The Scientific World Journal

Volume 2014 (2014), Article ID 265969, 7 pages

http://dx.doi.org/10.1155/2014/265969

Research Article

The Influence of Doping with Transition Metal Ions on the Structure and Magnetic Properties of Zinc Oxide Thin Films

Jenica Neamtu¹ and Marius Volmer²

¹National Institute for Research and Development in Electrical Engineering, Splaiul Unirii No. 313, Bucharest, Romania

²Transilvania University of Brasov, Eroilor No. 29, Brasov, Romania

Received 22 August 2013; Accepted 16 December 2013; Published 10 February 2014

Academic Editors: Y. Chai, C. He, and Y. Yun

Copyright © 2014 Jenica Neamtu and Marius Volmer. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

() and () thin films were synthesized by sol-gel method. The structure and the surface morphology of zinc oxide thin films doped with transition metal (TM) ions have been investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The magnetic studies were done using vibrating sample magnetometer (VSM) at room temperature. Experimental results revealed that the substitution of Ni ions in ZnO wurtzite lattice for the contents () leads to weak ferromagnetism of thin films. For with , the ions are magnetic coupling by superexchange interaction via oxygen ions in wurtzite structure. For () one can observe the increasing of secondary phase of ZnFe₂O₄ spinel. The Zn0.9Fe0.1O film shows a superparamagnetic behavior due to small crystallite sizes and the net spin magnetic moments arisen from the interaction between the iron ions through an oxygen ion in the spinel structure.

1. Introduction

Since the study of III–V semiconductors doped with transition metals by Ohno [1, 2], many researches were conducted to obtain the room temperature ferromagnetism of diluted magnetic semiconductors (DMS). Ferromagnetism in transition metal (TM)-doped ZnO is theoretically investigated by Sato and Katayama-Yoshida [3] using ab initio calculations based on local density approximation (LDA). The ferromagnetism of TM-doped ZnO is considered through a double-exchange mechanism, without requiring additional carrier incorporation. A few years ago, it turned out that most of incomplete 3d shell metal ions can be used to produce room temperature magnetism in ZnO doped with transition metal (Cu, Mn, Fe, Co, or Ni) [4–9]. It is a great interest because the DMS can be integrated for fabricating transparent spin-based devices [10]. Measurable ferromagnetism at room temperature was reported in cobalt-doped zinc oxide thin films [5] and Co-Mn doped zinc oxide [6]. Films consisting of were prepared by alternating-target laser ablation deposition, with Fe doping levels ranged from to 0.125 [8]. Also by pulsed laser deposition Ni-doped ZnO thin films were prepared with room temperature ferromagnetism [9].

However, the origin of this ferromagnetism is controversial. The studies on the origin of room temperature ferromagnetism in TM:ZnO films have been connected to substituting positions of TM ions in the ZnO lattice [11]. Further studies evidenced the origin of ferromagnetism as being TM precipitates [6] or clusters embedded in ZnO [12]. The room-temperature ferromagnetism of Co-doped ZnO is correlated with structural defects [13] or by incorporation of donor defects enhancing the ferromagnetic properties of Co:ZnO [14]. On the other hand, various chemical methods, as chemical precipitation, hydrothermal reaction, and sol-gel synthesis, have been developed to prepare nanoparticles and nanowires of zinc oxide doped with transition metal ions [15–17]. However, it is a great challenge to synthesize ZnO thin films doped with the transition metal ions using a simple process, with a low cost, as sol-gel preparation.

The goal of this work is the study of structural and magnetic properties of Ni-doped ZnO (, ) and Fe-doped ZnO (, ) thin films, both synthesized by sol-gel method. X-ray diffraction (XRD) and atomic force microscopy (AFM) were used to characterize their structure and the surface morphology. The magnetic studies were done using vibrating sample magnetometer (VSM) at room temperature. The VSM results revealed that the contents of (Ni²⁺) lead to weak ferromagnetism of thin films. The thin films with show a weak ferromagnetism, for Fe³⁺ hysteresis loop of thin films shows a superparamagnetic behavior. The structure and surface morphology of thin films were correlated with magnetic properties of TM:ZnO thin films.

2. Materials and Methods

2.1. Materials

(; 0.05; 0.10) and (; 0.05; 0.10; 0.15) thin films were synthesized by a sol-gel method. Stoichiometric amounts of zinc acetate-2-hydrate [Zn(CH₃COO)₂2H₂O] (Fluka 99.5%) and nickel nitrate Ni(NO₃)₂6H₂O (Aldrich 98%) or, respectively, Fe(NO₃)₃9H₂O (Aldrich 98%) were each dissolved in 20 mL propanol (C₃H₈O) by magnetic stirring at room temperature. Both homogeneous solutions of zinc acetate and nickel nitrate (or iron nitrate) were mixed together and then ethanolamine (NH₂CH₂CH₂OH) (Merk 99.5%) was added drop by drop under vigorous stirring. The resulting solution was then refluxed at 80°C for 4 h, until the solution was converted in a gel.

