Browsing by Author "Neelabh Srivastava"

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A study on exchange coupled structures of Fe/NiO and NiO/Fe interfaced with n- and p-silicon substrates
(2012) Neelabh Srivastava; P.C. Srivastava
Interfacial structures of ferromagnetic (FM)/antiferromagnetic (AF) (Fe/NiO) and AF/FM (NiO/Fe) on n- and p-Si substrates have been realized by sequential deposition of FM and AF layers on the silicon substrates by electron beam evaporation technique. The structures have been characterized from x-ray diffraction (XRD), atomic force microscopy (AFM), magnetic force microscopy (MFM), and M-H characteristics. It has been found that there is a strong interfacial intermixing to form the various oxide and silicide phases of Fe 2O 3, β-Fe 2O 3, β″-Fe 2O 3, NiSi, Ni 3Si, and Fe 5Si 3. AFM micrographs show the granular morphology of the top layer of the structure, with a large grain size of ∼400 nm, however, the XRD data show the crystallite size of ∼20 to 70 nm. It seems that the crystallites are clustered to form larger grains. MFM features show a large domain size corresponding to AFM grain size for Fe/NiO/Si structure and very small domain of nanometer size for NiO/Fe/Si structure (having NiO as a top layer). M-H characteristics show that the magnetic behavior is only significant for Fe/NiO/nSi structure with a significant coercivity and exchange bias than for all other interfacial structures of Fe/NiO/pSi, NiO/Fe/pSi, and NiO/Fe/nSi. Thus, it has been found that Fe/NiO/nSi structure can be used in magneto-electronic device applications. It seems that the observed result of significant exchange bias and coercivity is due to the microstructural and chemical structure changes in the antiferromagnetic layer along with the roughness (data as obtained from AFM). © 2012 American Institute of Physics.
Core level X-ray photoelectron spectroscopy study of exchange coupled Fe/NiO bilayer interfaced with Si substrate (Fe/NiO-nSi structure)
(2013) Neelabh Srivastava; T. Shripathi; P.C. Srivastava
The chemical interactions and surface chemical states of exchange coupled structures of Fe/NiO bilayer interfaced with silicon substrate has been studied from X-ray photoelectron spectroscopy (XPS) technique. The structure has been irradiated from swift (∼100 MeV) heavy ions (of Fe7+) with a fluence of ∼5 × 1012 ions/cm2. The surface of the samples has also been cleaned from ion beam etching for various durations to record the XPS spectra. On the irradiation, signals due to Fe2p and Ni2p are emerging along with the signal of Si2p whereas no signal of Si2p has been observed, prior to irradiation. The observation shows that the Si atoms are present within the probed surface layer due to irradiation induced interfacial intermixing. We also observed a shift in the binding energy position of Si2p which could be due to the formation of silicide phases as a result of irradiation induced interfacial intermixing. XPS results have indicated the presence of ionic Fe (Fe2+ or Fe3+) and metallic Ni at the Fe/NiO interface, which could be due to the different chemical reactions at the Fe/NiO interface. © 2013 Elsevier B.V.
Effect of interfacial modifications on magnetic, morphological and transport properties of CoFe/n-Si thin film structures using ion irradiation
(Elsevier B.V., 2019) Arvind Kumar; Neelabh Srivastava; P.C. Srivastava
Interfacial structures of ferromagnetic (FM) metal combined with semiconductor (SC) are key step for realizing spintronic devices where spin degree of freedom can also be utilized in addition to the charge of electron. Tailoring of such interfaces (of FM/SC) is known to tune the magnetic, optical, structural and transport properties. Swift heavy ion irradiation (SHI) can be used as a potential tool to tailor the interfaces. In our present work, effect of ion beam irradiation (100 meV Ni+7 ions) on the CoFe/n-Si interfacial structures had been studied. Thin films of CoFe was deposited by electron beam evaporation technique using CoFe metallic alloy on Si substrate. The realized interfacial structure was characterized from structural, morphological and magnetic point of view. The surface exhibits grainy structure with very fine domain (∼50 nm) on the irradiation. Electronic transport study across the interface shows an increase in current on irradiation with significant magnetic field sensitivity and a magnetoresistance of ∼7% has been observed. The observed results of increased magnetization and enhanced magnetic field sensitivity has been understood in the realm of irradiation induced interfacial intermixing across the interfaces. The chemical phases formed and nature of the interface as a result of irradiation was probed by X-ray photoelectron spectroscopy (XPS) study which confirms the formation of various silicide phases along with the metallic phase. The results were also compared with our earlier studies [15–16] on CoFe/n-Si system and found that fluence of 1 × 1011 ions/cm2 seems significant to result in interesting magnetic and transport properties without much damage to interface. © 2019 Elsevier B.V.
Effect of ion irradiation on magnetic property of exchange coupled interfacial structures of Fe/NiO and NiO/Fe on Si substrates
(Taylor and Francis Ltd., 2014) Neelabh Srivastava; P.C. Srivastava
The role of defects on the magnetic behaviour of exchange coupled interfacial structures of Fe/NiO and NiO/Fe on Si substrates has been studied. For introduction of defects in the structures, swift (∼ 100 MeV) heavy ion irradiation has been used, which is known to cause structural and microstructural modifications. In our earlier study [Srivastava, N; Srivastava, P.C. J. Appl. Phys. 2012, 111, 123909] on similar structures, the significant magnetic behaviour (of exchange bias (EB) and coercivity) for Fe/NiO/nSi interfacial structure was observed and discussed in the realm of interfacial structural modification in the antiferromagnetic layer of the structure. The irradiated interfacial structures have been characterized from X-ray diffraction and M-H characteristics. Structural investigation has shown the formation of various silicide and oxide phases due to the irradiation-induced interfacial intermixing. A significant enhancement in EB field and coercivity has been observed for Fe/NiO/nSi interfacial structure on the irradiation (as compared to unirradiated ones). The observed enhanced EB and coercivity on the irradiation has been understood due to the creation of domain wall pinning centres across the interface as a result of ion irradiation. Moreover, the present study confirms the role of defects in the antiferromagnetic layer to cause the significant change in EB and coercivity. The observation supports the domain state model of EB in the exchange-coupled structures. © 2014 Taylor & Francis.
Electronic and magneto-transport study across swift heavy ion irradiated exchange coupled Fe/NiO bilayer on Si substrate
(Elsevier, 2015) Neelabh Srivastava; P.C. Srivastava
Low temperature electronic and magneto-transport study across Fe/NiO bilayer on Si substrate has been reported. These bilayer structures have been irradiated by swift heavy ions (∼100 MeV Fe7+ ions with a fluence of 5×1012 ions/cm2). The electronic transport study across such bilayer (both unirradiated and irradiated) structures has shown the semiconducting nature of the interface. A significant decrease in current has been observed for the irradiated structure (as compared to unirradiated ones on the irradiation) which could be due to the irradiation induced introduction of defects/disorders in the structure. The magneto-transport study across unirradiated structure has shown the magnetic field sensitivity at low temperatures only whereas the irradiated structure has not shown any perceptible magnetic field sensitivity at low temperatures. Such observed intriguing feature of magnetic field sensitivity across the bilayer structures could be understood due to the motion of thermally assisted magnetic domain walls in the presence of external applied magnetic field. The observed high % MR could be related to spin-dependent electron scattering at the interfaces. © 2015 Elsevier B.V.
Exploring 32 % efficiency of eco-friendly kusachiite-based solar cells: A numerical study
(Elsevier Ltd, 2025) Sumit Choudhary; Rahutosh Ranjan; Manish Nath Tripathi; Neelabh Srivastava; Arvind Kumar Sharma; Mashamichi Yoshimura; Li Chang; Rajanish Nath Tiwari
Recently, solar cells have appeared as a promising solution to meet the increasing energy demand. However, their large-scale commercial use is limited by issues such as toxicity (Cd, Pb, etc), high manufacturing costs, lower stability, and low efficiency. In this perspective, kusachiite solar cells (KSCs) are environmentally friendly, long-term stable, and easy to manufacture. Thus, in this work, kusachiite (CuBi2O4) is used as an absorber layer, with NiO and SrTiO3 serving as the hole transport material (HTM) and an electron transport layer (ETL), respectively. The KSCs, with a structure of FTO/SrTiO3/CuBi2O4/NiO/Au, are numerically simulated. Maximum power conversion efficiency is achieved by optimizing several photovoltaic parameters, such as the thickness and doping density of the ETL, absorber, and HTM. The optimized thicknesses for the HTL, absorber layer, and ETL are 1.5 μm, 2.28 μm, and 0.02 μm, respectively. The designed KSCs exhibit an efficiency eta (η) of 31.89 %, an open-circuit voltage (Voc) of 1.31 V, a short-circuit current (Jsc) of 28.58 mA/cm2, and a fill factor (FF) of 84.99 %. © 2024
Heavy ion induced modifications on morphological, magnetic and magneto-transport behaviour of exchange-biased Fe/NiO and NiO/Fe bilayers with Si substrate for spintronic applications
(Kluwer Academic Publishers, 2015) Neelabh Srivastava; P.C. Srivastava
Exchange-coupled interfacial structures of Fe/NiO and NiO/Fe with pSi substrate have been studied and also the effect of swift heavy ion irradiation on the morphological, structural, transport and magnetic behaviour is reported. The interfacial structures have been characterised from X-ray diffraction (XRD), magnetic force microscopy/atomic force microscopy, X-ray photoelectron spectroscopy and magnetisation characteristics. XRD and X-ray photoelectron spectroscopy studies have shown the formation of various silicide and oxide phases due to the interfacial intermixing across the interfaces which is found to affect the transport and magnetic behaviour. A significant enhancement in exchange bias field and coercivity has been observed for Fe/NiO/pSi interfacial structure on the irradiation (as compared to unirradiated ones). The observed enhanced exchange bias and coercivity on the irradiation has been understood due to creation of uncompensated surface/pinned interfacial spins. Magnetic field-induced enhanced current has been observed at low temperatures (50–250 K) for the irradiated structure suggesting the spin-mixing effect. Low temperature magneto-transport study across the irradiated interface has shown negative magnetoresistance (MR) as compared to unirradiated ones for which positive MR is observed. The observed change in MR at low temperatures has been understood in terms of diffuse scattering at grain boundaries/spin-disorder scattering and/or magnetic polarons. Role of interfacial modification/changes in chemical environment across the interfaces is invoked for the observed changes in magnetic and transport behaviour of the structures. A possible explanation for the observed changes is given. © 2015, Springer Science+Business Media New York.
Injecting electrode controlled electronic transport across Fe 3O4 film-Si interfacial structure
(Elsevier Ltd, 2014) Surajit Ghosh; Neelabh Srivastava; P.C. Srivastava
A study of electronic transport has been carried out across a half-metallic Fe3O4 film-nSi interfacial structure in CPP mode with an interfacial SiO2 layer of <5 nm. It has been measured with and without applied magnetic field along the plane of the interface between 25 K and 300 K to investigate the role of half-metallic Fe3O4 layer on the transport. The electronic transport has shown a distinct behaviour for the reverse bias in which injection takes place from metallic film of Fe3O4 to silicon than for the forward bias in which injection takes place from silicon to metallic Fe3O4 side. It has been found that the reverse bias current shows a tunnel transport, Verwey transition like feature and low temperature positive GMR, evidencing spin involved transport from Fe3O4 to semiconductor side. Whereas the forward bias current has not shown any of the above features but a thermionic controlled transport, showing a change of resistance with temperature as of semiconductor silicon. Thus, it has been found that the transport is controlled by physical properties of the injecting electrode. The study has shown that the transport below Verwey transition temperature (TV) for the Fe3O4 film interfacial structure is also related to its electronic spins. The observed GMR of ∼200% at low temperature (for reverse bias current only) has been discussed as the effect of electronic spin scattering. Our unique observations from simple measurements of CPP I-V across the interfacial structure of Fe3O4-nSi seem significant to reveal that the Verwey transition is related to spins of half-metallic Fe 3O4. © 2014 Elsevier Inc. All rights reserved.
