Evolution of Structural and Magnetic Properties of Ar2+ Ion Irradiated TiO2 Thin Films Annealed under Argon Atmosphere

Abstract

TiO2 crystallizes in three different structures such as anatase, brookite and rutile [1-3]. In addition, Magneli phase is a titanium sub oxide (TinO2n-1, 4 ≤ n ≤ 10) have been explored as a promising candidate because of their high electrical conductivity, chemical inertness and corrosion resistance [4]. Among the Magneli phase compounds, Ti4O7 is important for its high electrical conductivity. In general, Ti4O7 is synthesised by thermal reduction of TiO2 [5]. These Magneli phases can easily accommodate oxygen deficiency through anionic vacancies, which leads to the partial reduction of Ti4+ to Ti3+. This Ti3+ gives local ferromagnetism in the material through hopping of the single 3d electron [6]. In this work, ion irradiation is used as a unique tool for the material modification depending upon the projectile ion, energy and the nature of the target. Ion irradiation has been found to amorphize, recrystallise and induce crystalline to crystalline phase transition or defects in various materials. In our earlier studies, we have used 500 keV Ar2+ ions to irradiate on the TiO2 thin films grown on Si substrate by e-beam evaporation technique and subsequently annealed under O2 environment at 500 C [7-8]. However, we have observed phase transformation from anatase to brookite through amorphisation and room temperature ferromagnetism (RTFM) in the films attribute to oxygen vacancies [7-8]. Hence, in the present study we have explored the role of 500 keV Ar2+ ion irradiation on the structural and magnetic properties of TiO2 thin films annealed under Ar at 900 C. The annealed films show Ti4O7 phase which transform to anatase phase upon irradiation with fluence 1 × 1014 ions.cm-2 as observed from the glancing angle x-ray diffraction (GAXRD). Further, irradiation with fluence 5 × 1016 ions.cm-2 recrystallizes to Ti4O7 phase. Raman spectroscopic studies also support the changes in phase. AFM show small change of surface morphology upon ion irradiation with roughness, 0.060, 0.062, 0.079 nm and grain size, 28, 28, 29 nm for the pristine film, films irradiated with ion fluenece 1 × 1014 and 5 × 1016 ions.cm-2, respectively. The origin of RTFM is coined with the oxygen vacancies concentration in the films.