Irradiated Multilayers Research Articles

Size-dependent irradiation tolerance and mechanical properties in WTaTiVCr/W multilayered films

Increasing the interface density in nuclear-material systems has been proposed to be a promising strategy for annihilating defects introduced by ion irradiation. The role of interface characteristics between the WTaTiVCr refractory high entropy alloy (RHEA) and metal tungsten (W) on the irradiation resistance is rather unclear because of the distinct defect energy that exists in these two kinds of materials. Here, the nano-crystalline and amorphous mixed structures were obtained in WTaTiVCr/W multilayers to investigate the irradiation tolerance and mechanical properties. After being exposed to a 60 keV helium (He) ion irradiation platform at a fluence of 1 × 1017 cm−2, the grain sizes in irradiated multilayers were slightly decreased from 12.4 nm to 7.2 nm with increasing individual thickness, which was explained from the perspective of He+ energy deposition. The dependence of bubble growth in multilayers on the individual thickness was disclosed based on the analysis of bubble size and its pressure in the RHEA and W layers. The phase structures and element distributions in WTaTiVCr/W multilayers were also revealed by the defect formation and crystallization. Also, a maximum irradiation hardening of 1.91 GPa in the RHEA(10)/W(7) multilayer was disclosed, and the irradiation hardening in the multilayers exhibited a decreasing trend with increasing monolayer thickness, and the corresponding mechanisms about the hardening behaviors in these multilayers were also discussed based on the interface density and irradiation-induced defects.

Effect of periodic thickness on the helium bubble evolution and irradiation hardening in Cu/W(Re) multi-layered films under helium ion irradiation

Based on the basic understanding of prominent helium (He) bubble storage and defect annihilation in the dual-phase heterogeneous interfaces designed in nano-meter materials, the Cu/W(5.31 at.% Re) multilayered films with nominal periodic thickness (h) ranging from 2 nm to 50 nm were irradiated by 60 keV He+ at irradiation temperature of 725 K. At the fluence of 2×1017 cm-2, the lamellated interfaces in the Cu/W(Re) multilayered film of h = 2 nm were broken down due to the irradiation-induced intermixing between the Cu-W(Re) neighbouring interfaces, resulting in a stable distribution of bubble in grains or at grain boundaries. However, at He concentration peak region, the bubble clusters inside the Cu layers and the blurred interfaces were formed in the Cu/W(Re) multilayered film of h = 10 nm. The W phase structure was changed by the aggregated bubbles in lattices, and the Cu layers were damaged due to the distribution scope of aggregated bubble-cluster approximately increased to a critical periodic thickness (10 nm). The lamellated interfaces still kept stable at low He concentration region. The calculated bubble pressure and He density inside bubbles can effectively explain the lattice distortion induced partial phase transformation from α-W to γ-W in the W(Re) layers. Moreover, the effects of interfaces, He bubbles and dislocations on the increase of nano-hardness in the irradiated multilayers were discussed in detail.

Experimental Study of Irradiation of Thin Oxide and Mo/Si Multilayers by High Brightness Broadband VUV/UV Radiation and Their Degradation

This study discusses the main features of the irradiation of prospective multilayer coatings by VUV/UV radiation from compressed plasma flows. Such radiation is characterized by a broadband spectrum and high brightness fluxes. Oxide and Mo/Si bilayers were used as the basis of the reflective multilayers for the visible and UV ranges. A gas-dynamic response from the irradiated surfaces was studied with schlieren photography. The properties of original and irradiated multilayers were described with ultra violet visible infrared spectroscopy (UV-Vis-IR), X-ray diffraction (XRD), X-ray reflectometry, scanning electron microscopy (SEM) and other techniques. Data on the degradation of optical properties and surface morphology were obtained.

Suppression of all-optical switching in He+ -irradiated Co/Pt multilayers: influence of the domain-wall energy

The observation of all-optical helicity-dependent switching (AO-HDS) in ferri- and ferro-magnetic materials depends not only on the saturation magnetization, but on the magnetic domain size which takes into account the competition between the demagnetizing energy and the domain wall (DW) energy. In this paper, we study the optical response of Co/Pt multilayers for which perpendicular magnetic anisotropy can be tuned using He+ -irradiation. This allows us to correlate the observation of AO-HDS with the magnetic properties of the He+ -irradiated multilayer. These results give direct evidence of the crucial role of the DW energy in the laser-induced switching process.

