Nanostructural origin of semiconductivity and large magnetoresistance in epitaxial NiCo₂O₄/Al₂O₃ thin films

Authors

Congmian Zhen, Hebei Normal UniversityFollow
Xiaozhe Zhang, University of Nebraska-Lincoln
Wengang Wei, Fudan University, Shanghai
Wenzhe Guo, Hebei Normal University
Ankit Pant, University of Nebraska–Lincoln
Xiaoshan Xu, University of Nebraska-LincolnFollow
Jian Shen, Fudan University
Li Ma, Hebei Normal University
Denglu Hou, Fudan University

ORCID IDs

Congmian Zhen https://orcid.org/0000-0002-9652-3257

Xiaoshan Xu https://orcid.org/0000-0002-4363-392X

Date of this Version

3-2018

Document Type

Article

Citation

Published in J. Phys. D: Appl. Phys. 51 (2018), 145308, 9 pp.

doi 10.1088/1361-6463/aab2a3

Comments

Copyright © 2018 IOP Publishing Ltd. Used by permission.

Abstract

Despite low resistivity (~1 mΩ cm), metallic electrical transport has not been commonly observed in inverse spinel NiCo₂O₄, except in certain epitaxial thin films. Previous studies have stressed the effect of valence mixing and the degree of spinel inversion on the electrical conduction of NiCo₂O₄ films. In this work, we studied the effect of nanostructural disorder by comparing the NiCo₂O₄ epitaxial films grown on MgAl₂O₄ (1 1 1) and on Al₂O₃ (0 0 1) substrates. Although the optimal growth conditions are similar for the NiCo₂O₄ (1 1 1)/MgAl₂O₄ (1 1 1) and the NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films, they show metallic and semiconducting electrical transport, respectively. Post-growth annealing decreases the resistivity of NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films, but the annealed films are still semiconducting. While the semiconductivity and the large magnetoresistance in NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films cannot be accounted for in terms of non-optimal valence mixing and spinel inversion, the presence of anti-phase boundaries between nano-sized crystallites, generated by the structural mismatch between NiCo₂O₄ and Al₂O₃, may explain all the experimental observations in this work. These results reveal nanostructural disorder as being another key factor for controlling the electrical transport of NiCo₂O₄, with potentially large magnetoresistance for spintronics applications.