2.2. Preparation of Zinc Oxide Thin Films

The and thin films have been deposited from the gels by spin coating method (1500 RPM, 30 seconds) on Si (100) and Crown glass substrates. This procedure was repeated four times. The preheated temperature for film stabilization after each layer deposition was 200°C/20 min. The final films have been calcined at 700°C in air, during 2 hours. The same sol-gel method was used for preparation of ZnO thin film, starting to the zinc acetate Zn(CH₃COO)₂2H₂O (Fluka 99.5%), propanol, and ethanolamine (NH₂CH₂CH₂OH). The same procedure was used for the deposition of ZnO thin films on Si (100) and Crown glass substrates and final treatment at 700°C in air.

2.3. Characterization

The thickness of the ZnO thin films, the and thin films, measured using a FILMETRICS F20 thin film analyzer, were ranged between 70 and 90 nm.

X-ray diffraction (XRD) of the films was carried out using Bruker-AXS type D8 ADVANCE X-ray diffractometer with Cu-kβ radiation of 1.5406 , at a step of 0.04°/s in the range . Surface morphology and roughness of the samples were investigated by atomic force microscopy using “Ntegra aura” microscope with NSG tip (10 nm resolution).

The characterization of magnetic properties at room temperature is done using a LAKESHORE 7300 vibrating sample magnetometer (VSM).

3. Results and Discussion

Figure 1 shows the XRD diffraction patterns of nickel doped zinc oxide (, where ) thin films, sintered at 700°C temperature, for 2 h, in air. From X-ray diffraction intensity distribution it is observed that the peaks of wurtzite structure are majorities, indicating that these thin films have a structure similar to ZnO, in agreement with the reported JCPDS card no. 36-1451 ( and ). One can conclude that Ni²⁺ ions occupy the Zn²⁺ sites into the crystal lattice of ZnO. As can be seen in this figure with the increase of Ni²⁺  ions content the diffraction peaks arise for a secondary phase of rhombohedral , and 43° and lattice parameters: , , and [18].

Figure 1: X-ray diffraction patterns of () thin film samples.

Table 1 shows that the lattice parameters of () are slightly smaller than those of pure ZnO, because of small difference between the ionic radius of the elements ( and in tetrahedral coordination). The mean crystalline size, calculated from the full-width at half maximum (FWHM) of XRD lines by using the Debye-Scherrer formula [18], increases from 30.9 nm to 47.3 nm with the increased concentration of ().

Table 1: Lattice parameters calculated from the XRD data of ( = ) and mean crystalline size (with Debye-Scherrer formula [18]).

( = )

nm average size

Figure 2 shows the XRD diffraction patterns of iron doped zinc oxide (, where ) thin films, sintered at a temperature of 700°C, for 2 h, in air. One can observe that the peaks of wurtzite structure ( and ) are majorities.

Figure 2: X-ray diffraction patterns of () thin film samples.

From the XRD diffraction patterns of (where ) one can observe the peaks of secondary cubic phase at , and . This phase, ZnFe₂O₄, is a normal spinel with tetrahedral (A) sites occupied by Zn²⁺ ions and octahedral (B) sites occupied by Fe³⁺ and Fe²⁺ ions [19]. In spinel structure Fe³⁺ has ionic radius 0.55  (in octahedral coordination) matching that of Zn²⁺ (0.6 Å in tetrahedral coordination) which occupy the tetrahedral holes. Recent studies of XPS spectra indicated a very small amount of Zn in the B-site [20].

Table 2 shows the lattice parameters of () comparatively with the lattice parameters of pure ZnO and the mean crystalline size, calculated from FWHM of XRD lines by using the Debye-Scherrer formula [18].

Table 2: Lattice parameters calculated from the XRD data of ( = ) and mean crystalline size (Debye-Scherrer formula [18]).

( = )

nm average size

From Tables 1 and 2 one can notice that the lattice parameters of doped are close of ZnO wurtzite lattice parameters. The good compromise between the ionic radius matches that of zinc (0.60 Å) versus Ni²⁺ (0.55 Å) and zinc (0.60 Å) versus Fe³⁺ (0.49 Å) both in tetrahedral coordination and it is followed of substitution of Zn²⁺ with Ni²⁺ and Fe³⁺ ions.

Figure 3(a) shows the morphology of Zn0.97Ni0.03O thin film and Figure 3(b) shows the morphology of Zn0.90Ni0.10O thin film, both deposited on Si (100) substrate.

(a)

(b)

Figure 3: AFM micrographs of thin film surface for (a) Zn0.97Ni0.03O and (b) Zn0.90Ni0.10O.

Average crystallite size increases from 35 nm for Zn0.97Ni0.03O thin film to 56 nm for Zn0.90Ni0.10O thin film. This tendency of increase is in good accordance with the observed increase of mean crystalline size with the concentration, calculated from the FWHM of XRD lines by using the Debye-Scherrer formula (Table 1).

