Energetic Particle Irradiation Effect On The Magnetic Properties Of The Single-crystal Rutile TiO₂

TiO₂ is a versatile material that has been widely used in solar cells,photocatalyst,solar hydrogen production and self-cleaning coatings because of its superior optical and electronic properties,physical and chemical stability,cost effectiveness,and nontoxicity,in particular with regard to their potential use in spintronics devices that accommodate both charge and spin of electrons in a single substance.Following the first observation of room temperature ferromagnetism?RTFM?in Co-doped TiO₂,TiO₂ based diluted magnetic semiconductors?DMSs?,in which a small quantity of transitionmetal ions are randomly introduced into Ti sites of the semiconducting host lattice,have attracted substantial interest.Up to now,throughout most of the published papers,the ever important question of how to precisely determinate the nature of defects responsible for the experimentally observed RTFM is still open,especially for the systems doped with magnetic elements,i.e.,Fe,Co,Ni,probably forming metallic clusters.Unfortunately,however,magnetic properties of the DMSs are in general sample parameter-sensitive,lacking in reliability and reproducibility?R&R?remaining a major challenge.Since metallic copper doesn't show any essentially FM behavior and neither do CuO and Cu₂O,Cu doping overcomes the critical issue of magnetic precipitates in a substrate.To avoid any extrinsic FM signals,we here therefore extend the study to Cudoped TiO₂ wafers via ion implantation technique for the purposes of high reliability and reproducibility.Besides the Cu ion implantation,Ar ion and D-D neutron irradiation were also carried out to run as a proper control.In this frame,the main content and experiment results are following as below.A.Experiment and methods.?1?High quality single-crystal rutile TiO₂ wafers with one side polished from MaTecK were employed in this work.Since the physical properties of films or powders are extremely sensitive to deposition parameters such as temperature,composition and precursor concentration,Commercial TiO₂ single crystals can run away from the issue and exclude some contamination that might give rise to FM signals.?2?Cu-doped TiO₂ single crystals were prepared via ion implantation technique.Ion implantation is a more promising technique than other methods?i.e.,sol-gel,sputtering deposition,pulsed laser deposition?,for one can inject accurately measured quantities of dopants and the depth concentrations can be adequately controlled.For the purposes of those,80 keV Cu ions were injected into the polished side of the samples with a nominal fluence of 1?1016 ions/cm2,resulting in a projected range of 37.8 nm with a longitudinal straggle of ?Rp=15 nm as calculated by the stopping and range of ions in matter?SRIM?code.Intentionally,in comparison with the results from Cudoped TiO₂ single crystals,in such case,55 keV Ar ion irradiation was accordingly performed on the single crystals with the SRIM calculated dose of 2?1016 ion/cm2.?3?In order to increase the defects in the wafers,Ar ion post-irradiation was perform on the Cu-doped TiO₂ samples.?4?The magnetic,microstructural and optical properties of the samples were characterized to demonstrate the Bound Magnetic Polaron?BMP?model first introduced by Coey.?5?Based upon the above results,D-D neutron irradiation was carried out to further investigate the microstructural,magnetic and optical properties of the rutile TiO₂ single crystals to clarify the issue.B.Conclusions?1?All the samples,namely the pure,Ar irradiated,Cu implanted,and Ar postirradiated samples,show the ferromagnetism and the saturation magnetization?MS?of each sample is gradually increased.It is noteworthy that the MS of Ar irradiated sample is nearly equal to the value of Cu implanted one.?2?Some implanted Cu ions substituted for Ti sites,showing long-range ordering,while the others were highly dispersed in the projected range.The substitution led to a strong absorption in the visible range.The post-irradiated TiO₂ shows a spongelike layer in which the defect concentrations increase drastically.However,according to the decreased intensity of the absorption,it can be concluded that some substituted Cu ions are absent from the sites after the post-irradiation.?3?It is found that the distribution of defects and the induced disorder of each sample structure are,in some sense,consistent with the behavior of the MS.Interestingly,the MS of post-irradiated sample dramatically increased due to the higher defect concentration,indicating that the absence of the Cu ions did not influence the MS.?4?The D-D neutron irradiation induced a contraction of the lattice which exhibited RTFM.?5?We suggest that RTFM is more appropriate to be associated with the intrinsic defects,namely oxygen vacancies,rather than any substitution effect.