Peng_ZnCoO

a e 1, ation e 1, 06; O �x O th he M , po ure o 1−x x concentrations.5,8 Recently, the investigations of the origin of studies of Zn Co O thin films fabricated by a dual beam APPLIED PHYSICS LETTERS 88, 192110 �2006� magnetism in Co-doped ZnO were reported. They studied the origin of ferromagnetism of Zn1−xCoxO thin films either with Co clusters7 or precluding the Co clusters by controlling processing parameters.8 It seems that the origin of magnetic behaviors for Zn1−xCoxO thin films is not clear, especially when the Co concentration is low �x�0.1�. To be a candi- date material to develop device in spintronics, it is essential to assure that the magnetism does not originate from the second phases. Thus, it is significant to unveil the origin of M-H hysteresis loops of Zn1−xCoxO thin films with low Co concentrations �no apparent Co clusters�. In this way, it is helpful to clear the origins of ferromagnetism for Zn1−xCoxO thin films.9 1−x x pulsed laser deposition �DBPLD� method,16 in which the films form a wurtzite structure, and possible precipitates have been ruled out by a high-resolution transmission elec- tron microscopy �HRTEM�. To change the carrier density in the thin films, a Hamamatsu light source with the energy of around 0.2 mW/cm2 and wavelength � of 313 nm was applied at room temperature. An alternating gradient magnetometer �AGM� with the model of 2900 Micromag™ was used to obtained the M-H curves before and after the light irradia- tion. To obtain AHE signals, a Hall effect system �Lakeshore 7500� was used to investigate Hall resistivity and carrier den- sity of the Zn1−xCoxO thin films with a six point Hall bar geometry at room temperature with the angle of 45° between the magnetic field and the normal to the sample. In our stud- ies, Hall bar specimen is of �1-2-2-1� configuration. The light source mentioned above was also used in the Hall effect a�Present address: School of Science, Hangzhou Dianzi University, Hang- zhou 310018, People’s Republic of China and Electrical and Computer Engineering Department, National University of Singapore, Singapore 119260, Singapore and State Key Lab of Silicon Materials, Zhejiang Uni- Anomalous Hall effect and origin of m at low Co content Y. Z. Peng,a� T. Liew, and T. C. Chong Data Storage Institute, DSI Building, 5 Engineering Driv and Electrical and Computer Engineering Department, N Singapore 119260, Singapore C. W. An and W. D. Song Data Storage Institute, DSI Building, 5 Engineering Driv �Received 30 November 2005; accepted 11 April 20 A small anomalous Hall effect signal of a Zn1−xCox that there presents sp-d interactions in the Zn1−xCox we did not obtain observable differences between t were changed. Integrated with our previous results Zn1−xCoxO thin films were proposed based on a pict Institute of Physics. �DOI: 10.1063/1.2202140� In recent years, diluted magnetic semiconductors �DMSs� have attracted much interest in materials with both charge and spin degrees of freedom. Hence, they are ex- pected to play an important role in spintronics devices.1 Though the success in the synthesis of Mn-doped III-V type diluted magnetic semiconductors such as InMnAs and GaMnAs makes them possible to be used in structures for electrical or optical control of ferromagnetism,2 DMSs based on transition metal doped-II-VI semiconductors �ZnO� are predicted to have Curie temperature that can be raised above 300 K.3 Such materials are potential candidates as room tem- perature ferromagnetic semiconductors for spintronics applications,4 and, so, were extensively studied in this field. It has been reported the observation of M-H loops of the Zn1−xCoxO thin films.5,6 Despite of much references found for corresponding experimental and theoretical activities, it is difficult to see that these accounts show consistency. For example, Zn1−xCoxO thin films were reported paramagnetic for x�0.12 in Ref. 7; by contrast, M-H hysteresis loops were observed for Zn Co O thin films with similar Co versity, Hangzhou 310027, People’s Republic of China; electronic mail: yingzip@hziee.edu.cn 0003-6951/2006/88�19�/192110/3/$23.00 88, 19211 Downloaded 07 Jun 2006 to 139.18.52.178. Redistribution subject to gnetism in Zn1−xCoxO thin films Singapore 117608, Singapore al University of Singapore, Singapore 117608, Singapore published online 12 May 2006� =0.05� thin film was observed, indicating in film. However, under a self-held setup, -H curves when the near band electrons ssible mechanisms for the magnetism of f spins in a carrier sea. © 2006 American Several theories can be used to explain the magnetism in the absence of Co clusters for DMSs.10–13 One important criteria for DMSs to be intrinsic has been suggested to be the observation of the anomalous Hall effect �AHE� in thin films.14 Though the criteria were questioned when both su- perparamagnetism and AHE were observed to coexist in highly reduced Co-doped rutile TiO2−� films.15 However, AHE testing is still a tool to show the spin-orbital interac- tions in the materials. In this case, the interactions need to be analyzed further. Photoinduced phenomena in diluted mag- netic semiconductors have attracted much attention due to the possible interpretations of the origin of magnetic behav- iors, in terms of the exchange interaction between the pho- togenerated carriers and magnetic ions, if the magnetization could be manipulated by light irradiation. Motivated by the desire for illuminating the physical mechanism underlying DMS ferromagnetism, we tested anomalous Hall effect and observed their M-H loops re- sponse to carrier densities. Details in the setups will be given in the next paragraph. These works followed our previous system to compare the Hall resistivity and carrier density with and without light irradiation. © 2006 American Institute of Physics0-1 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp In the Hall effect measurement for a ferromagnetic ma- terial with the B field lying in the plane of current and the normal of the surface, there are two Hall voltage components.17 The Hall resistivity �B can be defined by �B = R0B cos � + RAM cos � , �1� where R0 is the ordinary Hall coefficient, B is the magnetic flux density, RA is the anomalous Hall coefficient, M is the magnetization of the film, � is the angle between the applied field and the normal to the sample, and � is the angle be- tween the magnetization and the normal to the sample. The first term in Eq. �1� is the ordinary Hall effect �OHE�. The second term is the AHE and is due to spin dependent scat- tering mechanisms. Figure 1 shows a small Hall AHE signal of Zn1−xCoxO thin film with x=0.05 after proper data extractions under the condition of �=45°, though the signal is small. It is an evi- dence that AHE was observed in the Zn1−xCoxO thin film. It is known that there is no AHE reported in Zn1−xCoxO thin films in which Co is suggested to be more uniformly distrib- uted. In the light of our experimental conditions, the AHE signal is dominant for lower magnetic field and can be evalu- ated by subtracting the linear background. Under our experi- mental conditions, only a few samples show the AHE signals and they are not pronounced. In general, skew scattering provides a mechanism of AHE. Hence, the observation of AHE presents that there pre- sents sp-d interactions, indicating that the magnetization di- rection follows the field direction,18 though the magnetic field range did not reach the saturation value due to the limits of the experimental conditions. The evaluation of �� /�0 dependence on � gives voice to that dependence on the angles between the B field and the hexagonal axis of the film. The evaluation of ��/�0 = ��B − �0�/�0 �2� as a function of B for a given direction of I and B with respect to the crystal axes was obtained, where �B is the Hall resistivity at B field of B and �0 is the �B with B=0. To compare the effect in constancy, we evaluated it while setting B at 500 G. In this experimental arrangement, B rotates from �=0° to �=90°. The result of �� /�0 dependence on � is depicted by Fig. 2. It can be seen that �� /�0 decreases with �, almost following a cosine dependence on �. It coincides with the first term in Eq. �1�. It is well known that the OHE depends on the perpendicular component of the B field, and produces an electric field perpendicular to the perpendicular component of B and the current density. From Eq. �1�, at the FIG. 1. AHE signal of Zn1−xCoxO