Low temperature self-ion implantation into granular aluminium films

Radiation induced effects in glass-rods and fiberoptics have been studied to determine parameters affecting the application of these materials in nuclear technology, i.e. as fiberscopes for visual inspection in severe radiation environments. Different glass and fibertypes have been exposed to fission product gamma radiation. The radiation induced transmission loss was measured with a spectrophotometer and then different annealing methods were examined to improve the transmission properties again. Especially the changes in glass and fiber recovery vs. time as a function of radiation dose and annealing temperature were investigated. Annealing experiments were performed exposing the samples either to temperature treatment or to various light sources such as quartz lamp, arc lamp or UV-laser for optical annealing. The transmission recovery was then investigated either as a function of annealing temperature or of exposure time to the light sources. The results allow conclusions on the design and composition of optical fiber endoscopes to be used in severe radiation environment where image transmission is required in the presence of high level nuclear radiation.     

Magnetoresistance in granular films formed by CoFe and phase change material

Magnetoresistance having a field and current dependence like that of GMR (but a rather small magnitude) has been observed in Co₇₀Fe₃₀/GeTe and Co₇₀Fe₃₀/Ge₂Sb₂Te₅ granular films. Film stacks were fabricated using tandem (multilayer) deposition and annealing was required for the films to develop the GMR-type response. This GMR-type behavior is distinct from AMR, which is observed before annealing. With films having the structure [CoFe 4 nm/GeTe 6 nm]₁₀, a magnetoresistance (MR) of 0.19 %, which has the GMR-type character can be observed after an optimal annealing temperature of 450 °C. TEM and X-ray reflectometry suggest that columnar granules with layered Fe form during deposition and with annealing. Magnetoresistive transport is believed to be between the discontinuous layers in each columnar grain. These discontinuous layers are observed to be superparamagnetic in SQUID ZFC-FC measurements measured from 5–300 K. Magnetoresistance can be fitted by the quadratic relation appropriate for GMR in granular films, especially at higher GeTe compositions. When Ge₂Sb₂Te₅ is used instead of GeTe, higher anneal temperatures are required before the MR with GMR character appears. This GMR type response does not appear when pure Fe is used instead of the CoFe alloy, with the samples showing only AMR in this case. This is due to the absence of Co which seems to cause a more granular growth.     

Ag离子注入非晶SiO2的光学吸收、拉曼谱和透射电镜研究

能量为200keV的Ag离子,以1×1016,5×1016,1×1017cm-2的剂量分别注入到非晶SiO2玻璃,光学吸收谱显示:注入剂量为1×1016 cm-2的样品的光吸收谱为洛伦兹曲线,与Mie理论模拟的曲线形状一致;注入剂量较大的5×1016,1×1017 cm-2的谱线共振吸收增强,峰位红移并出现伴峰.透射电镜观察分析表明,注入剂量不同的样品中形成的纳米颗粒的大小、形状、分布都不同,注入剂量较大的还会产生明显的表面溅射效应,这些因素都会影响共振吸收的峰形、峰位和峰强.当注入剂量达到1×1017 cm-2时,Ag纳米颗粒内部可能还形成了杂质团簇.     

Random telegraph noise spectra in granular high-Tc films

Random telegraph noise voltages developing across current biased granular high-T_c films interact with the system of Abrikosov vortices pinned inside the grains through random Lorentz forces caused by the flow of random currents in the film. Viscous relaxation of vortices opposes random forces and leads to the attenuation of the low frequency part of the observed spectra of the voltage noise. The theoretical model has been created and discussed. The predictions of the model have been verified experimentally by observations of a peak in the random noise spectra recorded for granular YBaCuO thin film strips immersed in a weak magnetic field and biased with a dc current flow above the critical current. It has been found that the peak position and its changes with charging random telegraph switching rate are in a good agreement with the theoretical predictions.     

Giant Hall effect in (GaAs)Fe granular films

A series of (GaAs)_1 − xFe_x (x: volume fraction) films with Fe granules embedded in GaAs matrix were prepared by magnetron sputtering. Hall Effect of the films was characterized. The largest saturated Hall resistivity of was observed in (GaAs)₃₀Fe₇₀ film at room temperature, which is over 2 orders larger than that of pure Fe, about 1 order larger than that of (NiFe)–(Al₂O₃) and (NiCo)–(SiO₂) granular films prepared under the same preparation conditions, and 150% larger than that of Ge₃₀Fe₇₀.     

FMR studies in Al2+ implanted Fe60Al40 alloy

The Al²⁺ implanted layer (∼ 200 nm) in Fe₆₀Al₄₀ has been investigated by FMR. The magnetization is found to be 6400 G in the absence of anisotropy which agrees well with the value reported on cold worked Fe₆₀Al₄₀. Smaller line width for a sample implanted with the highest fluence of 1×10¹⁵ ion/cm² indicates better chemical and magnetic homogeneities. The g-factor is 2.06.     

Ag离子注入非晶SiO2的光学吸收、拉曼谱和透射电镜研究

能量为200keV的Ag离子，以1×10¹⁶，5×10¹⁶，1×10¹⁷ cm^-2的剂量分别注入到非晶SiO₂玻璃，光学吸收谱显示：注入剂量为1×10¹⁶ cm^-2的样品的光吸收谱为洛伦兹曲线，与Mie理论模拟的曲线形状一致；注入剂量较大的5×10¹⁶，1×10¹⁷ cm^-2的谱线共振吸收增强，峰位红移并出现伴峰. 透射电镜观察分析表明，注入剂量不同的样品中形成的纳米颗粒的大小、形状、分布都不同，注入剂量较大的还会产生明显的表面溅射效应，这些因素都会影响共振吸收的峰形、峰位和峰强. 当注入剂量达到1×10¹⁷ cm^-2时，Ag纳米颗粒内部可能还形成了杂质团簇. 关键词： 离子注入 纳米颗粒 共振吸收 红移     

Reversible photostructural change in chalcogenide glass films observed in infrared absorption spectra

Measurement of infrared absorption spectra in amorphous chalcogenide films revealed changes which can be closely related with the reversible photostructural change. The changes came out in intensity of absorption peak situated near 650 cm^-1 as well as definite change in infrared fundamental absorption peak. The absorption peak intensity near 650 cm^-1 showed considerable decrease by annealing, while it recovered initial value after light exposure. The change was shown to be in close correspondence with the well known reversible optical absorption edge shift.     

Optical property modification of PMMA by ion-beam implantation

Polymeric waveguides were fabricated by proton implantation on poly(methyl methacrylate) (PMMA). Depth profiles of the refractive indices of modified regions were obtained and were found to be in good agreement with the stopping power curve of protons in PMMA. It means that the waveguides are formed at the depths where the stopping power is the maximum value. Light losses for 635 nm wavelength were measured using planar waveguides to verify if the transmittance is enough for the application of the technique to optical devices.     

Properties of Al2O3: nc-Si nanostructures formed by implantation of silicon ions into sapphire and amorphous films of aluminum oxide

Physics of the Solid State - Photoluminescence, infrared Fourier spectroscopy, Raman scattering, transmission electron microscopy, and electron diffraction were used to study the luminescent,...     

Magnetic properties of Fe and Ni nanoparticles formed in triethoxysilane films by ion irradiation or thermal processing

Ion irradiation or heat treatments in vacuum of gel films prepared from mixtures of triethoxysilane with Fe and Ni nitrates permits to obtain a precipitation of metal particles in a glassy matrix, because these gels contain hydrido groups able to reduce Fe^{3 +} and Ni^{2 +} ions. The precipitation under irradiation is ascribed to the electronic excitations produced by the ions and the volume of metallic phase increases in proportion to the amount of energy transferred to electrons. The metal particles exhibit a narrower range of sizes than in films submitted to heat treatments in vacuum. Experiments of electron spin resonance indicate that the magnetic behavior of irradiated films is affected by a stress-induced anisotropy field. Films containing 3 to 7 at% Fe are in a superparamagnetic state in a given range of temperatures while the magnetic moments of nickel particles in films with similar metal contents exhibit a stronger correlation, due to a larger yield of precipitation and to the stress.Received: 4 December 2003, Published online: 2 April 2004PACS: 61.82.-d Radiation effects on specific materials - 75.70.-i Magnetic properties of thin films, surfaces, and interfaces - 76.50. + g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance - 75.50.Tt Fine-particle systems; nanocrystalline materials     

Particle size reduction and enhanced diffusion of Fe and Pt atoms in FePt-C granular films by N addition

N-doped FePt-C nanocomposite films were fabricated using facing-target sputtering method under different N₂ partial pressures (P_N) at room temperature. Annealing at 650 °C turns the amorphous films into ordered structures. Nitrogen doping not only make the ordering of FePt particles easier than the ordering in FePt-C films, due to the enhanced diffusivity of Fe and Pt atoms, but also effectively limits the growth of the FePt particles during the thermal induced ordering, especially for the annealed films fabricated at P_N = 40%, where the average size of well-isolated FePt particles is only ∼8 nm. The particle size reduction and the enhanced diffusion of Fe and Pt atoms can be ascribed to the desorption of doped N atoms and dissociation of FeN bonds during annealing. The room-temperature coercivity of the samples decreases with the P_N due to the particle size reduction and thus the enhancement of the thermal agitation for small particles during the magnetizing procedure.     

Anomalous Hall effect in granular Fe/SiO2 films in the tunneling-conduction regime

It is established that the Hall effect in Fe/SiO₂ nanocomposite films in the activational tunneling conduction range is anomalous, i.e., the Hall resistivity ρ_h is proportional to the magnetization and is due to the spin-orbit interaction. The parametric coupling of the Hall and longitudinal (ρ_xx) resistances ρ_h ∝ ρ _xx ^m (with temperature as the parameter) is characterized by a much lower value of the exponent m than in a uniform ferromagnetic metal. This circumstance is attributed to the characteristic features of the Hall effect mechanism in the hopping regime — in our case, the interference of the amplitudes of tunneling transitions in a set of three granules. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 87–92 (25 July 1999)     

Light emitting diode structure based on Si nanocrystals formed by implantation into thermal oxide

Light emitting diodes (LED), continuously operable at room temperature, have been fabricated by Si⁺ ion implantation into SiO₂ and subsequent annealing in order to form Si nanocrystals. A highly doped poly-Si layer was used to enhance injection into nanocrystals. Visible electroluminescence (EL) was observed from the LEDs with oxide thickness 180 Å for bias voltages above 8 V. The EL decay transient was similar to stretched-exponential decays observed for photoluminescence (PL) from Si nanocrystals.     

Mössbauer studies of granular Fe/Al2O3 films

Results from low temperature Mössbauer measurements upon several samples of dispersed small Fe particles in an alumina matrix are presented. The nature and population of the various iron species are deduced and discussed in relation to the particle mean diameters. An evaluation of the blocking temperature for one sample has been attempted.     

Optical waveguides in LiNbO3 formed by ion implantation

Abstract

Ion implantation in LiNbO₃ can be used to modify the refractive index. The change in indices, n ₀ n _e results from the damage formed by energy deposited by the (dE/dx)_nuclear collisions between the ions and the lattice and is independent of the ion species. A saturation change in index of some ?7% occurs after a deposition of 10²³ keV cm^?3 at 300K, greater changes of ?9 % occur with implants at 77K. Annealing studies indicate the optical absorption formed during irradiation is removed below 200°C whereas the index changes exist up to 400°C. For optical waveguide production a negative change in the index is not ideal as the damaged layer cannot directly act as the region of optical confinement. However the (dE/dx)_electronic term is unimportant and so we have been able to form optical waveguides by ion implantation with light energetic ions (e.g. MeV He⁺ ions) because the damaged layer is then formed beneath an unchanged high index surface layer. This retains the desirable electrooptic properties of the single crystal LiNbO₃. The experimentally observed waveguide modes are in accord with our theoretical predictions of the refractive index profiles.     

FMR investigation of magnetic anisotropies in Fe/Au superlattices

Fe(xML)/Au(xML) superlattices (1⩽x⩽4, ML: monatomic layer thickness) have been investigated by the ferromagnetic resonance method at room temperature. It has been confirmed that out-of-plane anisotropy field H_u shows oscillatory behavior as a function of Fe layer thickness with a period of 1 ML as reported previously from magnetization measurements. In addition, we have found that the in-plane fourfold anisotropy field H₂ oscillates in a similar manner. The easy magnetization axis for x⩾2.25 is Fe [1 1 0] in contrast with the case of bulk Fe, and the values of H₂ show maxima for x=2.5 and 3.5, suggesting that the atomic steps at interfaces are formed along the Fe [1 1 0] direction. Furthermore, the interface roughness for x=non-integer causes wide distributions of H_u and H₂ compared to those for x=integer with flat interfaces.     

Features of island nanostructures formed by melting Sn, Bi and Sn-Bi thin films on C substrates

We report the results of studying the morphological structure of island systems formed by melting Sn, Bi and eutectic Sn-Bi polycrystalline films 2-70 nm thick deposited on carbon substrates. Substrate coverage with a film is found to be non-uniform against film mass thickness after its melting and crystallization. The interval of film thickness is determined where the island systems with narrow enough particle size distribution are formed under melting.     

Conductivity, magnetoresistance, and the Hall effect in granular Fe/SiO2 films

We have investigated the conductance, magnetoresistance, and Hall effect in granular Fe/SiO₂ films with size of the iron grains around 40 Å, whose volume fraction x lies in the range 0.3–0.7. The conduction activation regime has been established for x<0.6. On the insulator side of the transition we observed a giant negative magnetoresistance, falling off sharply as the metal volume fraction decreases. For x<0.4 we observed a large positive magnetoresistance of premagnetized samples, showing up in fields; ～100 Oe and characterized by large response times. The field dependence of the Hall effect in the dielectric samples, as in the metallic samples, correlates with their magnetization. We found that the Hall resistance is proportional to the square root of the longitudinal resistance, which cannot be explained by known models of the anomalous Hall effect.