Defect-induced ferromagnetism in crystalline SrTiO3

In this work it is shown that ion irradiation of high-quality SrTiO₃ single crystals can lead to room-temperature ferromagnetism. Structural analysis revealed a polycrystalline SrTiO₃ layer, Sr₂Ti₆O₁₃, or Ruddlesden-Popper like secondary phase at the sample surface induced by the irradiation. The lack of any measurable ferromagnetic secondary phases suggests defects are the origin of the observed ferromagnetic signal.     

Characterization of superconducting YBaCuO-films by low temperature scanning electron microscopy

Low-temperature scanning electron microscopy has been performed for imaging the spatial distribution of the critical current densityj _c(x,y) and of the critical temperatureT _c(x,y) in polycrystalline superconducting YBaCuO films. Strongly inhomogeneous behavior has been observed, and the spatial resolution limit has been found to be 1–2 m. The local temperature increment in the specimen film caused by the electron beam scanning has been demonstrated experimentally as the underlying mechanism of the imaging principle, and the beam-induced thermal perturbation of the high-T _c film/substrate configuration is discussed in detail. The radiation hardness of the sample films against the electron beam irradiation in our imaging experiments has been evaluated. No radiation damage could be detected up to the maximum applied dose of well above 10²⁰ electrons/cm² for a typical beam energy of 26 keV.     

Radiation-dose dependence of E′1 centers in samples of quartz containing uncharged oxygen vacancies

EPR is used to study the generation of E′₁ centers (oxygen vacancies that have trapped one electron) in quartz samples containing uncharged oxygen vacancies as a function of irradiation dose. It is found experimentally that an irradiation dose of order 400 Gy is sufficient to allow every oxygen vacancy to trap two electrons apiece in essentially all such quartz samples. The linear segment of the dose dependences of E′₁ centers in samples annealed at 300 °C for 15 minutes can be used to reconstruct prior radiation doses up to 60–70 Gy. If the concentration of oxygen vacancies in the original sample is larger than 10¹⁸ cm⁻³, the signal intensity from E′₁ centers in the sample can be used to detect radiation doses as low as 1–3 Gy, which is significantly lower than the minimum radiation dose detectable by other paramagnetic centers in quartz. Fiz. Tverd. Tela (St. Petersburg) 40, 651–652 (April 1998)     

Quick X‐ray reflectivity using monochromatic synchrotron radiation for time‐resolved applications

A new technique for the parallel collection of X‐ray reflectivity (XRR) data, compatible with monochromatic synchrotron radiation and flat substrates, is described and applied to the in situ observation of thin‐film growth. The method employs a polycapillary X‐ray optic to produce a converging fan of radiation, incident onto a sample surface, and an area detector to simultaneously collect the XRR signal over an angular range matching that of the incident fan. Factors determining the range and instrumental resolution of the technique in reciprocal space, in addition to the signal‐to‐background ratio, are described in detail. This particular implementation records ～5° in 2gθ and resolves Kiessig fringes from samples with layer thicknesses ranging from 3 to 76 nm. The value of this approach is illustrated by showing in situ XRR data obtained with 100 ms time resolution during the growth of epitaxial La_0.7Sr_0.3MnO₃ on SrTiO₃ by pulsed laser deposition at the Cornell High Energy Synchrotron Source (CHESS). Compared with prior methods for parallel XRR data collection, this is the first method that is both sample‐independent and compatible with the highly collimated, monochromatic radiation typical of third‐generation synchrotron sources. Further, this technique can be readily adapted for use with laboratory‐based sources.     

Continuous X-ray induced Auger microprobe analysis and microscopy: First results

X-ray induced Auger electron spectroscopy can be applied to the chemical analysis of thin samples with a spatial resolution in the 10 μm range. The first experimental results obtained with various anode materials and various samples are reported. The signal intensities (in the range of 10³ counts/s) are similar to those obtained by using a conventional electron spectrometer with a resolution in the millimeter range. Theoretical considerations together with the results obtained make it possible to evaluate the sensitivity of this technique and suggest how the spatial resolution can be improved further (20 μm at present, 2–3 μm in the near future). The radiation damage and sample thickness constraints of this new technique and of conventional Auger electron spectros- copy are compared. Application of X-ray induced Auger analysis to biological objects is suggested and the effect of the present results on scanning X-ray radiography and characteristic X-ray absorption microanalysis is also pointed out.     