Comparisons of radiation damage in He ion and proton irradiated immiscible Ag/Ni nanolayers

Abstract We compare the evolution of microstructure and mechanical properties of Ag/Ni multilayers of varying layer thickness (1–200 nm) subjected to helium ion and proton irradiation at room temperature to similar dose, ∼2 displacements-per-atom on average. Layer structure remained intact after both types of irradiation although defects accumulated in the layers are different. Helium bubbles were the major defects in helium ion irradiated films, while dislocation loops were ubiquitous in proton irradiated multilayers. In He ion irradiated multilayers, radiation hardening was greater than that in proton irradiated specimens, and the magnitude of hardening decreased with decreasing individual layer thickness. In comparison no size dependent hardening was observed in proton irradiated films. Mechanisms of irradiation induced hardening were discussed.

Conditions for formation of germanium quantum dots in amorphous matrices by MeV ions: Comparison with standard thermal annealing

We investigate how MeV ions with different ion-beam parameters (ion type, electronic stopping power, and velocity) influence the formation, arrangement, and ordering quality in three types of ($\mathrm{Ge}+{\mathrm{SiO}}_{2})/{\mathrm{SiO}}_{2}$ multilayer films. The multilayers differ in total thickness, Ge-rich layer thickness, and Ge content. The results show that the most important parameter for structural manipulation with MeV ions is the electronic stopping power ${S}_{e}$. Ion velocity is found to be another crucial parameter, while at the same time it can be seen that the multilayer type does not play an important role. The temperature increase within the ion tracks is estimated using the thermal spike model and cluster separation distribution. The structural changes produced by ion beams and estimated temperatures are compared to those obtained by standard thermal annealing. It is concluded that the estimated temperatures within the ion tracks are in excellent agreement with the annealing temperatures and with the structural changes observed in the irradiated multilayers. Furthermore, a characteristic parameter of the temperature profile that presents the model-predicted temperature increase is determined for which the structural changes caused by ion beams are comparable to those achieved by standard annealing.

Interface-enabled defect reduction in He ion irradiated metallic multilayers

Metallic multilayers are good model systems to explore the effects of heterophase interfaces in reducing radiation damage in structural materials. We summarize recent studies on radiation damage in immiscible face-centered cubic/body-centered cubic metallic multilayers, in particular Cu/V and Cu/Nb. These multilayers have shown unique characteristics compared to bulk metals under irradiation, including several orders of magnitude higher He solid solubility, dramatic reduction of bubble density, interface confined growth of He bubbles, and much lower radiation hardening. The mechanisms for interface enhanced radiation tolerance are briefly discussed.

Generation of an ordered Ge quantum dot array in an amorphous silica matrix by ion beam irradiation: Modeling and structural characterization

We studied the generation of an ordered Ge quantum dot array in an amorphous silica matrix by ion beam irradiation. In particular we investigated the influence of the irradiation process on the nucleation of Ge clusters, on the correlations in their positions and on the crystalline quality of Ge quantum dots formed after subsequent annealing. We have developed a method for the description of the intensity of grazing-incidence x-ray small-angle scattering from irradiated multilayers, which enables a precise determination of the arrangement of quantum dots as well as their position correlation and size distribution. The analysis shows that the irradiation causes an ordering of Ge clusters along the irradiation direction, which substantially improves the correlations of the Ge dot locations in their three-dimensional array. The observed phenomena are explained and simulated by a Monte Carlo model based on the modification of local Ge density induced by ion tracks in the irradiated multilayers.

He ion irradiation damage in Al∕Nb multilayers

We investigate the evolution of microstructure and mechanical properties of sputter-deposited Al∕Nb multilayers with miscible fcc/bcc type interface and individual layer thickness, h, of 1–200nm, subjected to helium ion irradiations: 100keV He+ ions and a fluence of 6×1016∕cm2. Helium bubbles, 1–2nm in diameter, are observed. When h is greater than 25nm, hardnesses of irradiated multilayers barely change, whereas radiation hardening is more significant at smaller h. Transmission electron microscopy and scanning transmission electron microscopy studies reveal the formation of a thin layer of Nb3Al intermetallic phase along the Al∕Nb interface as a consequence of radiation induced intermixing. The dependence of radiation hardening on h is interpreted by using a composite model considering the formation of the hard Nb3Al intermetallic layer.

Atom probe tomography of swift ion irradiated multilayers

Nanometer scale layered systems are well suited to investigate atomic transport processes induced by high-energy electronic excitations in materials, through the characterization of the interface transformation. In this study, we used the atom probe technique to determine the distribution of the different elements in a (amorphous- Fe 2 Tb 5 nm / hcp - Co 3 nm ) 20 multilayer before and after irradiation with Pb ions in the electronic stopping power regime. Atom probe tomography is based on reconstruction of a small volume of a sharp tip evaporated by field effect. It has unique capabilities to characterize internal interfaces and layer chemistry with sub-nanometer scale resolution in three dimensions. Depth composition profiles and 3D element mapping have been determined, evidencing for asymmetric interfaces in the as-deposited sample, and very efficient Fe–Co intermixing after irradiation at the fluence 7 × 10 12 ion cm - 2 . Estimation of effective atomic diffusion coefficients after irradiation suggests that mixing results from interdiffusion in a molten track across the interface in agreement with the thermal spike model.

