Defect types and room-temperature ferromagnetism in undoped rutile TiO2 single crystals

Room-temperature ferromagnetism has been experimentally observed in annealed rutile TiO2 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,Ti3+-V O defect complexes(or clusters) have been identified in annealed crystals at a high vacuum.We elucidate that the unpaired 3d electrons in Ti3+ ions provide the observed room-temperature ferromagnetism.In addition,excess oxygen ions in the TiO2 lattice could induce a number of Ti vacancies which obviously increase magnetic moments.     

Preparation, structure and ferromagnetic properties of the nanocrystalline Ti1-xMnxO2 thin films grown by radio frequency magnetron co-sputtering

This paper reported that the Mn-doped TiO2 films were prepared by radio frequency （RF） magnetron cosputtering. X-ray diffraction measurements indicate that the samples are easy to form the futile structure, and the sizes of the crystal grains grow big and big as the Mn concentration increases. X-ray photoemlssion spectroscopy measurements and high resolution transmission electron microscope photographs confirm that the manganese ions have been effectively doped into the TiO2 crystal when the Mn concentration is lower than 21%. The magnetic property measurements show that the Ti1-xMnxO2 （x = 0.21） films are ferromagnetic at room temperature, and the saturation magnetization, coercivity, and saturation field are 16.0 emu/cm^3, 167.5 × 80 A/m and 3740 × 80 A/m at room temperature, respectively. The room-temperature ferromagnetism of the films can be attributed to the new futile Ti1-xMnxO2 structure formed by the substitution of Mn^4＋ for Ti^4＋ into the TiO2 crystal .lattice, and could be explained by O vacancy （Vo）-enhanced ferromagnetism model.     

Temperature Dependence of the Fluorescence Emission Intensity of Eu/TiO2 Nanocrystals

The samples of europium ions doped titanium dioxide （Eu^3＋/TiO2） nanocrystals are synthesized by a modified sol-gel method with hydrothermal treatment. The x-ray diffraction and scanning electron microscopy are used to characterize the sample. The temperature-dependent fluorescence emission effect of Eu^3＋-doped samples is investigated. It is found that under the excitation of 514.5nm light, the emission intensity of Eu^3＋ reaches a maximum value at 450K among various Eu^3＋ dopant concentrations in Eu^3＋ /TiO2 nanocrystals. The variation of the emission intensity may be attributed to the photon-assist absorption and the temperature-quenching effect.     

Structure and photoluminescence properties of Er3+-doped TiO2-SiO2 powders prepared by sol-gel method

Er 3+-doped TiO 2-SiO 2 powders are prepared by the sol-gel method,and they are characterized by high resolution transmission electron microscopy (HR-TEM),X-ray diffraction (XRD) spectra,and Raman spectra of the samples.It is shown that the TiO 2 nanocrystals are surrounded by an SiO 2 glass matrix.The photoluminescence (PL) spectra are recorded at room temperature.A strong green luminescence and less intense red emission are observed in the samples when they are excited at 325 nm.The intensity of the emission,which is related to the defect states,is strongest at the annealing temperature of 800 C.The PL intensity of Er 3+ ions increases with increasing Ti/Si ratio due to energy transfer between nano-TiO 2 particles and Er 3+ ions.     

Role of defects in magnetic properties of Fe-doped SnO2 films fabricated by the Sol--Gel method

This paper obtains the room temperature ferromagnetism in Sn1 xFexO2 films fabricated by the Sol-Gel method.X-ray diffraction results show that Fe doping inhibits the growth of SnO2 and Fe3+ ions occupy the Sn sites.The measurement of resistance excludes the free carrier inducing ferromagnetism.Moreover,the temperature dependence of magnetization has been better fitted by the Curie-Weiss law and bound magnetic polaron(BMP) theory.An enhancement of ferromagnetism is achieved by annealing the samples with x = 7.1% in H2,and a decrease of oxygen flow rate.All these results prove that the BMP model depending on defects can explain ferromagnetism in diluted magnetic oxides.     

Effect of tellurium(Te~(4+)) irradiation on microstructure and associated irradiation-induced hardening

The GH3535 alloy samples were irradiated using 15-Me V Te~(4+) ions at 650℃ to a dose of 0.5, 3.0, 10, and 20 dpa, respectively. The Te atoms distribution and microstructure evolution were examined by electron probe microanalysis(EPMA)and transmission electron microscopy(TEM). The nano-indenter was then used to measure the nano-hardness changes of samples before and after irradiation. TEM results showed the formation of dislocation loops in the irradiated samples.Their mean diameters increase with the increase of irradiation dose and tends to be saturated when irradiation dose exceeds10 dpa. The ratio of yield strength increments calculated by dispersed barrier hardening(DBH) model is basically consistent with that of nano-hardness increments measured by nano-indenter. In addition, the relationship between the nano-hardness increments and dpa for the GH3535 alloy irradiated by Te ions has been revealed in the study.     

Observation of Room Ferromagnetism in Cu-Implanted Crystal ZnO

Ion implantation technique is used to study the magnetic properties of Cu-dopecl ZnO. The room temperature ferromagnetism in the Cu-implanted ZnO samples is observed. From the photoluminescence spectrum of implanted samples we observe a broad green emission around 510nm, which is related to defects in the samples. X-ray photoelectron spectroscopy measurement shows that Cu ions are in the mixed oxidation state of 4-1 or ＋2 and substitute for the Zn^2＋ ions of the ZnO matrix. We argue that the ferromagnetism is related to these defects, and the substitution of Cu^2＋ into Zn^2＋ sites in crystal ZnO could contribute to the observed ferromagnetism.     

Structure, room-temperature magnetic and optical properties of Mn-doped TiO2 nano powders prepared by the sol-gel process

TiO 2 nano powders with Mn concentration of 0 at%-12 at% were synthesized by the sol-gel process,and were annealed at 500 C and 800 C in air for 2 hrs.X-ray diffraction (XRD) measurements indicate that the Mn-TiO 2 nano powders with Mn concentration of 1 at% and 2 at% annealed at 500 and 800 C are of pure anatase and rutile,respectively.The scanning electron microscope (SEM) observations reveal that the crystal grain size increases with the annealing temperature,and the high resolution transmission electron microscopy (HRTEM) investigations further indicate that the samples are well crystallized,confirming that Mn has doped into the TiO 2 crystal lattice effectively.The room temperature ferromagnetism,which could be explained within the scope of the bound magnetic polaron (BMP) theory,is detected in the Mn-TiO 2 samples with Mn concentration of 2 at%,and the magnetization of the powders annealed at 500 C is stronger than that of the sample treated at 800 C.The UV-VIS diffuse reflectance spectra results demonstrate that the absorption of the TiO 2 powders could be enlarged by the enhanced trapped electron absorption caused by Mn doping.     

Tailoring optical properties of TiO2 in silica glass for limiting applications

We present the linear and nonlinear optical studies on TiO2-SiO2 nanocomposites with varying compositions. Opti- cal band gap of the material is found to vary with the amount of SiO2 in the composite. The phenomenon of two-photon absorption （TPA） in TiO2/SiO2 nanocomposites has been studied using open aperture Z-scan technique. The nanocom- posites show better nonlinear optical properties than pure TiO2, which can be attributed to the surface states and weak dielectric confinement of TiO2 nanoparticles by SiO2 matrix. The nanocomposites are thermally treated and similar studies are performed. The anatase form of TiO2 in the nanocomposites shows superior properties relative to the amorphous and rutile counterpart. The involved mechanism is explained by rendering the dominant role played by the excitons in the TiO2 nanoparticles.     

Field-induced insulator-metal-insulator transitions in low-energy H2- ion-irradiated epitaxial La2/3Cal/3MnO3 thin films

Epitaxial La2/3Cal/3MnO3 thin films grown on LaA103 （001） substrates were irradiated with low-energy 120-keV H＋ ions over doses ranging from 1012 ions/cm2 to 1017 ions/cm2. The irradiation suppresses the intrinsic insulator-metal （I-M） transition temperature and increases the resistance by reducing the crystallographic symmetry of the films. No irradiation-induced columnar defects were observed in any of the samples. The specific film irradiated at a critical dose around 8 x 1015 ions/cm2 is in a threshold state of the electric insulator where the I-M transition is absent. In an external field of 4 T or higher, the I-M transition is restored and thus an enormous magnetoresistance is observed, while a negative temperature coefficient resumes as the temperature is reduced further. Magnetic relaxation behavior is confirmed in this and other heavily irradiated samples. The results are interpreted in terms of the displacement of oxygen atoms provoked by ion irradiation and the resulting magnetic glassy state, which can be driven into a phase coexistence of metallic ferromagnetic droplets and the insulating glass matrix in a magnetic field.     

