Ni3+-induced semiconductor-to-metal transition in spinel nickel cobaltite thin films,Physical Review B

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Ni3+-induced semiconductor-to-metal transition in spinel nickel cobaltite thin films
Physical Review B ( IF 3.2 ) Pub Date : 2021-09-23 , DOI: 10.1103/physrevb.104.125136
X. C. Huang ₁ , W.-W. Li ₂ , S. Zhang ₃ , F. E. Oropeza ₄ , G. Gorni ₅ , V. A. de la Peña-O’Shea ₄ , T.-L. Lee ₆ , M. Wu ₇ , L.-S. Wang ₈ , D.-C. Qi ₉ , L. Qiao ₃ , J. Cheng ₁ , K. H. L. Zhang ₁

Affiliation

State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People's Republic of China
MIIT Key Laboratory of Aerospace Information Materials and Physics, College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, People's Republic of China
School of Physics, University of Electronic Science and Technology of China, Chengdu 610054, People's Republic of China
Photoactivated Processes Unit, IMDEA Energy Institute, Parque Tecnológico de Móstoles, Avda. Ramón de la Sagra 3, 28935 Móstoles, Madrid, Spain
CELLS-ALBA Synchrotron, Carrer de la Llum 2-26, 08290 Cerdanyola del Vallès, Spain
Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom
Department of Physics, Xiamen University, Xiamen, 361005, People's Republic of China
Fujian Key Laboratory of Materials Genome, College of Materials, Xiamen University, Xiamen 361005, People's Republic of China
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland 4001, Australia

In this paper, we report insights into the local atomic and electronic structure of

{NiCo}_{2} O_{4}

epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined

{NiCo}_{2} O_{4}

epitaxial thin films with controlled properties on

Mg {Al}_{2} O_{4} (001)

substrates using pulsed laser deposition. Films grown at low temperatures (

< 400^{\circ} C

) exhibit a ferrimagnetic and metallic behavior, while those grown at high temperatures are nonmagnetic semiconductors. The electronic structure and cation local atomic coordination of the respective films were investigated using a combination of resonant photoemission spectroscopy, x-ray absorption spectroscopy, and ab initio calculations. Our results unambiguously reveal that the

{Ni}^{3 +}

valence state promoted at low growth temperature introduces delocalized

Ni 3 d

-derived states at the Fermi level (

E_{F}

), responsible for the metallic state in

{NiCo}_{2} O_{4}

, while the

Co 3 d

-related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and

\sim + 2

. Further structural and defect chemistry studies indicate that the formation of oxygen vacancies and secondary CoO phases at high growth temperature are responsible for the