Elsevier

Applied Surface Science

Volume 264, 1 January 2013, Pages 7-10
Applied Surface Science

Influence of annealing atmosphere on room temperature ferromagnetism of Mn-doped ZnO nanoparticles

https://doi.org/10.1016/j.apsusc.2012.08.108Get rights and content

Abstract

Zn0.92Mn0.08O nanoparticles were synthesized by co-precipitation technique, and annealed in H2, CO, Ar and O2, respectively. Room temperature ferromagnetism was found in samples annealed in H2 and CO, respectively. The absorption spectra and Raman spectra of the samples reveal that a high concentration of oxygen vacancies appears in CO-annealed and H2-annealed Zn0.92Mn0.08O nanoparticles. By Gaussian fitting PL spectra of the samples, a broader green–yellow emission band decomposed into the green emission band and yellow emission band, providing the evidence that singly ionized oxygen vacancies may play an important role in the origin of room temperature ferromagnetism. The exchange interaction between the donor electron trapped by the singly ionized oxygen vacancy and surrounding Mn ions is responsible for ferromagnetism of diluted magnetic semiconductors at room temperature.

Highlights

► Zn0.92Mn0.08O nanoparticles were synthesized by co-precipitation technique, and annealed in H2, CO, Ar and O2, respectively. ► Room temperature ferromagnetism was found in sample annealed in H2 and CO, respectively. ► The higher the carrier concentration, the larger the shift to the high energy direction in the optical absorption spectra. ► Singly ionized oxygen vacancies play an important role in the origin of ferromagnetism. ► We decomposed visible emission peaks into the green emission band and yellow emission band by Gaussian fitting.

Introduction

During the past few years, diluted magnetic semiconductors (DMSs) have attracted much attention because of their potential applications in spintronic devices as well as magneto-optic components. For practical application, DMSs with curie temperatures above room temperature should be required. Since Dietl et al. predicted a theoretical basis to exhibit ferromagnetism above room temperature on transition-metal-doped zinc oxides [1], ZnO-based DMSs have generated widespread interest. Room temperature ferromagnetism (RTFM) was observed in Mn, Co, Ni, Cu and Cr doped ZnO systems [2], [3], [4], [5], [6], [7]. However, recently, FM has also been found in pure ZnO system [8], due to the presence of intrinsic defects or oxygen vacancies.

There is still no definite agreement on the origin of FM in a Zn1−xMnxO system, and several reasonable theoretical models as well as some experiments have been proposed to explain the ferromagnetic mechanisms. Dietl et al. pointed out that FM is an exchange interaction between free holes and localized spins in strongly p-doped ZnO-based DMSs [1]. Some other related studies have shown that the origin of ferromagnetism is the inclusion of FM phases or clusters in the sample [9]. The Ruderman–Kittel–Kasuya–Yosida (RKKY) model, modified Zener model and double-exchange model were used to explain the ferromagnetic mechanisms in DMSs, but these models had some deficiencies. So far, the most popular mechanisms suggested for magnetic ordering are carrier-mediated exchange [3], [4], [10], and the bound magnetic polaron (BMP) model with respect to defects [6], [7]. Recently, Park et al. proposed that FM is caused by the coupling of hydrogen shallow donors and Mn ions in Mn-doped ZnO mediating short-range ferromagnetic spin–spin interaction [11]. In this regard, the origin of FM may be closely related to hydrogen [4], [11]. In our recent work, FM was also observed in CO-annealed Zn0.92Mn0.08O nanoparticles and the origin of FM excluded the influence of hydrogen. In this study, Zn0.92Mn0.08O nanoparticles were annealed in H2, CO and O2 atmospheres, respectively, and an obvious RTFM was observed in H2-annealed and CO-annealed Zn0.92Mn0.08O nanoparticles. Our results indicate that the origin of FM may be attributed to the exchange interaction between the donor electron trapped by the singly ionized oxygen vacancy and many surrounding Mn ions.

Section snippets

Experimental details

Zn0.92Mn0.08O nanoparticles were synthesized by the co-precipitation method [12]. Reagents were of analytical grade purity without further purification. Zn(NO3)2 aqueous solution was made from high-purity ZnO dissolved into dilute HNO3 solution. An amount of 50 wt% Mn(NO3)2 aqueous solution and Zn(NO3)2 aqueous solution were mixed at required molar ratios and stirred to mingle equably. 1 mol/L NaOH aqueous solution was then added drop by drop to the mixed solution with stirring until there were

Results and discussion

Fig. 1 shows the X-ray diffraction (XRD) patterns of as-synthesized Zn0.92Mn0.08O nanoparticles with different annealing atmospheres. According to the XRD data, all the peaks of the samples match the standard ZnO diffraction pattern well and are identified with a hexagonal wurtzite ZnO structure. The peak positions (2θ) of Zn0.92Mn0.08O nanoparticles annealed in H2, CO, Ar and O2 are smaller than that ZnO powders. For example, the inset shows (0 0 2) peaks of Zn0.92Mn0.08O under of different

Conclusion

In summary, nanoparticles of Mn-doped ZnO were successfully synthesized by co-precipitation technique at room temperature. Mn-doped ZnO nanoparticles annealed in H2 and CO atmospheres show the same order of magnitude of MS. Raman spectra and PL spectra reveal that VO plays an important role in the origin of FM in Mn-doped ZnO nanoparticles. By Gaussian fitting PL spectrum, a broader green–yellow emission band decomposed into the green emission band and yellow emission. Our studies provide the

Acknowledgment

The work is supported by NSFC (No. 50972139).

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