Chin. Phys. B Vol. 22, No. 3 (2013) 037504 Defect types and room-temperature ferromagnetism in undoped rutile TiO 2 single crystals ∗ Li Dong-Xiang() a)b) , Qin Xiu-Bo() c) † , Zheng Li-Rong() d) , Li Yu-Xiao() b) , Cao Xing-Zhong() a) , Li Zhuo-Xin() a) , Yang Jing( ) a) , and Wang Bao-Yi() a) a) Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China b) School of Physical Engineering, Zhengzhou University, Zhengzhou 450001, China c) Research Center for Application of Nuclear Technology, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China d) Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China (Received 5 September 2012; revised manuscript received 25 October 2012) Room-temperature ferromagnetism has been experimentally observed in annealed rutile TiO 2 single crystals when a magnetic field is applied parallel to the sample plane. By combining X-ray absorption near the edge structure spectrum and positron annihilation lifetime spectroscopy, Ti 3+ –V O defect complexes (or clusters) have been identified in annealed crystals at a high vacuum. We elucidate that the unpaired 3d electrons in Ti 3+ ions provide the observed room-temperature ferromagnetism. In addition, excess oxygen ions in the TiO 2 lattice could induce a number of Ti vacancies which obviously increase magnetic moments. Keywords: annealing, oxygen vacancies, Ti 3+ –V O defect complexes, ferromagnetism PACS: 75.50.Pp, 71.55.–I, 75.20.Hr DOI: 10.1088/1674-1056/22/3/037504 1. Introduction Dilute magnetic semiconductors (DMSs) with magnetic and semiconducting properties are regarded as new genera- tion materials due to their potential applications in spintronic devices. [1–5] Since the first discovery of room-temperature fer- romagnetism (RTFM) in Co-doped anatase TiO 2 thin films, [6] several studies have been performed on the magnetism of TiO 2 doped with transitional metals (TMs), such as Mn, V, Cr, Nb, Fe, and Ni. Different research groups [7–10] learned about the magnetic properties of TM-doped TiO 2 samples us- ing the first-principles method, but the origin of ferromag- netism in magnetic-transition-element-doped oxide semicon- ductors is still under debate as to whether ferromagnetism is intrinsic [11,12] or whether it comes from the precipitation of magnetic clusters. [13] Recently, Coey’s group [14] reported on magnetism observed in HfO 2 thin films on sapphire or silicon substrates. Hong et al. [15] and Yoon et al. [16] reported room- temperature ferromagnetism in pure TiO 2 films. These results are surprising and have generated great interest in this new phenomenon, also known as d 0 magnetism. [17] Its appearance can exclude the influence of magnetic elements (imported by doping transitional metal) from the origins of ferromagnetism. First-principles calculations indicated that the ferromag- netism of undoped rutile TiO 2 largely originates from the d orbitals of low-charge-state Ti ions converted from Ti 4+ ions induced by the surface oxygen vacancies. [18] However, few ex- perimental studies of exploring the origin of room-temperature ferromagnetism in undoped TiO 2 single crystals exist. In this study, the influence of annealing temperature in a high vac- uum on structure and magnetism has been addressed. We aim to answer two questions: (i) what is the defect types of un- doped TiO 2 single crystals annealed in a high vacuum; (ii) what is the origin of room-temperature ferromagnetism after high-temperature annealing. 2. Experiments Commercial rutile TiO 2 single crystals were obtained from the Institute of Shanghai Optics and Fine Mechanics, Chinese Academy of Sciences. The size of the samples was 5×5×0.5 mm 3 , corresponding to a mass of 53.25 mg. Some samples were annealed in a vacuum annealing furnace (vac- uum: 10 -4 –10 -5 Pa) from 1023 to 1273 K for 1 h. The structure and magnetism of TiO 2 single crystals were char- acterized by X-ray diffraction (XRD), X-ray absorption near the edge structure (XANES) spectrum, and superconducting quantum interference device (quantum design MPMS-XL5) at room temperature. The XANES analyses were performed on the beam lines 1W1B from the Beijing Synchrotron Radiation Facility (BSRF). Positron annihilation lifetime spectroscopy (PALS) was used to probe the vacancy-type defects of TiO 2 single crystals with 22 Na as the positron source. Two of the same samples were tightly clamped on both sides of 22 Na ra- dioactive sources, forming a sandwich structure. The prompt time resolution of the system was 190 ps. Each spectrum con- ∗ Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 61006066) and the National Natural Science Foundation of China (Grant No. 11175191). † Corresponding author. E-mail: qinxb@ihep.ac.cn © 2013 Chinese Physical Society and IOP Publishing Ltd http://iopscience.iop.org/cpbhttp://cpb.iphy.ac.cn 037504-1