Dilute magnetic semiconductors (DMSs) have been attracting much interest since they show their possibility of manipulating charge and spin degrees of freedom in a single material. Theoretical predictions of room temperature ferromagnetism in ZnO based DMS have caused intensive efforts on ZnO material doped with transition metal. The Mn-doped ZnO system is very promising due to its wide bandgap of ZnO host material and due to its high solubility of Mn atoms in ZnO matrix. Mn-doped ZnO has also attracted much attention because of disagreements about the existence and the origin of roomtemperature ferromagnetism. Sharma et al. [1] observed ferromagnetism behavior above room temperature for small atomic percentages of Mn doped in ZnO bulk and thin films. They proposed that room-temperature ferromagnetism behavior due to carrier-induced interactions between isolated Mn ions in ZnO. Contrary to this result, several authors have been argued that room-temperature ferromagnetism in Mn-doped samples originated from an oxygen-vacancy-stabilized metastable phase [2,3]. Some authors [4-6] believed that the secondary phases of Mn transition metals clusters and their oxides might be responsible for the observed ferromagnetism behavior. However, some recent studies showed the absence of ferromagnetic ordering in bulk single phase of Mn-doped ZnO down to 2 K [7,8]. Such incosistent results have also been observed for Mn-doped ZnO thin films, which extend from paramagnetic properties [9] to spin-glass behavior [10]. Most likely the differences in the reported results are due to different preparation methods and by different researchers suggesting that magnetic properties of this system are very sensitive to the preparation conditions.

In this work, we reported synthesis of Mn-doped ZnO in the form of nanocrystalline particles using a simple coprecipitation method. The structural and magnetic properties of the nanocrystalline particles were investigated using X-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), infrared absorption (FTIR), electron spin resonance (ESR) and vibrating sample magnetometer (VSM).

For the synthesis of Mn-doped ZnO nanoparticles in this study, manganese sulfate monohydrate (MnSO₄∙H₂O), Zinc sulfate hepta hydrate (ZnSO₄∙7H₂O), 25% aqueous Sodium hydroxide (NaOH) were used which are procured from Aldrich and Merck. All of the chemicals used are GR grade without further purification. ZnO nanoparticles were synthesized by using a co-precipitation method. The requisite amounts of ZnSO₄∙7H₂O and MnSO₄∙H₂O were dissolved in distilled water depending on the percentage of Mn doping to form solution. For the sake of convenience, these solution are designated as solution A. Solution A was then put into an ordinary ultrasonic cleaner using a 57 kHz operating frequency for 2 h. Simultaneously, 44 mmol NaOH was prepared in 440 ml of de-ionized water (solution B). Then, solution A was subsequently stirred with a magnetic stirrer at room temperature. In this solution, the solution B was added until the final pH of solution reached to 12 and then solution was further stirred for 0.5 h with constant stirring. So obtained solution was aged at room temperature for 18 h. This solution was centrifuged and washed several times with ethanol and distilled water in order to remove residual and unwanted impurities. The obtained product was dried in a vacuum oven at 200˚C for 1 h yielding dark brown Mn doped ZnO powder.

The x-ray diffraction (XRD) patterns of the Mn-doped ZnO powder samples were measured at room temperature with a standard x-ray diffractometer Philips PW 1710 and monochromatic Cu-K_α (λ = 1.54060 Å) radiation operated at 40 kV and 20 mA in the range from 10˚ to 80˚. The calibration of the diffractometer was done using Si powder. Elemental analyses of the samples have been done by energy dispersive x-ray spectroscopy (EDX) using scanning microscope. Fourier-transform infrared spectra of the powder samples were recorded using a Shimadzu Fourier-transform spectrometer in the range of 400 - 4000 cm^–1. Magnetic properties were experimentally studied by measuring magnetization as a function of external magnetic field at room temperature using Oxford Type 1.2 T vibrating sample magnetometer (VSM). These measurements were taken from 0 to ±1 Tesla field. To obtain information on oxidation state and site occupancy of the Mn ions in the ZnO matrix electron spin resonance (ESR) was carried out using X-band JEOL JES-RE1X at room temperature. X-band spectrometer equipped with 9.1 GHz field modulation unit.

Figure 1 shows the representative EDX spectra for undoped and Mn-doped nanocrystalline ZnO particles. The elemental analysis indicated that the samples contain target elements with carbon as impurities within the EDX analysis limit. Quantitative results of the Mn/Zn ratio are calculated from the area of the corresponding spectral K lines. The amount of Mn in the nanocrystalline ZnO particles has been found to vary between 6 - 30 at%.