with x=0.05 thin film. 192110-2 Peng et al. point of B=500 G, the AHE’s contribution is small com- pared to the OHE term. To clarify the origin of magnetism, we applied the light source in the Hall effect system, as shown in the inset of Fig. Downloaded 07 Jun 2006 to 139.18.52.178. Redistribution subject to 3. The relationship between the magnetism and carrier is directly reflected through the response of the Zn1−xCoxO thin film �x=0.05� to the light irradiation with the wavelength of 313 nm. Figure 3 shows that under the condition of light irradiation, the Hall resistivity decreases while the carrier density increases. In contrast, when the light is off, the Hall resistivity increases while the carrier density decreases. It indicates that the light irradiation can increase the carrier density of the film. The Zn1−xCoxO thin films are n-type semiconductors.16 When the film was irradiated with a light with photoenergy larger than the band gap of the film, electrons near the va- lence band will be excited into the conduction band. The amount of the electrons contributing to the unequlilium local spin-density distribution due to the double exchange mecha- nism will also be changed. Hence the variance in the carrier density characterized by the Hall effect measurement may be a point to the variance in the electrons contributing to mag- netism in DMS, if this mechanism works, and vice versa. Hence to clarify the spin-orbit interactions in Fig. 1, we re- peated the testing under a light irradiation with the wave- length of 313 nm. However, we failed to obtain an enhanced AHE signal after the light irradiation �not shown here�. If the correlations between the global ordering of the Co2+ local moments and carrier density work in this spin system, the magnetic moment can be mediated by carrier density, and the AHE signals should be enhanced. Otherwise, there is no evi- dence to show the correlations between magnetic and trans- port properties in the magnetic system. Furthermore, we applied the light source in the AGM system to compare the M-H curves at different carrier den- sities in the inset of Fig. 4. Figure 4 presents typical M-H curves of the Zn1−xCoxO thin film �x=0.05� before and after light irradiation with the wavelength of 313 nm. Here we give an explanation about the plotting. The M-H loops are plotted with the same scale of y axis but a shift of x axis in order to give a clear comparison. It can be seen that there are no observable differences between the M-H curves before FIG. 2. Dependence of �� /�0 on angle � obtained by the Hall effect mea- surement of the Zn1−xCoxO thin film with x=0.05. Appl. Phys. Lett. 88, 192110 �2006� FIG. 3. Hall resistivity and carrier density response to light irradiation ob- tained by a setup as shown schematically in the inset. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp 192110-3 Peng et al. Appl. Phys. Lett. 88, 192110 �2006� and under the condition of light irradiations. In our view, if the correlations between the global ordering of the Co2+ local moments and carrier density in this spin system exist, the magnetic moment should be mediated by carrier density. On account of our experimental results, we did not detect such correlations. Based on our experimental results, there exists spin- orbital interactions in the Zn1−xCoxO thin films at room tem- perature, but there is no evidence to show the correlations between the magnetism and the carriers. We suppose that the magnetism of Zn1−xCoxO thin films at room temperature does not originate from the spin-orbit interactions in a long range order, but that in a shot range order is related to the magnetic behaviors. The result is also a sequence of our previous results.16 In our previous report, the Zn1−xCoxO thin films exhibit discrepancy points on the field cooled �FC�/ zero-field-cooled �ZFC� curves at low magnetic fields. The M-H curves are not squareness. These results suggest that the magnetic behaviors of Zn1−xCoxO thin films at room tem- perature appear to be some extent of spin-glass properties. Probably we could explain it using a spin glass system, something like spins in a carrier sea.13 In the Zn1−xCoxO system with less Co concentrations, Co ions substitute some Zn ions. We take into account the second nearest atoms of Co ions in the Zn1−xCoxO with wurtzite structure, there are 12 second near neighbors of Co ions, they are arranged with hexagonal symmetry in three-dimensional �3D� lattice, as show in Fig. 5, where the light shadowed balls depict Zn and the heavy solid balls with arrows depict Co ions which sub- stitute Zn site in ZnO. The Co ions substitute Zn site in ZnO randomly. Let us suppose that over two Co ions happened to be nearest neighbors, such as B and C to A in Fig. 5. Once the direction of atom A is up, the direction of surrounding spins should be down, and once the direction of C is down, B should be up. Namely, there are two possibilities for the Co ion B spin orientation. Hence the spins cannot find a most favorable direction. By contrast, they randomly spaced, which results in the competing interactions to form the spin glass state. In a word, the exchange interaction failed to be propagated, thus leading to a frustrated spin system. In conclusions, a small AHE signal of the Zn1−xCoxO �x=0.05� thin films was observed. However, we did not ob- tain observable differences between the M-H curves when the near band electrons are changed. Integrated with our pre- vious results, possible mechanisms for the magnetism were FIG. 4. Comparative M-H curves before and under that light of a hand-held light source. The inset shows the schematic diagram of the measurement setup. Downloaded 07 Jun 2006 to 139.18.52.178. Redistribution subject to proposed based on a picture of spins in a carrier sea. This work has been supported by the Data Storage Insti- tute of Singapore Project titled magnetic semiconductor for spintronics materials with the project code of DSI/03- 200001. The authors are grateful to Dr. T. J. Zhou, X. A. Liang, and Dr. Tan Eilee for their valuable discussions and kindness help. The authors would like to thank Dr. J. Linde- muth for his kind and useful discussions during this study. 1Y. Ohno, D. K. Young, B. Beschoten, F. Matsukura, H. Ohno, and D. D. Awschalom, Nature �London� 402, 790 �1999�. 2H. Ohno, D. Chiba, F. Matsukura, T. Omiya, E. Abe, T. Dietl, Y. Ohno, and K. Ohtani, Nature �London� 408, 944 �2000�. 3T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287, 1019 �2000�. 4T. Fukumura, Z. W. Jin, A. Ohtomo, H. Koinuma, and M. Kawassaki, Appl. Phys. Lett. 75, 3366 �1999�. 5Y. Z. Peng, W. D. Song, C. W. An, J. J. Qiu, J. F. Chong, B. C. Lim, M. H. Hong, T. Liew, and T. C. Chong, Appl. Phys. A: Mater. Sci. Process. 80, 565 �2005�. 6N. A. Theodoropoulou, A. F. Hebard, D. P. Norton, J. D. Budai, L. A. Boatner, J. S. Lee, Z. G. Khim, Y. D. Park, M. E. Overberg, S. J. Pearton, and R. G. Wilson, Solid-State Electron. 47, 2231 �2003�. 7Jung H. Park, Min G. Kim, Hyun M. Jang, Sangwoo Ryu, and Young M. Kim, Appl. Phys. Lett. 84, 1338 �2004�. 8Zhigang Yin, Nuofu Chen, Chunlin Chai, and Fei Yang, J. Appl. Phys. 96, 5093 �2004�. 9T. Fukumura, Y. Yamada, H. Toyosaki, T. Hasegawa, H. Koinuma, and M. Kawasaki, Appl. Surf. Sci. 223, 62 �2004�. 10K. Ueda, H. Tabata, and T. Kawai, Appl. Phys. Lett. 79, 988 �2001�. 11E. Dagotto, T. Hotta, and A. Moreo, Phys. Rep. 344, 1 �2001�. 12S. Das Sarma, E. H. Hwang, and A. Kaminski, Phys. Rev. B 67, 155201 �2003�. 13An Experimental Introduction to Spin Glass, edited by J. A. Mydosh �Tay- lor and Francis, London, 1993�. 14H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, and Y. Iye, Appl. Phys. Lett. 69, 363 �1996�. 15S. R. Shinde, S. B. Ogale, J. S. Higgins, H. Zheng, A. J. Millis, V. N. Kulkarni, R. Ramesh, R. L. Greene, and T. Venkatesan, Phys. Rev. Lett. 92, 166601 �2004�. 16Y. Z. Peng, T. Liew, T. C. Chong, W. D. Song, H. L. Li, and W. Liu, J. Appl. Phys. 98, 114909 �2005�. 17The Hall Effect in Metals and Alloys, edited by C. M. Hurd �Plenum, New York, 1972�, Chap. 1. 18H. Ohno, Adv. Colloid Interface Sci. 71, 61 �1997�. FIG. 5. Schematic graph of arrangement of the Zn�Co� ions and its next nearest neighbors of Zn�Co� ions in the hexagonal close-packed lattice �wurtzite structure�, where the light shadowed balls depict Zn ions, heavy solid balls �A–C� depict Co ions, and the arrows on A–C depict the spin orientations. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp