P. C. Pradhan

Determining Chemically and Spatially Resolved Atomic Profile of Low Contrast Interface Structure with High Resolution

We present precise measurements of atomic distributions of low electron density contrast at a buried interface using soft x-ray resonant scattering. This approach allows one to construct chemically and spatially highly resolved atomic distribution profile upto several tens of nanometer in a non-destructive and quantitative manner. We demonstrate that the method is sensitive enough to resolve compositional differences of few atomic percent in nano-scaled layered structures of elements with poor electron density differences (0.05%). The present study near the edge of potential impurities in soft x-ray range for low-Z system will stimulate the activity in that field.

Fabrication and performance of a high resolution ultra-short period W/B4C multilayer structure

Microstructure limited reflectivity of sputter deposited ultra-short period W/B4C multilayers (MLs) has been studied using x-ray reflectivity, diffuse scattering and transmission electron microscopy (TEM). In order to understand the W-B4C interface in the near-discontinuous range, the thickness of the constituent layers was systematically varied between 0.5 nm and 2.5 nm. Good density contrast and minimal thickness error were observed in MLs with individual layer thickness higher than ~1.1 nm. However, below 1.1 nm one enters into a regime where the individual layers become discontinuous, leading to embedded growth of the two constituent layers. Consequently, a significant loss of x-ray contrast was observed. In addition, increased area of contact between W and B4C enhances the fraction of tungsten compound resulting in a layer thickness error. Based on these findings, large repetition (upto 400 bilayers) MLs with ultra-short period (down to 1.6 nm) were optimized to achieve reflectivity as high as 53% and resolving power of 149, at 8.047 keV.

Superconducting proximity effect in NiBi3-Ni-NiBi3 trilayer system with sharp superconductor-ferromagnet boundaries

We have studied the superconducting proximity effect in a series of e-beam evaporated Bi-Ni-Bi trilayers, where diffusion of Bi into Ni spontaneously formed superconducting NiBi3 layers at both Ni-Bi interfaces, effectively resulting in superconductor-ferromagnet-superconductor (S-F-S) trilayers. The thickness of top and bottom superconducting layers was found to be different with slightly different transition temperatures. Both resistive transition temperatures in the series of S-F-S trilayers showed 0-π crossover as a function of ferromagnetic Ni thickness. The zero bias conductance calculated from the in-plane current-voltage measurements also confirmed the 0-π crossovers. The possibility of proximity effect in the superconducting fluctuation regime, above transition temperature, was investigated via in-plane magneto-transport measurements at 4 K. We observed clear modulations in magneto-resistance (MR) and in low-field MR-hysteresis at 4 K, in contrast to their monotonic behavior at 10 K. Although the...

Progressive magnetic softening of ferromagnetic layers in multilayer ferromagnet-nonmagnet systems and the role of granularity

We report a study of the structural and magnetic behavior of the topmost magnetic layer in a ferromagnet-nonmagnet (Co-Au) multilayer system. Glancing angle X-ray diffraction measurements performed on a series of multilayers showed a gradual decrease in the grain size of the topmost magnetic layer with the increasing number of bilayers. Concurrently, the magnetic hardness and magneto-crystalline anisotropy of the top Co layer were found to decrease, as observed by magneto-optical Kerr effect measurements. This magnetic softening has been discussed in the light of Herzers random anisotropy model. Micromagnetic simulations of the multilayer system also corroborated these observations.

Element‐specific structural analysis of Si/B4C using resonant X‐ray reflectivity

Element-specific structural analysis at the buried interface of a low electron density contrast system is important in many applied fields. The analysis of nanoscaled Si/B4C buried interfaces is demonstrated using resonant X-ray reflectivity. This technique combines information about spatial modulations of charges provided by scattering, which is further enhanced near the resonance, with the sensitivity to electronic structure provided by spectroscopy. Si/B4C thin-film structures are studied by varying the position of B4C in Si layers. Measured values of near-edge optical properties are correlated with the resonant reflectivity profile to quantify the element-specific composition. It is observed that, although Si/B4C forms a smooth interface, there are chemical changes in the sputtered B4C layer. Nondestructive quantification of the chemical changes and the spatial distribution of the constituents is reported.

Soft X-ray Reflection Spectroscopy for Nano-Scaled Layered Structure Materials

We introduce a novel approach that addresses the probing of interfacial structural phenomena in layered nano-structured films. The approach combines resonant soft x-ray reflection spectroscopy at grazing incidence near the “critical angle” with angular dependent reflection at energies around the respective absorption edges. Dynamic scattering is considered to determine the effective electron density and hence chemically resolved atomic profile across the structure based on simultaneous data analysis. We demonstrate application of the developed technique on the layered model structure C (20 Å)/B (40 Å)/Si (300 Å)/W (10 Å)/substrate. We precisely quantify atomic migration across the interfaces, a few percent of chemical changes of materials and the presence of impurities from top to the buried interfaces. The results obtained reveal the sensitivity of the approach towards resolving the compositional differences up to a few atomic percent. The developed approach enables the reconstruction of a highly spatio-chemically resolved interfacial map of complex nano-scaled interfaces with technical relevance to many emerging applied research fields.

Fabrication and characterization of W/B4C lamellar multilayer grating and NbC/Si multilayer phase-shift reflector

We present fabrication and structural analysis of two different multilayer grating structures. W/B4C based lamellar multilayer grating (LMG) was studied for high resolution monochomator application near soft x-ray region (~1.5 keV). Whereas NbC/Si based multilayer phase-shift reflector (MPR) was studied for high reflection at normal incidence near Si L-edge (~99 eV) and simultaneously to suppress the unwanted vacuum ultraviolet / infrared radiation. The grating patterns of different periods down to D = 10 micron were fabricated on Si substrates by using photolithography, and multilayers (MLs) of different periodicity (d = 10 to 2 nm) and number of layer pairs (15 to 100) were coated using sputtering techniques by optimizing the process parameters. The LMG and MPR samples are characterized by x-ray reflectivity (XRR) and atomic force microscopy (AFM) measurements. XRR results show successive higher order Bragg peaks that reveal a well-defined vertical periodic structure in LMG, MPR and ML structures. The lateral periodicity of the grating and depth of the rectangular groves were analyzed using AFM. The AFM results show good quality of lateral periodic structures in terms of groove profile. The effect of the process parameters on the microstructure (both on vertical and lateral patterns) of ML, LMG and MPR were analyzed.

Element-specific structural analysis of Si/B4C using resonant X-ray reflectivity. Corrigendum

Errors in the article by Nayak, Pradhan & Lodha [J. Appl. Cryst. (2015), 48, 786–796] are corrected.

X-ray multilayer optics for Indus synchrotrons application

We present the state-of-the-art X-ray multilayer optics fabrication facilities at Indus synchrotrons complex. The facilities are regularly used for fabrication of high quality x-ray multilayer structures. The results on two representative materials combination of Mo/Si and W/B4C are presented. In Mo/Si multilayer system, we have achieved ∼70% of reflectivity (near normal incidence angle) at soft x-ray region. Large area (300mm×50mm) Mo/Si multilayers are also successively fabricated for monochromator application in hard x-ray region. Whereas in W/B4C system, we demonstrate the capability of these facilities to fabricate ultra short period multilayer (periodicity ∼15-20 A) with large number of layer pairs in the range of 200-400 for transmission polarizer near Fe L-edge and for monochromator application in hard x-ray region. Hard x-ray reflectivity of ∼54% is achieved from W/B4C MLs with periodicity ∼20 A and number of layer pairs 300.