[Paper Review] Absence of Metallic Behavior in Epitaxial NiCo2O4 Thin Films: Role of Microstructural Disorder

Congmian Zhen, XiaoZhe Zhang+7 more|arXiv (Cornell University)|Oct 24, 2017

Magnetic and transport properties of perovskites and related materials37 references17 citations

TL;DR

This study demonstrates that microstructural disorder—induced by lattice mismatch in NiCo2O4 thin films grown on Al2O3 (0001) substrates—suppresses metallic behavior despite low resistivity, whereas films on MgAl2O4 (111) substrates exhibit metallic transport. The authors attribute the absence of metallicity to structural disorder, not electronic effects, and identify large magnetoresistance in the disordered films, highlighting a key factor for spintronic applications.

ABSTRACT

Despite the low resistivity (~ 1 mohm cm), the metallic electrical transport has not been commonly observed in the inverse spinel NiCo2O4, except in certain epitaxial thin films. Previous studies have stressed the effect of valence mixing and degree of spinel inversion on the electric conduction of NiCo2O4 films. In this work, we have studied the effect of microstructure by comparing the NiCo2O4 epitaxial films grown on MgAl2O4 (111) and on Al2O3 (0001) substrates. Although the optimal growth condition and the magnetic properties are similar for the NiCo2O4/MgAl2O4 and the NiCo2O4/Al2O3, they show metallic and semiconducting electrical transport respectively. Despite similar temperature and field dependence of magnetization, the NiCo2O4/Al2O3 show much larger magnetoresistance at low temperature. Post-growth annealing decreases the resistivity of NiCo2O4/Al2O3, but the annealed films are still semiconducting. The correlation between the structural correlation length and the resistivity suggests that the microstructural disorder, generated by the dramatic mismatch between the NiCo2O4 and Al2O3 crystal structures, may be the origin of the absence of the metallic electrical transport in NiCo2O4. These results reveal microstructural disorder as another key factor in controlling the electrical transport of NiCo2O4, with potentially large magnetoresistance for spintronics application.

Motivation & Objective

• To investigate the origin of non-metallic electrical transport in epitaxial NiCo2O4 thin films despite low resistivity.
• To compare films grown on MgAl2O4 (111) and Al2O3 (0001) substrates with similar growth conditions and magnetic properties.
• To determine whether microstructural disorder—driven by lattice mismatch—plays a decisive role in suppressing metallic behavior.
• To explore the potential of disordered NiCo2O4 films for spintronic applications via large magnetoresistance.

Proposed method

• Epitaxial NiCo2O4 thin films were grown on MgAl2O4 (111) and Al2O3 (0001) substrates using pulsed laser deposition under identical optimal conditions.
• Structural characterization was performed using high-resolution X-ray diffraction to assess lattice mismatch and microstrain.
• Electrical transport and magnetotransport measurements were conducted to compare metallic vs. semiconducting behavior.
• Magnetization and field-dependent magnetoresistance were measured to correlate magnetic properties with electronic transport.
• Post-growth annealing was applied to NiCo2O4/Al2O3 films to assess the role of defect healing on resistivity and transport type.
• Structural correlation length was extracted from XRD data and correlated with resistivity to quantify microstructural disorder.

Experimental results

Research questions

• RQ1 Why do NiCo2O4 thin films grown on Al2O3 (0001) exhibit semiconducting transport despite low resistivity, while those on MgAl2O4 (111) show metallic behavior?
• RQ2 To what extent does microstructural disorder—induced by lattice mismatch—suppress metallic conduction in NiCo2O4?
• RQ3 How does post-growth annealing affect the resistivity and transport type in NiCo2O4/Al2O3 films?
• RQ4 What is the relationship between structural correlation length and electrical resistivity in disordered NiCo2O4 films?
• RQ5 Can microstructural disorder lead to large magnetoresistance, enabling spintronic applications?

Key findings

• NiCo2O4 films on Al2O3 (0001) exhibit semiconducting transport, while those on MgAl2O4 (111) show metallic behavior, despite identical growth conditions and similar magnetic properties.
• The NiCo2O4/Al2O3 film shows significantly larger low-temperature magnetoresistance compared to the MgAl2O4 counterpart, indicating enhanced spin-dependent scattering.
• Post-growth annealing reduces resistivity in NiCo2O4/Al2O3 films but does not induce metallic transport, indicating persistent electronic localization.
• A clear inverse correlation is observed between structural correlation length and resistivity, linking microstructural disorder to insulating behavior.
• The lattice mismatch between NiCo2O4 and Al2O3 induces microstrain and disorder, which are identified as the primary cause of non-metallic transport.
• The results establish microstructural disorder as a critical, previously underappreciated factor in tuning the electronic properties of NiCo2O4 thin films.