Journal of Superconductivity and Novel Magnetism

, Volume 25, Issue 5, pp 1515–1521

Magnetic Properties of Zn0.8(Fe0.1,Co0.1)O Diluted Magnetic Semiconductors: Experimental and Theoretical Investigation

Authors

  • O. Mounkachi
    • Institute of Nanomaterials and Nanotechnology, MAScIR (Moroccan foundation for Advanced Science, Innovation and Research)
  • M. Boujnah
    • Laboratoire de Magnetisme et Physique des Hautes Énergies, LMPHE (URAC 12), Departement de Physique, B.P. 1014, Faculty of ScienceUniversité Mohammed V-Agdal
  • H. Labrim
    • CNESTEN (National Centre for Energy, Sciences and Nuclear Techniques)
    • Institute of Nanomaterials and Nanotechnology, MAScIR (Moroccan foundation for Advanced Science, Innovation and Research)
  • A. Benyoussef
    • Institute of Nanomaterials and Nanotechnology, MAScIR (Moroccan foundation for Advanced Science, Innovation and Research)
    • Laboratoire de Magnetisme et Physique des Hautes Énergies, LMPHE (URAC 12), Departement de Physique, B.P. 1014, Faculty of ScienceUniversité Mohammed V-Agdal
    • Hassan II Academy of Science and Technology
  • A. El Kenz
    • Laboratoire de Magnetisme et Physique des Hautes Énergies, LMPHE (URAC 12), Departement de Physique, B.P. 1014, Faculty of ScienceUniversité Mohammed V-Agdal
  • M. Loulidi
    • Laboratoire de Magnetisme et Physique des Hautes Énergies, LMPHE (URAC 12), Departement de Physique, B.P. 1014, Faculty of ScienceUniversité Mohammed V-Agdal
  • B. Belhourma
    • CNESTEN (National Centre for Energy, Sciences and Nuclear Techniques)
  • M. Bhihi
    • Laboratoire de Magnetisme et Physique des Hautes Énergies, LMPHE (URAC 12), Departement de Physique, B.P. 1014, Faculty of ScienceUniversité Mohammed V-Agdal
  • E. K. Hlil
    • Institut NéelCNRS-UJF, B.P. 166
Original Paper

DOI: 10.1007/s10948-012-1416-5

Cite this article as:
Mounkachi, O., Boujnah, M., Labrim, H. et al. J Supercond Nov Magn (2012) 25: 1515. doi:10.1007/s10948-012-1416-5

Abstract

Structural and magnetic properties of Zn0.8(Fe0.1, Co0.1)O bulk diluted magnetic semiconductor have been investigated using X-ray diffraction (XRD) and magnetic measurements. TEM (Transmission Electron Microscopy) images confirmed the high crystallinity and grain size of Zn0.8(Fe0.1,Co0.1)O powder, the samples were characterized by energy dispersive spectroscopy (EDS) to confirm the expected stoichiometry. This sample has been synthesized by co-precipitation route. The study of magnetization hysteresis loop measurements infers that the bulk sample of Zn0.8(Fe0.1,Co0.1)O shows a well-defined hysteresis loop at Tc (200 K) temperature, which reflects its ferromagnetic behavior. Hydrogenation treatment was used for the control of phase separation. Based on first-principles spin-density functional calculations, using the Korringa–Kohn–Rostoker method (KKR) combined with the coherent potential approximation (CPA), the ferromagnetic state energy was calculated and compared with the local-moment-disordered (LMD) state energy. The mechanism of hybridization and interaction between magnetic ions in Zn0.8(Fe0.1,Co0.1)O is also investigated.

