Ferromagnetic ordering in Mn-doped ZnO nanoparticles
© Luo et al.; licensee Springer. 2014
Received: 7 November 2014
Accepted: 7 November 2014
Published: 22 November 2014
Zn1 - xMn x O nanoparticles have been synthesized by hydrothermal technique. The doping concentration of Mn can reach up to 9 at% without precipitation or secondary phase, confirmed by electron spin resonance (ESR) and synchrotron X-ray diffraction (XRD). Room-temperature ferromagnetism is observed in the as-prepared nanoparticles. However, the room-temperature ferromagnetism disappears after post-annealing in either argon or air atmosphere, indicating the importance of post-treatment for nanostructured magnetic semiconductors.
KeywordsDiluted magnetic semiconductor ZnO Room-temperature ferromagnetism Nanoparticles
Over the past few decades, diluted magnetic semiconductors (DMSs) have drawn extensive attention due to their potential application in spin-based electronic devices [1–3]. Eu chalcogenide and Eu oxide are pure magnetic semiconductors, which were discovered by Kasuya and Yanase in the 1960s . However, these materials all show a very low Curie temperature (77 K). In recent years, III-V-based DMSs, such as In1 - xMn x As and Ga1 - xMn x As, have been considered as the classical models for magnetic semiconductors, which have demonstrated very promising properties for future spintronics devices. Similar to pure magnetic semiconductors, the Curie temperature of these DMSs is still very low. The highest reported Curie temperature of Ga1 - xMn x As, for example, is around 200 K, which is still much lower than room temperature . In 2000, Dietl et al., using the mean field theory based on the Zener model, predicted that GaN and ZnO are two possible candidates of semiconductor host materials for achieving DMSs with a Curie temperature higher than room temperature .
Guided by this prediction, many works based on GaN and ZnO were carried out and indeed room-temperature ferromagnetism has been widely reported [7–12]. ZnO-based DMSs have attracted more attention due to their easy fabrication process and promising properties [13, 14]. Mn-doped ZnO is one of the typical examples, which shows room-temperature ferromagnetism [15, 16]. Most of the Mn-ZnO thin-film samples were fabricated using the vacuum deposition method, such as sputtering [17–19], pulsed laser deposition (PLD)  or molecular beam epitaxy (MBE) . However, the ferromagnetism in Mn-ZnO prepared by the physical method can only be observed when the films were deposited under an oxygen-deficient environment. Clusters or secondary phase is possible to be formed to contribute to the ferromagnetism . Therefore, whether the room-temperature ferromagnetism in these samples is intrinsic or not is still controversial. In fact, Mn-doped ZnO was once reported to exhibit paramagnetic behaviour above 1.83 K . Meanwhile, there is an increasing number of works indicating that the formation of second phases or defects may be the origin of ferromagnetism instead of the inherent property of the system [21, 24–28]. In addition, it shows that ferromagnetic ordering is strongly dependent on the parameters/condition of preparation, such as oxygen partial pressure and post-treatment . Compared to the physical deposition approach, chemical synthesis is one of the economical ways for the fabrication of DMSs. It can offer a better controllable material composition and prevent undesirable contamination, as well as avoid a high-temperature vacuum environment that may introduce segregation of metallic impurities [26, 29]. The thin-film state of Zn1 - xMn x O has been prepared via direct chemical synthesis by Norberg et al. . The film was formed by spin coating of colloidal Mn2+:ZnO quantum dots followed by post-treatment. Room-temperature ferromagnetism was observed in this film . However, compared to the physical preparation methods, the doping concentration of the sample by the chemical method is relatively low. The highest concentration of Mn in the ZnO matrix reported in ref.  is as low as 1.3%. It is widely accepted that the concentration of magnetic dopants effectively affects the magnetic property of DMSs . For example, for Ga1 - xMn x As, higher Mn concentration leads to higher Curie temperature . However, increasing the doping concentration of dopants using chemical synthesis is a challenge.
In this work, we have synthesized Zn1 - xMn x O nanoparticles with a modified fabrication process and optimized fabrication parameters to achieve a very high doping concentration of the magnetic dopant without segregation. We also systematically studied the post-treatment effect on the magnetic properties of these synthesized nanoparticles.
Synthesis of Zn1 - xMn x O nanoparticles
Zn1 - xMn x O nanoparticles were fabricated with a method similar to that described previously in ref. . All the chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) with 99.9% purity. Mn(OAc)2 · 4H2O and Zn(OAc)2 · 2H2O with a variety of ratios were dissolved in dimethyl sulfoxide (DMSO) with a concentration of 0.1 M. Then, the mixture solution was added into 0.55 M tetramethylammonium hydroxide (N(Me)4OH · 5H2O) dissolved in ethanol for a few seconds with vigorous stirring, which is different from ref. . The reaction was at room temperature which is also different from the temperature of 60°C used in ref. . The fast mixture of the two solutions and the reaction at room temperature are to prevent the growth of the nanoparticles to a large size and to facilitate the doping of Mn in a non-equilibrium process, which may lead to a high doping concentration without precipitation/segregation. Zn1 - xMn x O nanoparticles were then precipitated by adding ethyl acetate, and the precipitates were cleaned with ethanol many times using a centrifuge to make sure that all the excess reactants were removed. The final Zn1 - xMn x O nanoparticles were dried in an oven at 100°C for over 24 h. Post-annealing was performed at 500°C under argon (Ar) or air atmosphere for 1 h. In this work, three samples were mainly prepared. The three samples with Mn doping concentrations of approximately 1.0 at%, 4.9 at% and 9.1 at% were analysed by inductive coupled plasma (ICP). We assign the concentrations as 1 at%, 5 at% and 9 at%, respectively, for convenience.
Physical property measurement
X-ray powder diffraction data were collected by a PANalytical X'pert multipurpose X-ray diffraction system (PANalytical B.V., Almelo, The Netherlands) using Cu Kα radiation. All X-ray diffraction (XRD) scans were operated under 45 kV and 40 mA in the range of 20° ≤2θ ≤85°. Step size and time per step were 0.026° and 99.45 s, respectively. The crystalline size of the samples was determined using transmission electron microscopy (TEM) (Phillips CM200, FEI, Hillsboro, OR, USA). X-ray photoelectron spectroscopy (XPS; Thermo Scientific ESCLAB 250Xi X-ray photoelectron spectrometer, Thermo Fisher Scientific, Waltham, MA, USA) was performed using a monochromatized Al K-alpha X-ray source (hv) of 1,486.6 eV with 20-eV pass energy. Magnetic properties of the samples were taken using a superconducting quantum interference device (SQUID; XL-7, Quantum Design, San Diego, CA, USA). X-band (9.5 GHz) electron paramagnetic resonance (EPR) spectra were measured by a Bruker EMX X-band electron spin resonance (ESR) spectrometer (Bruker, Karlsruhe, Germany). The high-resolution synchrotron X-ray powder diffraction experiment was carried out on the PD beamline at the Australian Synchrotron using a wavelength λ = 0.6887 Å. Phase identification was carried out using the HighScore Plus program.
Results and discussion
K stands for the shape factor, which is set to 0.9. λ is the wavelength of X-ray radiation of Cu Kα (1.5405 Å). θ hkl is the most intensive peak. We use the (101) peak in the calculation. β presents the full width at half maximum (FWHM) of the peak. We choose peak (101) for the calculation due to its high intensity. The lowest doping sample (1 at% Mn) exhibits the largest crystal size (62.60 Å). With increasing Mn concentration, the crystallite size decreases until it reaches 37.76 Å in diameter for 9 at% Mn-doped ZnO, suggesting that Mn doping may induce distortion of the lattice, leading to disordering.
To deeply understand the role of Mn and O in the magnetic properties of the Mn-Zn-O system, the XPS spectra of the Mn 2p and O 1s core level were measured. As discussed previously, we used the distance of Mn 2p3/2 and Mn 2p1/2 to identify the oxidation states of Mn. Figure 7c indicates that the as-prepared samples and annealed samples in air have Mn2+ state, whereas the sample annealed in Ar atmosphere has a large distance between the peaks Mn 2p3/2 and Mn 2p1/2, indicating a lower valence of Mn. Since the ferromagnetism disappears when annealed under both air and Ar atmosphere, the change of Mn state is not attributed to the disappearance of ferromagnetism. From Figure 7d, the gross peak of O 1s can be divided into three components, which refer to Zn-O (approximately 529.8 eV), O2- vacancies (approximately 530.8 eV) and H-O (approximately 531.5 eV), respectively [41, 44–47]. After post-annealing in air, the amount of oxygen vacancy was reduced drastically. The result demonstrates the uptake of oxygen or repairing of oxygen-deficient regions during high-temperature annealing. Either case tends to result in a disruption of the oxygen-vacancy-stabilized metastable phase. It has been reported that the OH bond may also possibly be the origin of ferromagnetism in ZnO-based DMSs . However, in our XPS analysis, the amount of OH bond has a negligible reduction, suggesting that OH bonds may not be the origin of ferromagnetism in this work. Cation vacancy may be one of the origins of ferromagnetism [48–52]. However, in our ESR measurement, Zn vacancy (g = 2.013) cannot be observed . It may be overlapped with the hyperfine structure of Mn, whereas after annealing, there is no obvious change in the hyperfine structure, suggesting that Zn vacancy is not the origin of ferromagnetism. Oxygen vacancy may play an important role in ferromagnetism since the air annealing condition leads to the disappearance of oxygen vacancy (Figure 7d). It should be noted that for the sample annealed under Ar atmosphere, ESR analysis has shown that Mn dopants leave the substitutional site and transfer to the surface of ZnO nanoparticles, which may be the main reason for the disappearance of ferromagnetism (Figure 7a).
We have synthesized Zn1 - xMn x O nanoparticles with different doping concentrations. The Mn dopant can reach up to 9 at% without the formation of secondary phase and precipitates. The as-prepared samples all show room-temperature ferromagnetism. Post-annealing in either air or Ar atmosphere will deteriorate the ferromagnetic ordering, supporting the role of oxygen vacancies in ferromagnetism. In addition, post-annealing easily induces surface oxide precipitation. Hence, delicate control of post-annealing in nanostructured DMSs is of importance for achieving ferromagnetic ordering at room temperature.
This work is funded by the Australian Research Council discovery projects DP110105338 and DP140103041 and the Queen Elizabeth II fellowship. Synchrotron XRD was carried out using the powder diffraction beamline at the Australian Synchrotron.
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