Thermally activated decomposition of (Ga,Mn)As thin layer at medium temperature post growth annealing

The redistribution of Mn atoms in Ga1-xMnxAs layer during medium-temperature annealing, 250-450 oC, by Mn K-edge X-ray absorption fine structure (XAFS) recorded at ALBA facility, was studied. For this purpose Ga1-xMnxAs thin layer with x=0.01 was grown on AlAs buffer layer deposited on GaAs(100) substrate by molecular beam epitaxy (MBE) followed by annealing. The examined layer was detached from the substrate using a “lift-off” procedure in order to eliminate elastic scattering in XAFS spectra. Fourier transform analysis of experimentally obtained EXAFS spectra allowed to propose a model which describes a redistribution/diffusion of Mn atoms in the host matrix. Theoretical XANES spectra, simulated using multiple scattering formalism (FEFF code) with the support of density functional theory (WIEN2k code), qualitatively describe the features observed in the experimental fine structure

Experimental study of EUV mirror radiation damage resistance under long term free electron laser exposures below the single shot damage threshold

The durability of grazing and normal incidence optical coatings has been experimentally assessed under free electron laser irradiation at various numbers of pulses up to 16 million shots and various fluence levels below 10 of the single shot damage threshold. The experiment was performed at FLASH, the Free electron LASer in Hamburg, using 13.5 nm extreme UV EUV radiation with 100 fs pulse duration. Polycrystalline ruthenium and amorphous carbon 50 nm thin films on silicon substrates were tested at total external reflection angles of 20 and 10 grazing incidence, respectively. Mo Si periodical multilayer structures were tested in the Bragg reflection condition at 16 off normal angle of incidence. The exposed areas were analysed post mortem using differential contrast visible light microscopy, EUV reflectivity mapping and scanning X ray photoelectron spectroscopy. The analysis revealed that Ru and Mo Si coatings exposed to the highest dose and fluence level show a few per cent drop in their EUV reflectivity, which is explained by EUV induced oxidation of the surfac

Enhancement of Flux Pinning in the Superconductor-Ferromagnet Bilayer Nb(Co/Pd)

The magnetometry and the magnetic force microscopy are used to study the influence of the magnetic domain size on the flux pinning in a superconducting/ferromagnetic bilayer (SFB), in which the S layer is niobium and the F layer is a Co/Pd multilayer with perpendicular magnetic anisotropy. The domain size is pre-defined using the angle-dependent demagnetization. The enhancement of pinning is found to be the strongest, up to a factor of 6, for narrow domains and small magnetic fields. This result differs from the behavior observed in the SFB in which the F layer is Co/Pt. The difference may be attributed to the degree of the disorder in the domain pattern

Phase Diagram and Activation Energy for Vortex Pinning in Nb/(Co,Pd) Superconductor-Ferromagnet Bilayer

Using the magnetoresistance measurements we study the phase transition line and the activation energy for vortex pinning in superconductor/ferromagnet bilayer, built of a ferromagnetic Co/Pd multilayer with perpendicular magnetic anisotropy, and a niobium film, with insulating layer in-between to eliminate proximity effect. The domain width is reversibly pre-defined using the angle-dependent demagnetization. We find that the enhancement of the activation energy for vortex pinning by magnetic domains is rather modest, by a factor of about 2.1. We attribute this to large domain width, and large dispersion of the domain width in this bilayer

Polarized dependence of soft X-ray absorption near edge structure of ZnO films implanted by Yb