Magnetostrictive properties of Kr-ion irradiated multilayers

The influence of irradiation with high energy Kr ions has been experimentally studied on the magnetostrictive properties of TbFe/Co multilayers. Enhancement of magnetostriction at saturation can be obtained at an optimal fluence with a lower saturation field than for the non-irradiated multilayer. For high fluences, these properties are degraded.

Intermixing at interfaces of KrF laser irradiated Co/W multilayers

The interface reactions in Co/W multilayers (ML) with Co thickness of 1 and 2 nm and W thickness of 1, 2 and 5 nm in as-deposited state and under KrF laser irradiation with fluences F=50–200 mJ/cm 2 were studied by different X-ray methods. Superlattice satellites around W (1 1 0) direction were found in as-deposited and irradiated multilayers. The layered structure persists up to the highest applied fluence. Co 3W compound is formed at the interfaces. However, the intermixing is low.

Modification of Co/Pt multilayers by gallium irradiation—Part 1: The effect on structural and magnetic properties

Atomic force microscopy, transmission electron microscopy, optical, and magneto-optical microscopy have been used to study how structural and magnetic properties are changed when a Co/Pt multilayer is quasihomogeneously irradiated with Ga ions. Under low irradiation fluence, both grain size and texture in the multilayer increase. These effects continue for fluences in excess of 1×1015 Ga ions/cm2, but beyond this dose significant thinning of the multilayer is also observed. Three distinct irradiation-induced magnetic regimes with sharp transitions between each were identified. For Ga fluences less than 5×1012 ions/cm2, the irradiated region retains perpendicular uniaxial anisotropy but with coercivity lower than that of the as-grown film. For fluences between 5×1012 and 1×1013 Ga ions/cm2, a transition from perpendicular to in-plane magnetization was experienced. Very little change of the in-plane magnetic properties of irradiated multilayers is then observed until the sample experiences a ferromagnetic to paramagnetic transition at fluences around 1×1015 Ga ions/cm2. A brief comparison with the effect of irradiating with He ions is given.

Effect of swift heavy ion-irradiation on Cr/Fe/Ni multilayers

A multilayer film having overall composition Fe 50Cr 25Ni 25, was irradiated successively by 80 MeV Si ions and Ag ions of 150 and 200 MeV energy. The energy deposited in the multilayer in the form of electronic excitations results in significant modification at the interfaces. The interfacial roughness increases in the system after the irradiations as revealed by X-ray reflectivity measurement. Mössbauer measurements provide evidence of intermixing after the irradiation by 200 MeV Ag ions. Comparison of heavy ion irradiated multilayer has been done with annealed and low energy ion irradiated samples. Results suggest that the phases formed at the interfaces of iron as a result of electronic energy loss are similar to those in the cases of thermal diffusion and keV energy ion beam irradiation.

Initial stages of the ion beam mixing process

The grazing x-ray reflectometry technique, performed on irradiated periodic multilayers, was used to study the early stages of the ion beam mixing process. We present our first results, obtained on NiAu samples irradiated with He ions. The experimental fluence dependence of the effective diffusion coefficient is in good agreement with a calculation based on a purely ballistic process.

Electrical Response of Irradiated Multilayer Structures

A modelization for the calculation of the electrical response of irradiated multilayer structures is described. Almost all the general experimentally observed behaviours are reproduced with an acceptable accuracy. The code is simple enough to be used for a large number of practical situations.

Charge Deposition, Photoconduction, and Replacement Current in Irradiated Multilayer Structures

The electronic effects on an irradiated one dimensional dielectric-metal slab structure have been modeled. The effects considered include: radiation driven currents, charge deposition, applied electric fields, deposited charge fields, field driven conductivity currents and radiation induced photoconductivity. The time and spatial dependence of the electric current density, charge density and electric field are determined. The time dependence of the replacement current flowing between metal slabs is calculated. Results are given for x-rays of energies from 30 to 1250 keV. Interesting results predict electric fields > 106 V/cm, time varying replacement currents which even reverse sign, and multipole electric fields in one dimension. Experimental measurements produce time dependent replacement currents under 60Co irradiation.

Transition Radiation from an Irradiated Multilayer

A general formula for the transition radiation emitted by a uniformly moving charged particle normally incident on a multilayer, either self-supporting or deposited on a thick substrate, is calculated. This formula enables one to take into account the exact multilayer configuration of targets, e.g., oxide layers in the analysis of transition radiation data.