Positron annihilation lifetime characterization of oxygen ion irradiated rutile TiO 2 Homnath Luitel a , A. Sarkar b , Mahuya Chakrabarti c , S. Chattopadhyay d , K. Asokan e , D. Sanyal a,⇑ a Variable Energy Cyclotron Centre, 1/AF Bidhannagar, Kolkata 700064, India b Department of Physics, Bangabasi Morning College, 19 Rajkumar Chakraborty Sarani, Kolkata 700009, India c Department of Physics, Acharya Prafulla Chandra College, New Barrackpore, Kolkata 700131, India d Department of Physics, Maulana Azad College, 8 Rafi Ahmed Kidwai Road, Kolkata 700013, India e Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India article info Article history: Received 16 October 2015 Received in revised form 5 April 2016 Accepted 5 April 2016 Available online xxxx Keywords: Magnetic semiconductor Ion beam irradiation Density-functional theory Positron annihilation spectroscopy Defect induced ferromagnetism abstract Ferromagnetic ordering at room temperature has been induced in rutile phase of TiO 2 polycrystalline sample by O ion irradiation. 96 MeV O ion induced defects in rutile TiO 2 sample has been characterized by positron annihilation spectroscopic techniques. Positron annihilation results indicate the formation of cation vacancy (V Ti , Ti vacancy) in these irradiated TiO 2 samples. Ab initio density functional theoretical calculations indicate that in TiO 2 magnetic moment can be induced either by creating Ti or O vacancies. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction In recent past, it has been observed that defect can induce fer- romagnetic ordering in transition metal oxides e.g., ZnO, TiO 2 , SnO 2 , MgO etc., which are otherwise non-magnetic at room tem- perature [1–5]. Introduction of room temperature ferromagnetism (RTFM) in ZnO, TiO 2 , MgO, and SnO 2 is very challenging since these semiconductors have potential application in different technolo- gies, like, spintronics, magneto and opto-electronic based devices [6,7]. The rutile TiO 2 (hereafter referred as R-TiO 2 ) has a band gap of 3.0 eV [8]. Additionally R-TiO 2 is a non-toxic and environ- mental friendly material. Observation of RTFM has been reported in undoped TiO 2 [5,9–15] by several groups. It has been understood that most probable origin of RTFM in TiO 2 is due to presence of vacancy like defects or defect clusters [2,3]. In most of the cases such vacancies have been identified as oxygen vacancies (V O ) in TiO 2 . However, density functional theoretical (DFT) calculations have predicted [13,16] ferromagnetic moments due to both Ti and O vacancies in TiO 2 . In this work, a clear evidence of presence of Ti vacancies in O ion irradiated (and ferromagnetic) TiO 2 using positron annihilation spectroscopy. It is worth mentioning that energetic ions create several possible defect combinations which can be purposefully used to tailor the physical properties of solid materials [17,18]. Positron annihilation spectroscopy, a nuclear solid state tech- nique, is a sensitive probe to characterize defect in solid [19–22] specially, the vacancy like defects. Present study uses positron annihilation lifetime technique for characterizing irradiation induced defects and also identify the nature of defects by coinci- dence Doppler broadening (CDB) spectroscopic technique. The details about these experimental techniques can be found in liter- ature [19–22]. 2. Experimental outline 99.998% pure R-TiO 2 (Alfa Aesar, Johnson Mathey, Germany) polycrystalline powder has been pelletized in the form of 0.5 mm thick pellets and annealed at about 700 °C before irradiation. Irra- diation experiment involving 96 MeV of 16 O 8+ ions from IUAC 15 UD pelletron was carried out in the Materials Science Beamline of IUAC. The irradiation fluence is 6  10 13 ions/cm 2 . The beam has been scanned uniformly on 1 cm  1 cm area of the sample. The temperature of the sample during irradiation was in between 300 and 320 °K. The magnetic measurements were carried out using Quantum Design make MPMS-XL SQUID magnetometer. http://dx.doi.org/10.1016/j.nimb.2016.04.014 0168-583X/Ó 2016 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: dirtha@vecc.gov.in (D. Sanyal). Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb Please cite this article in press as: H. Luitel et al., Positron annihilation lifetime characterization of oxygen ion irradiated rutile TiO 2 , Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.04.014