X-ray absorption spectroscopic and magnetic characterization of cobalt-doped zinc oxide nanocrystals prepared by the molten-salt method

Abstract

The local atomic arrangement and electronic structure of the Co-doped Zn_1−xCo_xO nanocrystal have been quantitatively examined along with its magnetic properties. According to our analysis using powder X-ray diffraction, electron microscopy, and Zn K-edge X-ray absorption spectroscopy (XAS), phase-pure wurzite-structured Zn_1−xCo_xO nanocrystals have been successfully synthesized via the molten-salt method. The Co K-edge XAS analysis clearly demonstrates that all the Co²⁺ ions are substituted for the tetrahedral Zn sites of the Wurzite structure with a coordination number of 3.9 and a bond distance of 1.97 Å, ruling out the presence of magnetic impurity phase and Co-metal cluster. Magnetization measurements reveal that the present Zn_1−xCo_xO sample does not show any ferromagnetic transition down to 2 K. In this regard, we can conclude that Co-doped zinc oxide is not ferromagnetic but the previously reported ferromagnetism in this phase would be an extrinsic property.

Introduction

Over the last decade, diluted magnetic semiconductors (DMSs) have attracted special attention because of their unique ferromagnetism at room temperature, applicable for spintronics [1]. The discovery of ferromagnetism above room temperature in Co-doped TiO₂ has triggered a worldwide pursuit of new DMS materials [2]. The theoretical calculations by Sato and Katayama-Yoshida [3] showed that ZnO doped with several 3d transition metal ions such as V, Cr, Fe, Co and Ni may exhibit ferromagnetic ordering. In fact, many researchers have reported experimental evidence on the ferromagnetism of Co-doped ZnO materials near or above room temperature [4], [5], [6]. However, there are considerable numbers of contradictory reports claiming the absence of ferromagnetism in this system [7], [8]. Also, it was supposed that the ferromagnetism of Zn_1−xCo_xO would stem from extrinsic factors like the existence of magnetic impurity phase. This suggestion was supported by the first principle calculation results showing that Co-doped ZnO prefers to be in a spin-glass state due to antiferromagnetic super-exchange interactions [9]. The above discussions demonstrate that the magnetic properties of Co-doped ZnO are by no means established. To obtain an insight into the relationship between magnetic properties and preparation condition, we made careful survey on the existing literatures about the DMS, which reveal that most Zn_1−xCo_xO samples showing ferromagnetism were prepared by a high-temperature heat treatment. The sintering at elevated temperature might produce Co-based magnetic impurity phases. Furthermore, magnetization values reported by many workers are too small to be attributable to the bulk ferromagnetism of Zn_1−xCo_xO [10], supporting the presence of magnetic impurities. In this regard, it is of crucial importance not only to synthesize single-phase Zn_1−xCo_xO without the formation of the Co-containing impurity phase but also to quantitatively characterize the local crystal structures of doped cobalt ions for clarifying uncertainty about the ferromagnetism of Co-doped Zn_1−xCo_xO.

In this study, we have quantitatively studied the local structure and oxidation state of zinc and cobalt ions in phase-pure Co-doped Zn_1−xCo_xO samples prepared by a molten-salt method. We chose the molten-salt flux method because it is known to be capable of producing highly crystalline phase-pure materials in large scales, and has been employed in the syntheses of various oxide materials, where features are desirable for the purpose of the present study [11], [12]. With a reference to the experimental data of reference CoO, Co K-edge extended X-ray absorption fine structure (EXAFS) analysis allows us to exactly determine the coordination number and local atomic arrangement of cobalt ions doped in zinc-oxide lattice.