快重离子辐照效应的穆斯堡尔谱学研究

概述了利用穆斯堡尔效应开展的固体材料快重离子辐照效应研究的部分结果 ,并对建立在兰州重离子加速器 (HIRFL)上的在束穆斯堡尔谱学研究装置及其应用作了简要的介绍.Mssbauer spectroscopy study of irradiation effects induced by swift heavy ions in solid materials were briefly presented.Amorphization phenomenon of yttrium iron garnet irradiated by 1 GeV Ar ions has been investigated. For the first time, the nearly complete amorphous state was observed. Stainless steel 316L samples were irradiated with 54 MeV C ions and phase transformation of the samples was observed. HT 9 ferrite steel was irradiated with 510 MeV C ions. Its phase...     

Room-Temperature Ferromagnetism in Semiconducting TiO2-δ Nanoparticles

TiO2-δ nanoparticles are synthesized by the sol-gel method and annealed under different reducing atmosphere. The x-ray diffraction patterns show that anatase is the dominant phase with small amounts of the futile phase of TiO2-δ for all the samples. Magnetic measurements indicate that the samples annealed in reducing atmosphere exhibit unprecedented room-temperature ferromagnetism, in particular, the saturation magnetization Ms is up to about 8.6 × 10^-3 emu/g for the sample annealed in H2/Ar mixture. Analysis of the x-ray photoelectron spectroscopy spectra for the samples processed under different conditions indicates that the amounts of Ti^3＋ or Ti^2＋ cations, namely, the concentration of oxygen vacancies, increase with intensifying reducing atmosphere during processing, which shows that ferromagnetism in this material strongly depends on the concentration of oxygen vacancies. The relationships between the ferromagnetism and the crystal structure as well as the grain size in this material are also discussed.     

Defect types and room temperature ferromagnetism in N-doped rutile TiO2single crystals

The magnetic properties and defect types of virgin and N-doped TiO2 single crystals are probed by superconducting quantum interference device （SQUID）, X-ray photoelectron spectroscopy （XPS）, and positron annihilation analysis （PAS）. Upon N doping, a twofold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancy, rather to unpaired 3d electrons in Ti3＋, arising from titanium vacancies and the replacement of O with N atoms in the futile structure. The production of titanium vacancies can enhance the room temperature ferromagnetism （RTFM）, and substitution of O with N is the onset of ferromagnetism by inducing relatively strong ferromagnetic ordering.     

Field-induced insulator-metal-insulator transitions in low-energy H_2~+ ion-irradiated epitaxial La_(2/3)Ca_(1/3)MnO_3 thin films

Epitaxial La 2/3 Ca 1/3 MnO 3 thin films grown on LaAlO 3 (001) substrates were irradiated with low-energy 120-keV H 2 + ions over doses ranging from 10 12 ions/cm 2 to 10 17 ions/cm 2 . The irradiation suppresses the intrinsic insulator-metal (I-M) transition temperature and increases the resistance by reducing the crystallographic symmetry of the films. No irradiation-induced columnar defects were observed in any of the samples. The specific film irradiated at a critical dose around 8 × 10 15 ions/cm 2 is in a threshold state of the electric insulator where the I-M transition is absent. In an external field of 4 T or higher, the I-M transition is restored and thus an enormous magnetoresistance is observed, while a negative temperature coefficient resumes as the temperature is reduced further. Magnetic relaxation behavior is confirmed in this and other heavily irradiated samples. The results are interpreted in terms of the displacement of oxygen atoms provoked by ion irradiation and the resulting magnetic glassy state, which can be driven into a phase coexistence of metallic ferromagnetic droplets and the insulating glass matrix in a magnetic field.     

Defect-induced ferromagnetism in rutile TiO_2 : A first-principles study

Based on first-principles calculations, the electronic and magnetic properties of undoped and Li-doped rutile TiO2 have been studied. The results demonstrate that a cation vacancy can arouse ferromagnetism in TiO2 and the magnetic moment mainly comes from p orbitals of O atoms around the Ti vacancy. However, the Ti vacancy under normal conditions is very difficult to form due to its high formation energy. Our calculations indicate that Li-doped TiO2 can reduce the formation energy while keeping the magnetism. The large magnetization energy indicates that Li-doped TiO2 is a promising room-temperature diluted magnetic semiconductor.     