Keywords

Diluted magnetic semiconductorsFerromagnetismHydrogen treatmentAb-initio calculationZnOSpintronic

1 Introduction

During the last few years, there has been extensive interest in the diluted magnetic seminconductors (DMS) for their potential application in spintronic devices that allow the control of both the spin and charge carriers [1]. Diluted magnetic semiconductor have been obtained by doping a non-magnetic semiconductor with transition metal ions. The first diluted magnetic semiconducting property was observed in intrinsically p-type Mn-doped GaAs around 110 K [2]. Dietel et al. predicted that Mn doped p-type ZnO should be ferromagnetic above room temperature [3]. Due to its wide bandgap (3.44 eV) and large excitation binding energy (60 MeV), transition metal doped ZnO is expected to play an important role in multidisciplinary areas of materials science and future spintronic devices [4].

Room temperature ferromagnetism in TM doped ZnO is proved by ab-initio calculation [59] and experimentally [10, 11]. However, the controversy between the intrinsic or extrinsic ferromagnetism must be clarified before one can really design the related devices. A number of researchers have successfully reported a Curie temperature Tc higher than room temperature in many TM-doped ZnO samples [10, 11]. On the contrary, many groups have failed to observe the room temperature ferromagnetism in these systems [12, 13]. Experimentally, the prediction of high temperature ferromagnetism depends on a large number of experimental parameters: preparation methods, measurement techniques, substrate choice, unexpectedly carriers introduced during the synthesis, for example, H or O, and acceptor or donor defects. From this point of view, the effects of native defects in ZnO, carrier doping on Zn0.80Co0.10Fe0.10O are investigated in this paper.

The present study focuses the magnetic behaviors of the ZnO co-doped by Fe and Co. In order to seek a situation where the material gets half-metallic behavior and has high magnetic moment, the Co and Fe concentrations are fixed at 10%. The synthesis of nominal 10% of Co and Fe co-doped ZnO sample has been done by co-precipitation method.

X-ray diffraction data related that Zn0.80Co0.10Fe0.10O crystallizes in the wurtzite structure with the presence of ZnFe2O4 spinel ferrite, following hydrogen treatment. DC magnetization measurement showed that the sample is ferromagnetic. However, a large increase in the magnetization is observed below 200 K. The origin of ferromagnetism is likely to be the intrinsic characteristics of the Zn0.80Co0.10Fe0.10O with the presence of donor defect (O vacancies) confirmed by energy stability for ferromagnetic state with and without donor defect.

2 Experimental Procedure

Co-precipitation is a process in which a solid is precipitated from a solution containing other ions. These ions are included into the solid by substitution in the crystal lattice. Adsorption on the surface of the growing particles is one of the principle mechanisms of co-precipitation.

(Co and Fe)-doped ZnO powders were successfully prepared by a simple chemical precipitation method using (CoCl2⋅6H2O), FeCl2, and (ZnCl2⋅6H2O) chloride as the sources of Co, Fe, and Zn, respectively. The precursors were added in de-ionized water and mixed homogeneously and refluxed under air atmosphere to yield a uniform mixture of precursor at 80. Precipitation was done using aqueous NaOH solution, the reactants were constantly stirred using a magnetic stirrer until a pH level of >7 was achieved. The dropping rate must be well controlled for the chemical homogeneity. The precipitates were washed several times to remove the water-soluble impurities and free reactants and dried at 100 ∘C for 10 h. The spongy contents were filtered, dried and then powdered. Heating treatments of the synthesized powders were conducted at 600 ∘C for 4 h, the synthesis methods used were previously applied for simple doping in ZnO like Mn doped ZnO [16] and double doping like Mn and Ni in ZnO [17].

3 Results and Discussion of Experimental Investigation

The physical characterization was performed by X-ray diffractometer (Model: D8 Discover Bruker AXS Detector 2D Hi-Star Turbo Source Rotating Anode X-Ray Cu), while the magnetic characterization was done by Magnetic Properties Measurement System (MPMS-7XL, Quantum Design, Inc). The morphology and particle size of the as-prepared samples were determined by transmission electron microscopy (TEM) observations with an accelerating voltage of 200 kV. The stoichiometric compositions of the obtained powders were determined by EDS.

