Magneto-Optical Properties of ZnO:Co Nanocrystalline Films

Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008, pp. 16211624

Magneto-Optical Properties of ZnO:Co Nanocrystalline Films

Luc Huy

Hoang

and Nguyen The

Khoi

Faculty of Physics, Hanoi National University of Education, 136 Xuan Thuy, Cau Giay, Hanoi, Vietnam

Nguyen Hoang

Hai

Center for Materials Science, Hanoi University of Science, Vietnam National University, Hanoi, Vietnam

Pacuski

W.

Institute of Experimental Physics, Warsaw University, Poland

In-Sang

Yang

Division of Nano-Sciences and Department of Physics, Ewha Woman University, Seoul 120-750

(Received 18 July 2007, in nal form 22 August 2007) Co doped ZnO lms were synthesized from the precursors Zn(CH3 COO)2 .2H2 O and Co(CH3 COO)2 .4H2 O by using a \High Voltage Spray Pyrolysis" technique. The physical properties of the prepared lms were characterized by using scanning electron microscopy (SEM), X-ray diraction (XRD) and physical property mesurement system (PPMS) measurements. The lms studied were of a wurtzite phase with grain sizes of about 20 nm. The 5 % Co-doped ZnO lms exhibited ferromagnetic behavior at room temperature. The transmission and the optical magnetic circular dichroism (MCD) measurements con rmed that Co2+ was located at the tetrahedral sites of the ZnO wurtzite structure. MCD results showed that the observed ferromagnetism was less likely related to a carrier-induced mechanism.

PACS numbers: 75.50.Pp, 78.20.Ls, 78.66.Hf Keywords: Diluted magnetic semiconductor, Ferromagnetism, Optical magnetic circular dichroism I. INTRODUCTION

In recent years, ferromagnetism in semiconductors has received much attention, partly due to the interest in spintronic device concepts [1]. Due to transition metal solubility and technological interest, contemporary research has primarily focused on II-VI and III-V materials. The works [2{5] reported that the wide-gap semiconductor ZnO doped with a few atomic percent of cobalt exhibits ferromagnetism with a Curie point above room temperature. Later, several transition-metal (such as V, Fe)-doped ZnO systems were reported to have Curie temperatures higher than 300 K [6, 7]. In spite of extensive studies on II-VI-based semiconductors doped with transition metals (TM) and recent experimental success, the results are sensitive to the preparation methods and the fundamental description of ferromagnetism in oxide semiconductors remains incomplete. Considering these inconsistent and confusing results among dierent research groups, we attempted to clarify the origin of ferromagnetism in ZnO-based diluted  E-mail: [email protected]; Fax: +84-4-7548442

magnetic semiconductors (DMS). For this purpose, we prepared ZnO and Co-doped ZnO thin lms, studied the eect of Co doping on the structural properties and examined the dierences in magnetic behavior among these lms. The s,p-d exchange interaction in ZnO:Co was studied using optical transmission and magnetic circular dichroism (MCD) measurements.

II. EXPERIMENTS

ZnO:Co lms were deposited on quartz substrates by using a \High Voltage Spray Pyrolysis" (HVSP) technique. The substrates were ultrasonically cleaned in acetone, rinsed in deionized water and subsequently dried in a owing gas before deposition. As a starting material, zinc acetate dihydrate Zn(CH3 COO)2 .2H2 O and cobalt acetate tetrahydrate Co(CH3 COO)2 .4H2 O were used. Zinc acetate dihydrate was rst dissolved in a mixture of methoxyethanol and distilled water (molar ratio 1 : 2) to get a 0.2 M solution of zinc acetate. The molar ratio of cobalt acetate in the solution was 5 %. The mixed solution was then stirred at 60  C for 4 h and

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Fig. 1. X-ray diraction patterns of ZnO lm (a) and ZnO lm doped with 5 % Co (b).

Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008

Fig. 3. Hysteresis curves measured at room temperature of (a) ZnO and (b) ZnO:5 % Co lms. III. RESULTS AND DISCUSSION

Fig. 2. SEM images of (a) ZnO and (b) ZnO:5 % Co.

a transparent solution was obtained. The nal solution was sprayed (by HVSP) on a quartz substrate, which was maintained at 200  C. After the deposition, the layer was kept at 200  C for 10 min in order to let the solvent residuals evaporate. The procedures were repeated ve times until the desired thickness of 400 { 450 nm was reached. The lms were then annealed at 500  C for 60 min in an Ar environment. A Siemens D-5005 X-ray diractometer (XRD) with CuK radiation was used for structural determination. The surface morphology was investigated using a scanning electron microscope (FESEM Hitachi S4800). Optical properties were tested by using a UV-VIS-NIR double beam spectrophotometer (Perkin-Elmer Lambda 900). A Quantum Design physical property measurement system (PPMS) was used to investigate the magnetic properties of these samples. The optical magnetic circular dichroism spectra were measured at a temperature 4 K in a wavelength range from 550 to 700 nm by applying magnetic elds up to 4 T along the light propagation direction (Faraday con guration). Alternating  + and  circular polarizations of light were produced by using a quartz stress photoelastic modulator.

The typical XRD patterns of the ZnO and the Codoped ZnO lms are shown in Figure 1(a). It can be seen from the X-ray diraction patterns (Figure 1(a)) that all peaks can be well indexed to the wurtzite phase of ZnO. No peaks from any other phase of ZnO or from impurities were observed, which indicates the high purity of the obtained wurtzite ZnO lms. The subsequent XRD (Figure 1(b)) showed that the Co doping had not changed the wurtzite structure of ZnO for doping concentrations of 5 %. The crystallite sizes of ZnO and ZnO:Co calculated by using Scherrer's equation from the width of the XRD peaks were around 5 nm. Figure 2 shows the SEM images of the surface of the ZnO and ZnO:5 % Co lms. It can be seen that ZnO lm reveals a smooth surface, consisting of nano-size crystallites having sizes of 15 { 20 nm. The apparent grain sizes observed by the SEM are larger than the values deduced from the XRD patterns by using Scherrer formula. It seems that the grains seen in the SEM micrographs are composed of several small crystal grains. In the Codoped lms, the grain sizes are slightly larger. However, it should be noted that at such a small percentage of Co dopant concentration (5 %), no signi cant change in morphology shoud be observed. Figure 3 shows the magnetic eld dependence of the magnetization of the ZnO and the Co-doped ZnO lms measured at 300 K. We see that the pure ZnO lm is diamagnetic. The 5 % Co-doped ZnO lm is ferromagnetic with a coercive eld of 500 Oe and a saturation magnetization of 7:5  10 2 emu/cm3 . In order to study the origin of the magnetic behavior in the ZnO:Co lms, we performed optical absorption and magnetic circular dichroism (MCD) measurements. The optical transmission spectra for the ZnO and the ZnO:5 % Co thin lms are compared in Figure 4. The

Magneto-Optical Properties of ZnO:Co Nanocrystalline Films { Luc Huy Hoang et

Fig. 4. Transmission spectra of (a) ZnO and (b) ZnO:5 % Co lms at room temperature.

Fig. 5. Zeeman splitting of the 4 A2 (F ) and the 2 E (G) levels in a magnetic eld and the optical transitions observed in dierent circular polarizations (solid arrows: + ; dotted arrows:  ).

band edge of the ZnO lm is at 3.26 eV while that of the ZnO:5 % Co lm shifts to lower energies. In the doped lm, there are three absorption peaks, 1.88 eV, 2.02 eV and 2.19 eV, related to d-d transitions of Co2+ ions. They are assigned to transitions from the 4 A2 (F ) state to the 2 E (G), 4 T1 (P ) and 2 A1 (G) states, respectively, in Co2+ at tetrahedral sites [8]. These facts provide a clear evidence that Co exists at the tetrahedral sites of the ZnO wurtzite structure as Co2+ . The doping of transition metal (TM) ions in semiconductors gives rise to DMS. In these materials, interactions between carriers (electrons and holes) and the localized spins of TM ions, called s,p-d interactions, play a special role in enhancing the magneto-optical eect [9]. The orientation of localized spins in an external magnetic eld, that is, the magnetization related to these spins, results in an internal eective magnetic eld, by far exceeding the external one, leading to large Zeeman splitting of the energy levels. Hence, optical transitions

al.