Microstructure and Mechanical Properties of Ti3SiC2 Irradiated by Carbon Ions

Thanks to its noteworthy mechanical properties, excellent damage tolerance and good thermal stability, the Ti3SiC2 ternary compound has attracted great concern and has been considered as a potential structural component material for the 4th generation of reactors （e.g., gas fast nuclear reactors） and future fusion reactors. The outstanding properties are due to the nanolameIlar structure which imparts characteristics of both metals and ceramics to this material In our work, Ti3SiC2 samples have been irradiated by C^＋ ions with a high fluence of 1.78 × 10^17 ions/cm^2 at a range of temperatures from 120℃～850℃. Subsequently, series of characterization techniques including synchrotron irradiation x-ray diffraction, scanning electron microscopy and nano-indentation are carried out to understand the changes of microstructure and mechanical properties. The composition exhibits high damage tolerant properties and a high recovery rate through the analysis, especially at high temperature. The minimum damage to an irradiated sample appears around 350℃ in the temperature range 120℃-550℃. At a high irradiation temperature, a significant reduction in the damage can be achieved and an almost complete lack of damage compared with an un-irradiated sample is revealed at the temperature of 850℃.     

First-Principles Study on Magnetic Properties of V-Doped ZnO Nanotubes

Electronic and magnetic properties of V-doped ZnO nanotubes in which one of Zn^2＋ ions is substituted by V^2＋ ions are studied by the first-principles calculations of plane wave ultra-soft pseudo-potential technology based on the spin-density function theory. The computational results reveal that spontaneous magnetization in Vdoped （9,0） ZnO nanotubes can be induced without p-type or n-type doping treatment, and the ferromagnetism is isotropic and independent of the chirality and diameter of the nanotubes. It is found that V-doped ZnO nanotubes have large magnetic moments and are ferromagnetic half-metal materials. Moreover, the ferromagnetic coupling among V atoms is generated by O 2p electron spins and V 3d electron spins localized at the exchanging interactions between magnetic transitional metal （TM） impurities. The appearance of ferromagnetism in V-doped ZnO nanotubes gives some reference to fabrication of a transparent ferromagnet which may have a great impact on industrial applications in magneto-optical devices.     

Structural and Magnetic Properties of Fe-Doped Anatase TiO2 Films Annealed in Vacuum

Structural and magnetic properties of Fe-doped anatase TiO2 films fabricated by sol-gel spin coating are investigated. X-ray diffraction measurements reveal that Fe^3＋ ions are incorporated into the TiO2 lattice. No ferromagnetism-related secondary phases and magnetic nanopaxticles are observed in the films. The presence of electron paramagnetic resonance signals at 9- 2.0 supports oxygen vacancies and/or defects generated in the films after annealing in vacuum. Magnetic measurements indicate that Fe-doped anatase TiO2 films are ferromagnetic at room temperature. These observations suggest that oxygen vacancies and/or defects axe energetically favorable for the long range Fe^3＋-Fe^3＋ ferromagnetic coupling in Fe-doped anatase TiO2 films.     

Multiferroic ZnO obtained by substituting oxygen with nitrogen

N-doped ZnO films were prepared in nitrogen plasma by pulsed laser deposition.Clear room temperature ferro-magnetism has been observed in the film prepared at a substrate temperature of 500 C.The structural characterizations of X-ray diffraction,Raman,and X-ray photoelectron spectroscopy confirm the substitution of O by N in ZnO,which has been considered to be the origin of the observed ferromagnetism.Furthermore,ferroelectricity has been observed at room temperature by piezoelectric force microscopy,indicating the potential multiferroic applications.     

Study on the optical properties of PPV／TiO2 nanocomposites

The optical properties of poly(Phenylenevinylene)(PPV)/TiO2 nanocomposites,prepared via the sol-gel process,were investigated by UV-VIS,FT-IR,FL spectroscopy and TEM.TEM images indicated that the surfaces of TiO2 nanoparticles became rough,and finally formed the connected network as the TiO2 content increased.FT-IR spectra showed the titanium butoxide hydrolyzed to from Ti organic compound,which can result in the connected network of TiO2 nanoarticles.The conjugation of PPV polymer chains can be interrupted by the TiO2 network structure,as evdenced in UV-VIS spectra.The investigation of the PL spectra revealed that the emitted light of the PPV/TiO2 nanocoposites blue shifted wihout fine structure,and the PL intensity enhanced,when the TiO2 net work fromed.These phenomena suggested that the optical properties of the PPV/TiO2 nanocomposites were dependent on the interfacial structure between PPV and TiO2 nanoparticles.2001 Elsevier Science B.V.All rights reserved.