Figure 4(a) shows the morphology of Zn0.97Fe0.03O thin film and Figure 4(b) shows the morphology of Zn0.90Fe0.10O thin film, both deposited on Si (100) substrate.

(a)

(b)

Figure 4: AFM micrographs of thin film surface for (a) Zn0.97Fe0.03O and (b) Zn0.90Fe0.10O.

Average crystallite size decreases from 46 nm for Zn0.97Fe0.03O thin film to 19.5 nm for Zn0.90Fe0.10O thin film. This tendency of decrease is similar to the observed decrease of mean crystalline size with the concentration, calculated from the FWHM of XRD lines by using the Debye-Scherrer formula (Table 2).

From AFM characterization it is found that all analyzed thin films are formed by close package of crystallites with the holes. It is found that the volume of holes decreases for by comparison with thin films.

Figure 5 shows the magnetization versus the magnetic field measured at room temperature by vibrating sample magnetometer (VSM) for (a) ZnO, (b) Zn0.97Ni0.03O, (c) Zn0.95Ni0.05O, and (d) Zn0.90Ni0.10O thin film samples. The experimental values are not corrected with respect to the diamagnetic contribution of the substrate. Comparatively with undoped ZnO film, all Ni-doped ZnO films show distinctly hysteresis loops, indicating that samples have room-temperature ferromagnetism. The magnetic moments increase with the content of Ni ions from (for ), (for ), and (for ). The coercivity () shows decreasing values with the content of Ni ions: (for ), (for ), and (for ). The sample Zn0.90Ni0.10O is magnetically unsaturated at maximum magnetic field. This observation can be analyzed in terms of change in lattice spacing from the secondary rhombohaedral phase and in terms of superexchange interaction.

(a)

(b)

(c)

(d)

Figure 5: Magnetic hysteresis curves of (a) ZnO, (b) Zn0.97Ni0.03O, (c) Zn0.95Ni0.05O, and (d) Zn0.90Ni0.10O thin films.

Figure 6 shows the magnetization versus the magnetic field measured at room temperature by VSM for (a) ZnO (b) Zn0.97Fe0.03O, (c) Zn0.95Fe0.05O, and (d) Zn0.90Fe0.10O thin films.

(a)

(b)

(c)

(d)

Figure 6: Magnetic hysteresis curves of (a) ZnO, (b) Zn0.97Fe0.03O, (c) Zn0.95Fe0.05O, and (d) Zn0.90Fe0.10O thin films.

The experimental values are not corrected with respect to the diamagnetic contribution of the substrate. By comparison with undoped ZnO film, the Fe-doped ZnO films show hysteresis loops, indicating the room-temperature ferromagnetism of thin films. The magnetic moments increase with the amount of Fe ions from (for ), (for ), and (for ). The coercivity () shows decreasing values with the content of Fe ions, (for ), (for  ), and (for ).

A question can arise from superparamagnetic “hysteresis” behavior of Zn0.90Fe0.10O thin films. This observation can be explained in terms of small crystallite size (approximately 20 nm from AFM visualization of grains) and in terms of superexchange interaction between Fe³⁺ and Fe²⁺ ions in octahedral (B) sites of ZnFe₂O₄ spinel phase.

4. Conclusions

Ni-doped ZnO (, ) and Fe-doped ZnO (, ) thin films have been synthesized using a simple sol-gel method. The results from the structural AFM and magnetic characterization reveal that the levels of doping ions are crucial for obtaining the magnetic properties at room temperature. The XRD analysis shows ZnO wurtzite structure for the thin films. A possible mechanism for room-temperature ferromagnetism of is magnetic coupling between Ni²⁺ ions by superexchange interaction via O²⁻ in wurtzite structure. The observation that Zn0.90Ni0.10O film is magnetically unsaturated, at maximum magnetic field, can be interpreted in terms of change in lattice spacing due to the secondary rhombohedral phase of , and in terms of superexchange interaction.

From XRD diffraction patterns of iron doped zinc oxide one can observe that the peaks of wurtzite structure are majorities. For samples, where , a magnetic coupling takes place between Fe³⁺ ions by superexchange interaction via oxygen atoms.

The Zn0.9Fe0.1O film shows a superparamagnetic behavior due to small crystallite sizes and a superexchange interaction between the resultant and original ions through an oxygen ion in the B-site. As reported in a number of established papers [21, 22] in the B-site, however, the difference between Fe³⁺ and Fe²⁺ ions cannot be recognized and the spin directions in Fe³⁺ and Fe²⁺ ions are the same, by double-exchange interaction leading to electron hopping between vicinal Fe³⁺ and Fe²⁺ ions through a hybrid orbital formed with the 2p orbital of an oxygen ion.

Our experimental studies confirm that the origin of room temperature ferromagnetism in TM:ZnO films can be connected to substituting positions of TM ions in the ZnO lattice.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This work was supported by Grants of the Romanian National Education and Scientific Research Ministry Project no. PCCE-ID 76/2010 and Project PCCA no. 204/2012.

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