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Fig. 6. MCD spectrum of ZnO:5 % Co under a magnetic eld of 2 T at 4 K.

from Zeeman-split levels can give information about this part of the magnetization. In our experiment, the absorption line at about 1.9 eV comes from the transition 4 A2 (F ) ! 2 E (G). In a magnetic eld, the levels split as shown in Figure 5. According to the selection rules, there are four optical transitions allowed, giving rise to two lines in each circularly polarized light ( + and  ). However, in each polarization the two lines are very near each other in energy; hence, only one line is observable. Besides, at low external magnetic eld, the splitting of the lines in dierent polarizations is too small and so is too dicult to detect. Here, we used magnetic circular dichroism (MCD) to study the Zeeman splitting. The MCD intensity can be determined as

I + I ; (1) I + + I where I+ and I are, respectively, the intensities of transmitted light in right ( + ) and left ( ) circular poMCD =

larizations. The MCD spectrum of ZnO:5 % Co at 4 K is shown in Figure 6. Among the structures in the spectrum, those with a dispersion shape result from the Zeeman splitting of narrow absorption lines. In particular, the structure around 1.9 eV comes from the absorption line at the same energy. Figure 7 shows the results for the MCD structure around 1.9 eV at dierent magnetic elds up to 1 T. For small Zeeman splitting, the MCD intensity can be considered to be proportional to the energy dierence between the split levels. Thus, the linear dependence of the MCD intensity on the external magnetic eld shown in Figure 8 may indicate that the magnetization of the Co2+ ions is proportional to the external magnetic eld, which is well known to be true for paramagnetism. MCD measurements performed at several photon energies in magnetic elds with reversed direc-

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Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008

because the XRD measurements may not be sensitive enough to detect small amounts of CoO and Co clusters. IV. CONCLUSIONS

Fig. 7. MCD signals of ZnO:5 % Co at 4 K, under various magnetic eld.

ZnO and ZnO: 5 % Co lms have been prepared on quartz substrates by using a high voltage spray pyrolysis technique. The lms are polycrystalline with a wurtzite structure. The ZnO:5 % Co lm exhibits ferromagnetic order at room temperature. The d-d transitions of tetrahedrally coordinated Co2+ in a ZnO:Co lm are observed in the transmission spectra. The MCD results show the paramagnetic behavior of the s,p-d exchange interaction in ZnO:Co. This is an argument for the observed ferromagnetism not being related to the carrier-inducedferromagnetism mechanism. The results of our study may contribute to elucidating the nature of the ferromagnetism in transition-metals-doped large-gap oxide semiconductors.

ACKNOWLEDGMENTS

This work was supported by the Vietnam Program for Basic Research under project 40-09-06, The Korea Research Foundation Grant No. C00511 (100806) and Asia Research Center, Vietnam National University, Hanoi Research Project.

REFERENCES

Fig. 8. MCD signal intensity of the 1.9-eV line vs magnetic eld.

tions revealed no hysteresis. From this, we can conclude that the MCD results con rm the paramagnetic behavior of the Co2+ ions doped in ZnO. The MCD results for ZnO:Co lm are not in accordance with those of the magnetization measurements. Following Ref. 2, there are three possible origins of the ferromagnetism in ZnO:Co: (i) carriers-induced ferromagnetism (RKKY or double exchange mechanism), (ii) the ferromagnetism of CoO and (iii) the ferromagnetism of Co clusters. From the results of our study, it is less likely that the observed ferromagnetism of the lms is related to carrier-induced ferromagnetism because the ion-carrier interaction has a paramagnetic character, as seen above. However, we also cannot assign the ferromagnetism to any of the two remaining mechanisms. We may only suggest not excluding them from consideration

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