Temperature-dependent magnetoresistance of ZnO thin film D.F. Wang a, b , Y. Ying a , V.T.T. Thuy a , J.M. Kim a , M.S. Seo a , F. Gao a , T.J. Zhang b , K.W. Kim c , Y.P. Lee a, ⁎ a Quantum Phtonic Science Research Center, Hanyang University, Seoul 133-791, Republic of Korea b Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials and Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, China c Sunmoon University, Asan, Republic of Korea abstract article info Article history: Received 29 April 2010 Received in revised form 17 July 2011 Accepted 25 July 2011 Available online 1 August 2011 Keywords: Zinc oxide Magnetoresistance Weak localization Spin splitting A ZnO film was deposited, and the magnetic and the magnetoresistive (MR) properties were studied. The MR measurements reveal negative MR at 80, 50, 20, 10 and 6 K, which is supposed to be induced by the weak- localization effect, based on a logarithmic dependence of the electrical conductivity on temperature. When temperature was reduced to be 2 K, a positive MR was observed. We suggest that it is related to the spin splitting induced by exchange interaction between itinerant electrons and vacancy defects in ZnO. Through the magnetic measurement, it is found that ZnO shows ferromagnetism. It is suggested that the observed ferromagnetism is correlated with the exchange interaction. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Diluted magnetic semiconductor (DMS) has attracted much atten- tion due to the possibility to manipulate the spin and the charge simultaneously in future spintronics [1–3]. Following the theoretical prediction on the room-temperature ferromagnetism in p-type ZnO- based DMS, ZnO-based DMS has been studied extensively to achieve ferromagnetism with a high Curie temperature [4]. Up to now, diverse magnetic properties, such as ferromagnetism, paramagnetism and spin glass, have been observed by different groups [5–8]. More recently, ferromagnetism was reported in pure ZnO without doping any 3-d transition-metal atom [9,10]. It is known that the polarization and the manipulation of electron spin is a key issue for the utilization of spintronic devices [11–13]. Therefore, the study on the magnetotransport properties of DMS materials is rather important. However, most of the efforts, done on DMS currently, are focused on the enhancement of magnetism and boosting of Curie temperature [14,15]. In this work, we studied the temperature-dependent magnetoresistance of ZnO thin film. It gives valuable information on the magnetotransport properties of ZnO and also shed a light on understanding of the mechanism of observed ferromagnetism in undoped ZnO. 2. Experimental details ZnO film with a thickness of 180 nm was deposited on a p-type silicon wafer, by radio-frequency magnetron sputtering, at a growth ratio of 1.8 nm/min. Before the deposition, silicon wafer was treated with HF to remove the SiO 2 layer on the surface. The resistivity of Si wafer employed was about 5 Ω-cm. A ZnO ceramic target was used for the sputtering. The base pressure in the chamber prior to sputtering was 1.1×10 −6 Pa. During the sputtering process, high-impurity (99.9999%) argon was introduced and the working pressure was 6.7 × 10 −1 Pa. The substrate was rotated at a constant rate to make the film uniform, and the temperature of target was kept at 300 °C. The magnetic properties were measured with a Quantum Design Magnetic Property Measure- ment System (MPMS). The electron transport properties, including the magnetoresistivity and the temperature-dependent resistivity, were obtained using the Quantum Design Physical Property Measurement System (PPMS) with a magnetic field up to 5 T and at a temperature down to 2 K. All the electronic measurements were performed in four- probe geometry. 3. Results and discussion The crystalline properties of the sample were investigated by X-ray diffraction (XRD). All the diffraction peaks in the XRD pattern correspond to ZnO as labeled in Fig. 1. Fig. 2(a) depicts the mag- netoresistance (MR) of ZnO film, measured at 80, 50, 20, 10 and 6 K in a magnetic field perpendicular to the film plane. The MR is defined as MR =(R h −R 0 )×100%/R 0 , where R h and R 0 is the resistivity of ZnO film, measured with and without, respectively, the application of magnetic field. As seen, ZnO shows a negative MR and the magnitude increased when temperature is decreased, with a maximum of 10.9% at 6 K. Since ZnO thin film was deposited on a semiconducting Si wafer, the observed MR might originate, even partially, from the Si wafer. Therefore, additional MR measurement on a bare Si wafer, with a Thin Solid Films 520 (2011) 529–532 ⁎ Corresponding author. Tel.: + 82 2 22815572; fax: + 82 2 22815573. E-mail address: yplee@hanyang.ac.kr (Y.P. Lee). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.07.053 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf