Origin of metallic behavior in NiCo2O4 ferrimagnet.

Jheng-Cyuan Lin,Qian Zhan,Yuanmin Zhu,Ying-Hao Chu,Heng-Jui Liu,Chien-Te Chen,Qing He,Chien Nguyen Van,Yugandhar Bitla,Hong-Ji Lin,Yi-Ying Chin,Ruirui Liu

doi:10.1038/srep15201

Abstract

Predicting and understanding the cation distribution in spinels has been one of the most interesting problems in materials science. The present work investigates the effect of cation redistribution on the structural, electrical, optical and magnetic properties of mixed-valent inverse spinel NiCo2O4(NCO) thin films. It is observed that the films grown at low temperatures (T < 400 °C) exhibit metallic behavior while that grown at higher temperatures (T > 400 °C) are insulators with lower ferrimagnetic-paramagnetic phase transition temperature. So far, n-type Fe3O4 has been used as a conducting layer for the spinel thin films based devices and the search for a p-type counterpart still remains elusive. The inherent coexistence and coupling of ferrimagnetic order and the metallic nature in p-type NCO makes it a promising candidate for spintronic devices. Detailed X-ray Absorption and X–ray Magnetic Circular Dichroism studies revealed a strong correlation between the mixed-valent cation distribution and the resulting ferrimagnetic-metallic/insulating behavior. Our study clearly demonstrates that it is the concentration of Ni3+ions and the Ni3+–O2−Ni2+ double exchange interaction that is crucial in dictating the metallic behavior in NCO ferrimagnet. The metal-insulator and the associated magnetic order-disorder transitions can be tuned by the degree of cation site disorder via growth conditions.

Highlights

Predicting and understanding the cation distribution in spinels has been one of the most interesting problems in materials science
The epitaxial growth of NCO(I) film on MAO(001) substrate is evident from the room temperature highresolution x-ray diffraction (HR-XRD) measurements shown in Fig. 1(b) (Fig. S1 in supporting information for NCO(M))
The detailed XAS analysis suggests the presence of more Ni3+ ions at the octahedral sites in NCO(M) than in NCO(I)

Summary

Introduction

Predicting and understanding the cation distribution in spinels has been one of the most interesting problems in materials science. Detailed X-ray Absorption and X–ray Magnetic Circular Dichroism studies revealed a strong correlation between the mixedvalent cation distribution and the resulting ferrimagnetic-metallic/insulating behavior. Cation distribution in spinels can influence many fundamental properties such as saturation magnetization, exchange couplings, magnetic ordering temperatures and electronic conductivity which result primarily from the valence state preferences of the transition metal ion for different interstitial sites[24]. A comparative study on single crystalline spinel NCO films representative of metallic and insulating phases is undertaken to understand the correlation between cation (Td/Oh)-site disorder and observed intriguing behaviors. Allowing us to elucidate the underlying mechanism responsible for observed disparities in different phases We show that both the systems have mixed-valent inverse spinel structure but with different cation distribution or disorder which directly correlates with their optical, structural, magnetic and transport properties. Our study paves an elegant route to tune the fascinating phenomena by modulating the interplay among various interactions and triggers significant interest to explore new multifunctional systems

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