3.1 Structures Properties

The structure analysis of Zn0.8Fe0.1Co0.1O was done by XRD with Cu kα radiation on the samples prepared by co precipitation method. Figure 1 shows the XRD pattern indicating the coexistence of wurtzite ZnO and spinel ZnFe2O4. This result shows that the Fe-doped ZnO DMS materials prefer to occupy the ferric state of Fe ion, namely, ZnFe2O4. In order to eliminate ZnFe2O4 secondary phase and obtain a single wurtzite phase, the sample has been annealed in Ar/H2 bal. gas ambience. Similar property has been reported by Ahn et al. [14] for Fe-doped ZnO powder calcined at higher temperature. After hydrogenation and annealing, there were no clustering and other phases observed in the XRD spectrum (see Fig. 1), and hence magnetic ordering may be intrinsic and may not be due to any other magnetic phases. The EDS analysis gives the qualitative composition of Zn0.8Fe0.1Co0.1O powder and also indicates the quantitative presence of Zn, Co, Fe, and O in the prepared sample (Fig. 2), the value of at.% metal ion calculated is near the value obtained with EDS (8.8% for Fe, 9.2% for Co, and 82% for Zn), confirming the required stoichiometry. Figure 2 shows the spectrum obtained from EDS measurements. The samples have been analyzed by the TEM as shown in Fig. 3, the particles are regular and uniform in size. The average particles size is 30 nm.
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Fig. 1

X-ray diffraction pattern for the Zn0.80(Co0.10Fe0.10)O with and without hydrogen treatment

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Fig. 2

EDS measurements of Zn0.80(Co0.10Fe0.10)O DMS

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Fig. 3

The TEM micrograph patterns of Zn0.80(Co0.10Fe0.10)O DMS

3.2 Magnetic Properties

We have investigated the magnetic properties of Co–Fe co-doped ZnO sample using a Magnetic Properties Measurement System in the temperature range 5–300 K. To probe the magnetic properties of the sample Zn0.8Fe0.1Co0.1O, magnetization measurement were preformed on the sample for two different temperature 10 K and 180 K and are shown in Fig. 4, ferromagnetic hysteresis loops were observed in Fe and Co doped ZnO at low temperature 10 K and 180 K. The magnetization and corresponding coercive field Hc decrease with increasing temperature. The higher magnetic moments observed in Zn0.8Fe0.1Co0.1O may be due to the indirect exchange interaction among TM (Co, Fe) ions, mediated by O ions. Figure 5 shows the temperature dependence of magnetization measured at a fixed applied magnetic field of 500 Oe. It clearly shows a large magnetization up to 200 K, indicating the presence of ferromagnetic order at low temperature.
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Fig. 4

Magnetic hysteresis loops of Zn0.80(Co0.10Fe0.10)O for different temperatures; T=10 K and 180 K

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Fig. 5

Magnetization versus temperature measured for Zn0.80(Co0.10Fe0.10)O in an external magnetic field (500 Oe)

4 Results and Discussion of Theoretical Investigation

4.1 Computational Details

The calculations in this paper were based on the Korringa–Kohn–Rostoker (KKR) Green’s function method and the coherent potential approximation (CPA) combined with the parameterization of Vosko, Wilk and Nusair (VWN) [15]. The VWN functional predicts a bandgap of 1.8 eV (Fig. 6) for bulk ZnO to be compared to the experimental value of 3.3 eV. The KKR-CPA code MACHIKANEYAMA2002v08 package produced by H. Akai of Osaka University1 has been used, higher K-points up to 456 in the irreducible part of the first Brillouin zone are considered. Recall that the wurtzite structure is the crystalline structure associated to pure ZnO, with a=3.26 Å and c=5.21 Å obtained by our experimental data. The hexagonal unit cell is constituted from tetrahedrons where the zinc atoms occupy the center and oxygen cations are placed in its four corners. Co and Fe impurities are introduced randomly into cation sites of the ZnO semiconductor. To simulate the ferromagnetic state of Zn0.8(Fe0.1,Co0.1)O, all Fe and Co atoms are substituted randomly and of parallel moment Zn0.8(Fe0.1,up,Co0.1,up)O, then the system has a finite magnetization. The LMD state is described as Zn0.8(Fe(0.05,up), Fe(0.05,down); Co(0.05,up), Co(0.05,down))O, the magnetic moment of Fe and Co atoms point randomly with respect to each other and the system has no magnetization.
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Fig. 6