Virgin and Yb-implanted epitaxial ZnO films grown using atomic layer deposition (ALD) were investigated by X-ray absorption spectroscopy (XAS). XAS study revealed a strong polarization dependence of films determined by the orientation of the polarization vector of the synchrotron radiation to the sample surface. It also indicated that the implantation and subsequent annealing have an important influence on the native point defect complexes in the ZnO. Comparison of experimental spectra with the modelled ones, which are computed based on the linear combination of model spectra corresponding to the selected point defects and their complexes, confirmed the presence of donor-acceptor complexes (mVZn - nVO, m = 1,4; n = 1,2) in the samples under study. The mechanism of vacancy complexes formation is unclear as it takes place under non-equilibrium conditions, for which any theoretical method has not been well established. Exploring the 3 d → 4 f absorption, it was found that oxidation state of Yb in ZnO is 3+, which is consistent with the XPS findings and previously conducted Resonant Photoemission Spectroscopy (RPES) investigations. The inversion of the polarization dependence for samples with different Yb fluences visible in Yb M5 spectra can be associated with a tilt of the oxygen pseudo octahedra or/and with their distortion. The analysis of the presented data suggests that the donor-acceptor complexes are present both in as grown and implanted films and may influence their electrical properties. This suggestion was confirmed by previous Hall measurements showing that the resistivity of annealed ZnO:Yb film with a fluence of 5e15 ions/cm2 decreases by about one order compared to the one with a fluence of 5e14 ions/cm2

Magnetic Nature of a Ni Dopant in La1.85Sr0.15CuO4La_{1.85}Sr_{0.15}CuO_4: Spin-Glass Behavior

The magnetic properties of $La_{1.85}Sr_{0.15}CuO_4$ doped with Ni was investigated in the field up to 5 T and in the temperature range from 2 K to 400 K using both dc and ac techniques. For Ni content larger than 0.05 the system exhibits irreversibility of low-field susceptibility χ(T) below a certain temperature depending on y and a cusp at $T_{g}$ in χ(T) measured after zero-field cooling. The decay of remnant magnetization below $T_{g}$ with time is described by a stretched-exponential function. In accordance with scaling theory, all the χ(T) data for y = 0.50 sample taken in the vicinity of $T_{g}$ at different fields collapse onto two separate curves when plotted as $q|t|^{-β}$ vs. B$^2 |t|^{-β - γ}$, where q is the spin-glass order parameter, t = (T - $T_{g}$)/$T_{g}$, and β and γ are the critical exponents. All these features taken together reveal existence of spin-glass phase below $T_{g}$. Variation of $T_{g}$ with y is linear below y = 0.25 and $T_{g}$ extrapolates to 0 K for y → 0 what strongly suggests that spin-glass phase extends into superconducting region of the phase diagram

Optimization of the Superconducting Properties of Laser Ablated YBa2Cu3O7−δYBa_2Cu_3O_{7-δ} Films on CeO2CeO_2-Buffered Sapphire

We use pulsed laser deposition to grow $YBa_2Cu_3O_{7-δ}$ (YBCO) superconducting films for microwave applications. The films are grown on R-cut sapphire substrates, with $CeO_2$ buffer layers, which are re-crystallized at high temperature prior to YBCO growth. Using the atomic force microscopy (AFM) and X-ray diffractometry we determine the optimal temperature for recrystallization (1000°C) and the optimal buffer layer thickness (30 nm). The properties of YBCO films of various thickness, grown on the optimized $CeO_2$ buffer layers, are studied using several methods, including AFM, magnetooptical imaging, and transport experiments. The YBCO film roughness is found to increase with the increasing film thickness, but the magnetic flux penetration in the superconducting state remains homogeneous. The superconducting parameters (the critical temperature and the critical current density) are somewhat lower than the similar parameters for YBCO films deposited on mono-crystalline substrates

Growth and Characterization of YBa2Cu3O7−δYBa_2Cu_3O_{7-δ} Films Deposited by Laser Ablation on CeO2CeO_2-Buffered Sapphire

In this work we study the growth, by pulsed laser deposition, of $YBa_2Cu_3O_{7-δ}$ (YBCO) films on the $CeO_2$-buffered R-cut sapphire substrates, with the buffer layer recrystallized prior to the deposition of superconductor. We find that the superconducting critical temperature and the critical current density of the films are very close to similar parameters for the YBCO films grown on lattice-matched single crystalline substrates. It appears that the structural defects in the buffer layer affect the microstructure of YBCO films, resulting in high values of the critical current density, suitable for applications