Room-temperature ferromagnetism and optical properties of Zn1−xMnxO nanoparticles

doi:10.1016/j.ssc.2009.02.016

Solid State Communications

Volume 149, Issues 17–18, May 2009, Pages 682-684

https://doi.org/10.1016/j.ssc.2009.02.016 Get rights and content

Abstract

The room-temperature ferromagnetism is observed in Zn_0.98Mn_0.02O nanoparticles, which is related to the host-lattice defects induced by doping Mn. The ferromagnetism in Zn_0.95Mn_0.05O nanoparticles can be suppressed by Mn clusters. The additional peak at 519 cm⁻¹ is observed in Raman scattering spectra of the Zn_1−xMn_xO nanoparticles associated with intrinsic host-lattice defects, which become activated due to the Mn doping. The decrease in band gap and the weak intensity of absorption peak in the nanoparticles may be due to the sp–d interaction between transition metal and Zn anions.

Introduction

The diluted magnetic semiconductors have been developed for spintronic technology, which exhibit both room-temperature ferromagnetism and semiconducting properties. The Zn_1−xMn_xO system is very promising because of the wide band gap of the ZnO host material (3.37 eV at room temperature, which is used for UV optoelectronic devices), and because of the high solubility of Mn in ZnO [1]. It seems reasonable to expect that the luminescence of Zn_1−xMn_xO should be located in blue and ultraviolet regions, according to the corresponding optical absorption spectra [2]. However, it has been shown that Mn doping suppresses the luminescence of bulk or powder Zn_1−xMn_xO even at a very low doping level [3], [4]. The multiphonon optical modes of Mn-doped ZnO compounds were identified as originating from two phonon processes in the disorder lattice and secondary phase at high Mn concentrations [5], [6]. The large absorption of Mn-doped ZnO is due to the charge transfer transition between donor and/or acceptor ionization levels of Mn ions and band continuum. Ferromagnetism has been observed at room temperature in transition-metal (such as V [7], Co [8], Ni [9] or Mn [5], [10])-doped ZnO nanocrystals. Studying defects and lattice disorder in host semiconductors is helpful for understanding the origin of the ferromagnetism in the diluted magnetic semiconductors [11], [12]. Raman scattering is a versatile technique for studying impurity incorporation, induced defects and disorder in the host lattice. Most of the peaks in the Raman spectra, reported for Zn_1−xMn_xO, directly reflect the wurtzite-lattice vibration modes of pure ZnO [13]. Recently, the origins of additional features (such as the local vibrational mode, disorder-induced silent modes, or phonon modes) were investigated [14], which do not present in the Raman spectra of pure ZnO. In this work, room-temperature ferromagnetism is observed in Zn_0.98Mn_0.02O nanoparticles, while Zn_0.95Mn_0.05O nanoparticles show paramagnetism. The ferromagnetism of the Zn_0.98Mn_0.02O nanoparticles is found to relate with the host lattice defects induced by doping Mn, corresponding to the peak at 265 and 519 cm⁻¹ in Raman scattering. The optical band gap of ZnO is decreased by doping Mn for low Mn concentration, due to the sp–d interaction between transition metal and Zn anions.

Section snippets

Experimental

For the synthesis of Mn²⁺ doped ZnO nanoparticles, suitable Zn(NO₃)₂⋅6H₂O, and Mn(NO₃)₂⋅6H₂O aqueous solution were mixed, then stoichiometric NaOH solution was added to the mixed solution with stirring. The collected precipitate was washed by distilled water several times and dried at room temperature for 3–4 days in air. The Zn_1−xMn_xO nanoparticles were obtained by annealing the dried precipitate in air at 450–800 ^∘C for about 2 h. By changing the amount of Zn(NO₃)₂⋅6H₂O and Mn(NO₃)₂⋅6H₂O, one

Results and discussions

Fig. 1 shows the XRD patterns of Zn_0.98Mn_0.02O and Zn_0.95 Mn_0.05O nanoparticles. It is clearly seen that all XRD peaks correspond to ZnO with the wurtzite structure, without any secondary phase, up to the doping of 5% Mn in ZnO. The annealing temperature for preparing good crystalloid nanoparticles is increased from 450 ^∘C to 700 ^∘C with increasing Mn content in ZnO (corresponding to Zn_0.98Mn_0.02O and Zn_0.95Mn_0.05O, respectively). The average particle size of Zn_1−xMn_xO is estimated to be 80 nm,

Conclusions

In conclusion, the room-temperature ferromagnetism of Zn_0.98Mn_0.02O nanoparticles is observed, while Zn_0.95Mn_0.05O nanoparticles show paramagnetism due to the existence of Mn clusters. The ferromagnetism of the Zn_1−xMn_xO nanoparticles is strongly related to the intrinsic host defects, which corresponds to the Raman vibration mode of 265 and 519 cm⁻¹. The Raman line at 519 cm⁻¹ becomes weak in Zn_0.95Mn_0.05O nanoparticles, because these intrinsic host-lattice defects will reduce the activation

Acknowledgements

This work has been supported by the National Natural Science Foundation of China under Grant No. 50831006 and 50331030.

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Room-temperature ferromagnetism and optical properties of Zn1−xMnxO nanoparticles

Abstract

Introduction

Section snippets

Experimental

Results and discussions

Conclusions

Acknowledgements

J. Cryst. Growth

Appl. Phys. Lett.

J. Lumin.

J. Cryst. Growth

Acta Crystallogr. Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr.

Appl. Phys. Lett.

Appl. Phys. Lett.

J. Appl. Phys.

J. Appl. Phys.

Appl. Phys. Lett.

Appl. Phys. Lett.

Phys. Rev. B

Room-temperature ferromagnetism and optical properties of Zn_1−xMn_xO nanoparticles