Total density of states of ZnO

4.2 Results and Discussion

Figure 7 shows that the impurity bands originated from 3d states of TM impurities (Co and Fe) are located in the band gap. Figures 7 and 8 show the calculated electronic structure of Zn0.8(Fe0.1,Co0.1)O for the Ferromagnetic state (a); and the local-moment-disordered (LMD) state (b). For 10% (Fe, Co) concentrations, the total energies for the ferromagnetic states are −4.99×10−3 eV and 6.62×10−3 eV, with and without defect (O vacancies), respectively, compared with the LMD state; this indicates that the ferromagnetic state is more stable with defect by O vacancies. From the calculated density of states of Zn0.8(Fe0.1,Co0.1)O, it is clear that the ferromagnetic state realizes a half-metallic behavior: the spin-down states are metallic. For LMD state (Fig. 8) we can only see the influence of the crystal environment: It degenerates atomic 3d levels into the e and t subgroups and the exchange splitting is null. The TM (Fe, Co) impurity has \(d^{5}(d^{5}=e^{2}_{+}t^{3}_{+}e^{0}_{-}t^{0}_{-})\) electron configuration due to the substitution of Zn2+ by TM2+ ions which get high magnetic moment of TM. From band structure calculations presented in Figs. 7 and 9, one can see that the d–d hybridization of TM for Zn0.8(Fe0.1,Co0.1)O is believed to result from half-metallic property, the introduced d orbitals of TM (Fe, Co) impurity will interact with the host p states of O, forming hybrid p–d orbitals, this hybridization occurs because of the Co d and Fe d states in a tetrahedral crystal field. This hybridization between the Fe-3d, Co-3d, and O-2p states and the host valence bands produces effective magnetic field to align Fe and Co magnetic moments and stabilize the half-metallic ferromagnetic state. This is called the p–d exchange mechanism and is one of the important ferromagnetic mechanisms.
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Fig. 7

Total and partial density of states of a ferromagnetic state of Zn0.80(Co0.10Fe0.10)O with O vacancies

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Fig. 8

Total and partial density of states of a spin glass state of Zn0.80(Co0.10Fe0.10)O

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Fig. 9

Partial density of states (p-state of O) for ferromagnetic state of Zn0.80(Co0.10Fe0.10)O with O vacancies

5 Conclusions

In conclusion, single crystalline Zn0.8(Fe0.1,Co0.1)O have been synthesized through a co-precipitation method. X-ray diffraction revealed that the Zn0.8(Fe0.1,Co0.1)O with hydrogenation treatment possess wurtzite structure. The samples were characterized by TEM and EDS to confirm the expected stoichiometry, high crystallinity, and grain size. Zn0.8(Fe0.1,Co0.1)O are ferromagnetic with Curie temperature ≈180 K. By electronic structure and energy stability for ferromagnetic state with and without donor defect, we demonstrated that the origin of ferromagnetism is likely to be the intrinsic characteristics of the Zn0.80(Co0.10Fe0.10)O with the presence of donor defect (O vacancies).

Footnotes
1

MACHIKANEYAMA2002v08: H. Akai, Department of Physics, Graduate School of Science, Osaka University, Machikaneyama 1-1, Toyonaka 560-0043, Japan, akai@phys.sci.osaka-u.ac.jp.

 

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© Springer Science+Business Media, LLC 2012