Atomic Structure and Properties of the (310) Symmetrical Tilt Grain Boundary (STGB) in SrTiO3 Part II: Comparison with Experimental Studies

The atomistic simulation results presented in Part I for SrTiO₃ (310) symmetrical tilt grain boundary (STGB, the so-called = 5 GB with 36.8° symmetrical misorientation about [001]) are analyzed in the context of available experimental studies. In particular, atomic imaging studies of SrTiO₃ GBs via high resolution TEM and incoherent Z-contrast STEM imaging; and determination of oxygen positions by combining electron energy loss spectroscopy (EELS) and bond-valence-sum rules, are compared with simulation results. The atomistic simulation data on the GB energies are compared with relative experimental estimates obtained via a novel approach of faceting of focused ion beam (FIB) induced microvoids.While there are considerable differences in details of simulation and experimental results, some basic trends seem to emerge about the core structural framework of GBs in SrTiO₃. The paper highlights the limitations of both, experimental and simulation techniques, and argues in favor of synergistic use of diverse experimental and simulation approach to determine the atomic structure and properties of GBs.     

Damage behavior and atomic migration in MgAl2O4 under an 80 keV scanning focused probe in a STEM

With the dramatic improvement in the spatial resolution of scanning transmission electron microscopes over the past few decades, the tolerance of a specimen to the high-energy electron beam becomes the limiting factor for the quality of images and spectra obtained. Therefore, a deep understanding of the beam irradiation processes is crucial to extend the applications of electron microscopy. In this paper, we report the structural evolution of a selected oxide, MgAl₂O₄, under an 80 keV focused electron probe so that the beam irradiation process is not dominated by the knock-on mechanism. The formation of peroxyl bonds and the assisted atomic migration were studied using imaging and electron energy-loss spectroscopic techniques.     

Study of the critical current in YBaCuO films under reactor-neutron irradiation in the low-temperature helium circuit in the 25–300 K range

Changes in the temperature and dose dependences of I-V characteristics and of the critical current j _c of YBaCuO films on MgO and SrTiO₃ substrates under neutron irradiation has been studied at 25–80 K. The transport properties of YBaCuO films on MgO (M1) and SrTiO₃ (S1) substrates were found to behave differently. It is shown that the M1 films have granular structure, and their transport properties were considered within the concept of percolation over weak intergranular links. The S1 are single-crystal films, and their properties are analyzed within a resistive-state theory associated with thermal activation of Abrikosov vortices. It is shown that the degradation rate of the critical temperature T _c of S1 films is 3.5 times smaller than that of the M1 films. The dose dependence of j _c has an exponential character, j _c=j _c(0)exp(−kΦ), where k is related to the number of displaced atoms per neutron and is the same for the M1 and S1 films, irrespective of the irradiation temperature. The pinning energy has been derived from the I-V characteristics and it has been found that the U/kT ratio lies within 20–25 and is independent of neutron flux. It is shown that the radiation-induced disorder changes the pinning mechanism, from pinning at the boundaries of misoriented crystals to that at spatial inhomogeneities, apparently radiation induced. Fiz. Tverd. Tela (St. Petersburg) 40, 1961–1967 (November 1998)     

Charge distribution in surface layers of polarized electret ceramics according to electron spectroscopy measurements

The anomalous electron emission (AEE) from the negative surface of a polarized SrTiO₃ ceramic sample is studied both experimentally and theoretically. The form of the low-energy edge and the fine structure of AEE spectra are interpreted within the “brick wall” model. The distribution of electric potential over the charged surface is calculated using Green’s functions under the assumption of p-type conduction of intergrain boundaries. It is demonstrated that the AEE spectra of SrTiO₃ ceramic surface mainly reflect the charge distribution in the polarized electret SrTiO₃ sample.     

Electron-induced Ti-rich surface segregation on SrTiO3 nanoparticles

Atomic surface structures of nanoparticles are of interest in catalysis and other fields. Aberration-corrected HREM facilitates direct imaging of the surfaces of nanoparticles. A remaining concern of surface imaging arises from beam damage. It is important to identify the intrinsic surface structures and the ones created by electron beam irradiation in TEM. In this study, we performed aberration-corrected HREM and EELS to demonstrate that TiO and bcc type Ti islands form due to intense electron irradiation. The formation of Ti-rich islands is in agreement with previous high temperature annealing experiments on the surfaces of SrTiO₃ single crystals.     

