Indigenous development of ultra high vacuum (UHV) magnetron sputtering system for the preparation of Permalloy magnetic thin films

We have designed and developed an indigenous ultra high vacuum (UHV) sputtering system which can deposit magnetic thin films with high purity and good uniformity. The equipment consists of state-of the-art technologies and sophistication. With this system it is possible to deposit coatings of various materials on a sample size of 3”3” 3”. The Ni81Fe19 ferromagnetic thin films, with Tantalum (Ta) as a buffer and cap layers have been deposited on silicon substrates using this ultra high vacuum (UHV) sputtering system. The magneto transport measurement study indicated a significant variation in the AMR values of the films for varying thicknesses of tantalum and NiFe layers

Investigation of interface properties of sputter deposited TiN/CrN superlattices by low-angle X-ray reflectivity

Approximately 1.8 m thick nanolayered multilayer coatings of TiN/CrN (also known as superlattices) were deposited on silicon (100) substrates at different modulation wavelengths (4.6–12.8 nm), substrate temperatures (50-400 °C) and substrate bias voltages (-50 to -200 V) using a reactive direct current magnetron sputtering system. X-ray reflectivity (XRR) technique was employed to determine various properties of the multilayers such as interface roughness, surface roughness, electron density, critical angle and individual layer thicknesses. The modulation wavelengths of the TiN/CrN superlattice coatings were calculated using modified Bragg’s law. Furthermore, the experimental X-ray reflectivity patterns were simulated using theoretically generated patterns and a good fit was obtained for a three layer model, i.e., (1) top surface roughness layer, (2) TiN/CrN multilayer coating (approximately 1.8 m) and (3) Ti interlayer (~ 0.5 m) at the film-substrate interface. For the superlattice coatings prepared at a modulation wavelength of 9.7 nm, a substrate bias of -200 V and a substrate temperature of 400 C the XRR patterns showed Bragg reflections up to 5th order, indicating well-defined periodicity of the constituent layers and relatively sharp interfaces. The simulation showed that the superlattice coatings prepared under the above conditions exhibited low surface and interface roughnesses. We also present the effect of substrate temperature and substrate bias, which are critical parameters for controlling the superlattice properties, onto the various interface properties of TiN/CrN superlattices

Structure and wear mechanisms of nano-structured TiAlCN/VCN multilayer coatings

Dry sliding wear of transition metal nitride coatings usually results in a dense and strongly adhered tribofilm on the worn surface. This paper presents detailed electron microscopy and Raman spectroscopy characterizations of the microstructure, a newly developed multilayer coating TiAlCN/VCN and its worn surface after pin-on-disc sliding wear against an alumina ball. The friction coefficient in a range of 0.38–0.6 was determined to be related to the environmental humidity, which resulted in a wear coefficient of the coating varying between 1017 and 1016 m3 N1 m1. TEM observation of worn surfaces showed that, when carbon was incorporated in the nitride coating, the formation of dense tribofilm was inhibited

Studies on hot-filament chemical vapor deposition grown graphene sheets

Graphene was grown on high purity Cu foils using hot-filament chemical vapor deposition method. The foils were kept directly below the tungsten filament and the whole assembly was kept inside a vacuum chamber. CH4 and H2 were used as precursor gases and were allowed to shower on a hot filament, which was kept at a predetermined temperature. The optimization of the process parameters such as gas flow rates, temperature, durations, etc. was done to grow single layer and multilayer graphene. The graphene was characterized using optical microscopy, field emission scanning electron microscopy and micro-Raman spectroscopy techniques. The graphene layers grown at different methane flow rates are shown in Figure 1. By varying the methane flow rates, graphene domains of different sizes and shapes were achieved and are clearly evident from Figures 1a-c. The curved white lines (Figure 1a) present in the FESEM micrographs correspond to Cu terraces. The graphene grown on Cu foils was successfully transferred to SiO2 substrate and the micrograph of which is shown in Figure 1d. The presence of D, G and G’ bands in the Raman spectrum confirmed the growth of graphene in the Cu foil (Figure 2)

Structural and optical properties of graphene oxide prepared by modified hummers' method

Graphene oxide was synthesized from graphite flakes using modified Hummers' method. The interlayer spacings of graphite, graphite oxide and graphene oxide were measured using X-ray diffraction technique. The C/O atomic ratios of graphite oxide and graphene oxide were calculated from XPS measurements. The transformation of graphite to graphite oxide and finally to graphene oxide was clearly observed from the micro-Raman spectroscopy data and was confirmed from the FESEM micrographs. UV-VIS-NIR spectrophotometer was used to study the absorbance of graphene oxide and reduced graphene oxide samples. Finally, the chemically reduced graphene oxide was heat-treated in air to obtain chemically modified graphene

Enhanced defect emission in Co doped ZnO nanorods synthesized by electrochemical route

We present the effects of Co doping on the morphology, structural and optical properties of ZnO nanocoatings in detail. Highly oriented wurtzite ZnO nanorods were fabricated by electrodeposition method using Zn(NO3)2 as the electrolyte. Again, the Co doped (0.03 mM CoSO4) ZnO nanocoatings were synthesized on ITO coated glass substrates. The surface morphology and the structural properties of the coatings were studied using field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD), respectively. The FESEM data (Fig. 1) showed the top view of oriented hexagonal ZnO nanorods. The XRD pattern showed a shift in (002) diffraction peak of ZnO towards higher 2Ѳ values by the substitution of dopant, which indicates that the Zn2+ site is successfully replaced by the dopant Co2+. Moreover, the photoluminescence spectroscopy (PL, 325 nm excitation) was used to study the UV emission and the defect emission of the doped and undoped ZnO nanostructures. The PL data (Fig. 2) revealed a blue shift in the UV emission for the doped sample which signifies that for ZnO coating prepared with 0.03 mM CoSO4, the energy gap increased from 378 nm (3.27 eV) to 375 nm (3.30 eV). Also, the intensity of deep level emission (~ 600 nm) increased for the doped one as compared to the pure ZnO coating. Our results demonstrate that Co doping may be responsible for the blue shift in band gap of ZnO nanocrystals. The oxygen defects or Zn interstitials may be responsible for the broadening of the PL peak at 600 nm

Development of pinned electrode for magnetic tunnel junction with perpendicular magnetic anisotropy

The magnetic electrodes with perpendicular magnetic anisotropy (PMA) have gained a great deal of attention in magnetic information storage technology. The use of perpendicular magnetic tunnel junctions (p-MTJs) enhances the storage density due to the reduction in area required for a unit bit. Among two electrodes in a MTJ structure (ferromagnetic/insulator/ferromagnetic), one electrode should be fixed or pinned to a specific direction of magnetization in order to have a unidirectional magnetic anisotropy with higher exchange field. This can be achieved by a phenomenon of exchange bias observed between the ferromagnetic (FM) and the anti-ferromagnetic (AFM) materials as a shift in a hysteresis loop when they are deposited in a presence of an external applied field