Vantari Siva

Spontaneous formation of superconducting NiBi3 phase in Ni-Bi bilayer films

We report the spontaneous formation of superconducting NiBi3 phase in thermally evaporated Ni-Bi bilayer films. High reaction-diffusion coefficient of Bi is believed to drive the formation of NiBi3 during the deposition of Bi on the Ni film. Cross sectional transmission electron microscopy and glancing incidence X-ray depth profiling confirmed the presence of NiBi3 throughout the top Bi layer. Superconducting transition at ∼3.9 K, close to the bulk value, was confirmed by transport and magnetization measurements. The bilayers were irradiated with varying fluence of 100 MeV Au ions to study the robustness of superconducting order in presence of large concentration of defects. Superconducting parameters of NiBi3, such as transition temperature and upper critical field, remained unchanged upto an ion dose of 1 × 1014 ions/cm2. The diffusive formation of NiBi3 in Ni opens the possibility of studying superconducting proximity effect at a truly clean superconductor-ferromagnet interface.

Enhanced surface and interface diffusion in Ni–Bi bilayers by swift heavy ion irradiation

In this report, the effect of 100 MeV Au ion irradiation on diffusion in Ni/Bi bilayers has been studied. Normally, both of the layers across the interface need to reach their molten state to observe ion beam induced diffusion and mixing. However, we experimentally observed enhanced diffusion at the interface, though the thermal spike model calculations suggest that Ni does not reach its molten state at this energy regime. Spontaneous formation of a NiBi3 layer at the interface of Ni/Bi was found via the reaction–diffusion mechanism during the deposition. The evolution of porous like structures on the surface has been observed after irradiation of the films. This porous structure evolution was driven by the possible out-diffusion of Bi, as it reaches the molten state. The interface NiBi3 layer improves the mixing of the Ni and Bi layers and increases the ion fluences observed in the depth profiles obtained using cross-sectional transmission electron microscopy and Rutherford backscattering spectroscopy. Such an enhancement can be attributed to the combined swift heavy ion induced athermal effects and ballistic effects. The detailed mechanism of the ion induced surface and interface modifications were explained on the basis of thermal spike model calculations.

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.

Ion induced dewetting of Au–Si on a SiO2 surface: composite nanodot evolution and wettability transition

A nanodot array morphology gradually develops on SiO2 surface when a thin bi-layer of Au and Si undergoes ion irradiation. An increasing amount of gold silicide is detected as islands on the insulator surface evolve into nanodots as a function of increasing ion fluence. Different stages of evolution from islands to nanodots are found to be driven by the localized melting of Au along the ion-track and dewetting of the metal film. Dewetting is accompanied by sputter-erosion and mixing of Au and Si at the bi-layer interface due to ion energy deposition. Interestingly, a gradual transition in wettability of the surface from the hydrophilic to the hydrophobic one is observed with the growth of nanodots, which is correlated with the compositional variation. The experimental results indicate a route towards the controlled growth of composite nanodots on an insulator surface having hydrophobic properties using ion irradiation.

Unusual ferromagnetic behaviour of embedded non-functionalized Au nanoparticles in Bi/Au bilayer films

There is a growing consensus through various experimental and theoretical studies that gold can exhibit magnetic properties at low dimensions in contrast to its well-known diamagnetic nature when in bulk form. Although theoretical simulation studies show that bare gold nanoclusters can be intrinsically magnetic, experimental reports to prove this theory are scarcely available since most of the studies are based on functionalized gold nanoparticles. In this article, we report unusual ferromagnetic behaviour that is observed in embedded non-functionalized Au nanoparticles using superconducting quantum interference device (SQUID) magnetometry. These nanoparticles are obtained after irradiating thermally deposited Au/Bi double bilayer films using 1.5 MeV Au ions at different fluences from 5 × 1014 to 1 × 1016 ions per cm2. A detailed study of cross-sectional high resolution transmission electron microscopy (X-HRTEM) results for the irradiated sample confirms the presence of embedded Au nanoparticles with an average size of 2.61 nm. The unusual ferromagnetic behaviour is attributed to these embedded Au nanoparticles that were formed after ion irradiation.

Nanoscale interfacial mixing of Au/Bi layers using MeV ion beams

We have studied nanoscale mixing of thermally deposited double bilayer films of Au/Bi after irradiating them by 1.5 MeV Au2+ ions. Post irradiation effects on the morphology and elemental identification in these films are studied by Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDS). Glancing angle X-ray diffraction (GAXRD) of the samples indicate marginal changes in the irradiated samples due to combined effect of nuclear and electronic energy loss. The interfacial mixing is studied by Rutherford backscattering (RBS).

Solid state reaction induced phase evolution of Ni/Bi thin films

Effect of thermal annealing on the structural and electrical properties of Ni/Bi layer deposited on Si substrates are studied as a function of temperature. Agglomeration of the thin films into island-like structures is observed after annealing whereas grazing angle x-ray diffraction studies shows formation of both NiBi and NiBi3 phases. We discuss the phase evolution in terms of diffusion of Bi and NiBi3/NiBi phase formation at the interfaces. Semi-metallic nature of the as-deposited film is revealed by resistivity versus temperature study, which transforms to metallic behavior after annealing.