 ⁰C exhibit Ferri-Metallic behavior while that grown at 400 ⁰C is an insulator with lower transition temperature. NCO exhibits mixed-valent inverse spinel structure. Detailed X-ray Absorption and X–ray Magnetic Circular dichroism study reveals a strong correlation between mixed-valent cation distribution at octahedral and tetrahedral sites and the resulting ferri-metallic behavior. The present study demonstrates that the metal-insulator and the associated magnetic order-disorder transitions can be tuned by the degree of cation site disorder via growth temperature. It is the concentration of Ni(III) ions and the percolative B-site double exchange interaction that is crucial in dictating the ferri-metallicity in NCO. Hence, the metal-insulator transition in NCO is merely due to competing double exchange and inherent super exchange interactions where in temperature plays a decisive role in tilting the balance in favor of one of them.

Keywords: Spinel, Thin films, Ferrimagnetism, Metal-insulator transition