Cluster spin-glasslike behavior in nanoparticles of diluted magnetic semiconductors ZnS:Mn
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Zn1−xMnxS nanoparticles with x ⳱ 0.08, 0.16, and 0.32 were synthesized by a coprecipitation reaction between nitrate and sodium sulfide at room temperature in air. The magnetic properties of the Zn1−xMnxS nanoparticles were investigated by alternating-current (ac) susceptibility and direct-current (dc) magnetization measurements. The Mn3O4 phase was observed to exist in the Zn1−xMnxS nanoparticles as x 艌 0.16. The actual concentrations (x) of Mn-doped ZnS nanoparticles were determined by energy-dispersive x-ray analysis (EDAX) to be 0.06, 0.11, and 0.20, respectively, corresponding to the initial concentrations x ⳱ 0.08, 0.16, and 0.32. All the nanoparticles had the cubic structure and the lattice constant of Zn1−xMnxS phase increased with increasing Mn dopant concentration. For the Zn0.68Mn0.32S nanoparticles, there was evidence for appearance of cluster spin-glasslike behavior, as indicated by two maxima around 15 and 25 K in temperature dependence of ac susceptibility. The frequency independence of the peak at higher temperature is related to the intracluster ferromagnetic (FM) interactions, and the frequency dependence of the peak at lower temperature is associated with the spin glass. All the results revealed that the concentration of Mn2+ in Mn–ZnS and the amount of Mn3O4 were crucial for the cluster spin-glass behavior, which was not found when the real concentration (x) was unequal to 0.20 in Zn1−xMnxS.
I. INTRODUCTION
Magnetic semiconductors have been intensively studied because of the application in spintronics.1 On the other hand, nanostructured materials have attracted interest because of their unique properties. Recently, nanoparticles of sulfide compounds prepared by precipitation techniques have been investigated.2–4 The ZnS doping with Mn has been studied intensively as an important electroluminescent material. Bhargava et al. first reported on luminescence properties of Mn-doped ZnS nanocrystals prepared by a chemical process at room temperature, 5 which initiated investigation on this topic.6–8 Both transition-metal ions (e.g., Mn2+) and rareearth ions (e.g., Eu3+) were incorporated into ZnS nanostructures by physical and chemical methods.9,10 Zinc sulfide nanoparticles doped with Mn2+ ions are of special interest, due to the highly effective luminescence. The
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0317 2376
J. Mater. Res., Vol. 22, No. 9, Sep 2007
introduction of magnetic Mn2+ ions into nanoparticles of nonmagnetic ZnS semiconductor allows for the generation of diluted magnetic semiconductors (DMS), which can exhibit interesting magnetic and magneto-optical properties. The recombination of the effects of magnetic spins and electron charges in such magnetic nanostructure also provides opportunities for the new field of spintronics.1 Magnetic measurements are useful to determine the ionic state in DMS. The magnetic susceptibilities were measured in the temperature range of 20 to 500 K for ZnS single crystals doped with Mn2+.11 The total magn
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