Effects of sputtering pressure on the field emission properties of N-doped SrTiO3 thin films coated on Si tip arrays

The influence of sputtering pressure on the electron emission properties of Si tips coated with N-doped SrTiO₃ ultrathin films was investigated. X-ray diffraction studies revealed that the N-doped SrTiO₃ films deposited at different pressures remain the perovskite structure. However, the threshold electric field of electron emission decreased markedly when the sputtering pressure is increased, and reached a minimum value of 17.37 V/μm while deposited at 1.6 Pa. The decrease in the threshold field is attributed to the narrowed band gap and the lowered surface energy of SrTiO₃ thin films with nitrogen doped, as confirmed using spectroscopic ellipsometry and water contact angle measurement. Furthermore, it is revealed using XPS that such sputtering pressure dependence is accompanied with the change of nitrogen bonding state in the films, which changes from poorly screened γ-N₂ state to atomic β-N state when the sputtering pressure is increased. A mechanism of bonding and band-forming was proposed for the enhanced electron emission with nitrogen incorporation in the sputtered SrTiO₃ films.     

碲镉汞辐射损伤的微观过程

利用高分辨率的电子显微技术,在原子和晶格尺度上,研究了碲镉汞晶体结构在大剂量高能电子作用下,晶体结构产生损伤、扩展和重构的过程。研究结果表明,碲镉汞结构损伤是在强电子束作用下,[(Hg,Cd)Te₄]四面体的形变,在{111}面产生并沿{111}面呈阶梯形扩展的过程。从而在[110]方向观察到了新的更大周期的两维重构网格。 关键词：     

Measurements of gas desorption from polyethylene-UHMWPE irradiated by 5 MeV electrons

Gas desorption in vacuum from electron irradiated ultra high molecular weight polyethylene (PE) is measured with a high sensible mass quadrupole spectrometer. Measurements are performed in thick PE irradiated with 5 MeV electron beams at doses of the order of tens of kGy. The irradiation modifies the PE molecules producing dehydrogenation, emission of different C _x –H _y groups, C-enrichment and carbon cross-linking processes. Results indicate that the radiation damage depends on the dose and that a significant change of chemical and physical polymer properties is reached for a critical dose of 18 kGy.     

Influence of radiation damage on lattice vibration parameters in crystalline and vitreous SiO2

The influence of radiation damage on a number of lattice vibration parameters in two SiO₂ modifications was investigated using IR reflectance spectroscopy. The radiation kinetics of changes in spectral characteristics of SiO₂ fundamental vibrations in crystalline and glassy states were determined. The reflectivity coefficient and the frequency of degenerate vibrations as functions of dose showed minima, the locations of which were governed by the type of sample. At high neutron irradiation doses (10²¹ cm⁻²), certain characteristics of the bands had the same values for both modified materials. Features of the radiation kinetics of sample dynamic parameters were determined. It was deduced that the specific character of the observed radiation-induced changes in spectral and dynamic parameters of vibrations near degenerate modes was due to both the accumulation of radiation damage and a change in the force field surrounding bridging bonds, which is related to a change in the SiO₂ structure. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 354–358, May–June, 2008.     

Charge ordering modulations in Bi0.4Ca0.6MnO3 film with a thickness of 110 nm

Low temperature sample stage in transmission electron microscope is used to investigate the charge ordering behaviours in Bi_0.4Ca_0.6MnO₃ film with a thickness of 110 nm at 103 K. Six different types of superlattice structures are observed using selected-area electron diffraction (SAED) technique, while three of them match well with the modulation stripes in high-resolution transmission electron microscopy (HRTEM) images. It is found that the modulation periodicity and direction are completely different in the region close to the Bi_0.4Ca_0.6MnO₃/SrTiO₃ interface from those in the region a little far from the Bi_0.4Ca_0.6MnO₃/SrTiO₃ interface, and the possible reasons are discussed. Based on the experimental results, structural models are proposed for these localized modulated structures.     

A study of the charging and dissociation of SiO2 Surfaces by AES

AES is used to determine the initial spectrum of a vacuum-broken SiO₂ surface and to follow its dissociation under the electron beam probe. Both Auger peaks heights and energies are affected by the irradiation. The change in stoichiometry is accompanied by a decrease of the surface charge by 5–8 V. The relation between stoichiometry and charge is explained by the influence of radiation-induced defects on secondary electron emission. The reduction of SiO₂ is characterized in terms of irradiation dose, dissociation cross-section and electron impact efficiency. Resistance to radiation damage is increased by surface carbon contamination. The chemical contribution to the Auger peak energy can be distinguished from the charging effect leading to a shift between element and compound of 12 eV for the silicon peak.     

PNP输入双极运算放大器在不同辐射环境下的辐射效应和退火特性

研究了PNP输入双极运算放大器LM837在1 MeV电子和⁶⁰Coγ源两种不同辐射环境中的响应特性和变化规律. 分析了不同偏置状态下其电离辐照敏感参数在辐照后三种温度 (室温, 100 ℃, 125 ℃)下随时间变化的关系, 讨论了引起电参数失效的机理. 结果表明: 1 MeV 电子辐照LM837引起的损伤主要是电离损伤, 并且在正偏情况下比⁶⁰Coγ源辐照造成的损伤大; 辐照过程中, 不同辐照源正偏条件下的偏置电流变化都比零偏时微大; 在不同的辐照源下, LM837辐照后的退火行为都与温度有较大的依赖关系, 而这种关系与辐照感生的界面态密度增长直接相关. 关键词： PNP 输入双极运算放大器 60Coγ源')" href="#">电子和⁶⁰Coγ源 偏置条件 退火     

Investigation of metabolic changes in blood and tissue of mice γ-irradiated with sublethal doses by direct observation of EPR signals from Hb-NO complexes

The metabolic changes in probes of blood and tissue (spleen, liver and kidney) of mice under total γ-irradiation with the doses varied in the interval of 1–10 Gy at the dose rate of 0.073 Gy/min were studied in the early postirradiation period by ex vivo electron paramagnetic resonance (EPR). It was established that the impact with the lower dose rate leads to more intensive nitric monoxide biosynthesis in comparison with higher dose rates. In the early postirradiation period (from 2 up to 6 h), irradiation with doses higher than 2 Gy brings about an increase of the NO concentration and, hence, the appearance of nitrosyl complexes which were registered directly by EPR in blood and spleen. The observed line is identified as the signal from α-(Fe²⁺-NO)₂β(Fe³⁺)₂ or α-(Fe²⁺-NO) α(Fe²⁺)β(Fe³⁺)₂ complexes since the methemoglobin concentration also increases in comparison with the control level. The concentration of Hb-NO complexes in blood and spleen depends on the dose and individual radiosensitivity of the organism. Therefore, the intensity of the Hb-NO signal may serve as a criterion of the radiation injury level during the first hours after the irradiation. 30 h after the impact, the Hb-NO complexes were no longer detected. For the first day, the concentration of Fe³⁺-transferrin in blood increases with the dose and time passed after the irradiation. The intensity of the EPR signal from Fe³⁺-transferrin in blood may also serve as a measure of the radiation injury level.     

Chiral surfaces and metal/ceramic heteroepitaxy in the Pt/SrTiO3(621) system

Low-energy electron diffraction (LEED), atomic force microscopy (AFM), and X-ray diffraction (XRD) have been used to investigate the structural and morphological character of a naturally chiral ceramic SrTiO₃(621) substrate and of Pt and Cu thin films deposited on its surface. AFM experiments showed that as-received chirally-oriented SrTiO₃(621) substrates display atomically smooth surface morphologies, while LEED patterns revealed that the surface structure has a net chirality. Pt(621) and Cu(621) thin films were grown heteroepitaxially on SrTiO₃(621) substrates, as confirmed by XRD. AFM showed that the film surfaces were atomically smooth and LEED illustrated that the Pt films exhibit surface chirality, and by implication that the atomically-flat chirally-oriented Cu films also have chiral surfaces. The characteristics of the observed LEED patterns, where splitting of diffraction spots is considered to arise from the kinked step features of naturally chiral fcc metal surfaces, are discussed with respect to existing models. These results indicate that the chiral SrTiO₃(621) ceramic surface drives the growth of single-enantiomer, chiral, metal (621) thin films.