Irradiation induced modifications in microstructural and magnetic property of Fe 50Ni 50 (permalloy)/Si interfacial structure
(2014) P.C. Srivastava; P.S. Pandey; Neelabh Srivastava; M.K. Srivastava
The interfacial structures of Fe50Ni50/Si have been prepared by electron beam evaporation of the alloy on silicon substrates. X-ray diffraction data confirm the formation of the alloy phase of Fe50Ni50 and intermixed silicide phases of Fe5Si3, Ni3Si2, Ni2Si and NiSi. The structures have also been irradiated from swift heavy ions to study the effect of irradiation induced interfacial intermixing. Magnetic force microscopy (MFM) and magnetisation characteristics have been used to characterise the magnetic behaviour of as-prepared and irradiated structures. It is observed from the MFM that the magnetic domains are of sub-micron scale and there is a significant magnetic signal from the surface of the structure which becomes stronger for the irradiated structures. The observed magnetisation (M-H) characteristics show the presence of in-plane and out-of-plane magnetic anisotropy in the structure which has been observed to be removed on the irradiation. The magnetisation characteristics has been understood as the magnetic behaviour of nano-magnetic grains of silicides (and of the alloy, Fe50Ni50) which are formed as a result of interfacial chemistry and modified by irradiation induced interfacial intermixing. The observed magnetic behaviour seems interesting and significant for many applications related to magnetics. © 2013 © 2013 Taylor & Francis.
Magnetic and Structural Properties of Exchange Coupled Heusler Alloy NiO/Co2FeAl Interfaces with n-and p-Type Silicon Substrates
(Springer, 2020) Arvind Kumar; Neelabh Srivastava; P.C. Srivastava
Exchange coupled structures of half metallic ferromagnets, Heusler Alloy (FM), Co2FeAl (CFA) and antiferromagnet (AF), NiO were fabricated by electron beam evaporation technique on n- and p-type Si substrates. These fabricated bilayer structures were further characterized for structural, morphological and magnetic point of views using x-ray diffraction (XRD), atomic/magnetic force microscopy (AFM/MFM) and vibrating sample magnetometer (VSM) techniques. Structural study shows the presence of CFA alloy and NiO phase. Surface morphology shows the granular feature of the top surface with grain size of 200–300 nm due to clustering of smaller grains. Smaller sized magnetic domains have been observed for the structures. Hysteresis behaviour of NiO/CFA/Si structures shows the ferromagnetic behaviour for in-plane orientation with negligible exchange bias whereas a significant exchange bias for out of plane orientation has been observed. The observed result could be understood due to surface roughness and arrangements of spins near the interface of ferromagnetic and anti-ferromagnetic layers. © 2019, The Minerals, Metals & Materials Society.
Microstructural, magnetic and magneto-transport properties of NiO thin film deposited on Si (100) substrates
(Springer New York LLC, 2016) Neelabh Srivastava; P.C. Srivastava
NiO thin film on p- and n-type Si (100) substrates were fabricated by electron beam evaporation technique and studied by structural, morphological, magnetic and magneto-electronic transport characterization techniques. The room temperature interfacial chemistry across the interfaces is found to affect the morphology, transport and magnetic behaviour of the structures. Structural study has shown the presence of magnetic nickel silicide phases along with some metallic phases as a result of interfacial intermixing. Isolated nano-granular features of magnetic grains along with the presence of clustered grains are observed. Strong magnetic signal strength is observed for NiO/nSi structure as compared to the interface on pSi substrate which suggests the formation of more magnetic silicide phases resulting in a higher magnetization value with increased coercivity. Magnetization characteristics of the NiO thin films on Si substrates have shown a nearly superparamagnetic behaviour with weak ferromagnetic contribution as compared to bulk antiferromagnetic nature of NiO. Electronic transport study measured across NiO/nSi interface have shown the enhancement in conductivity up to 2–3 orders of magnitude than for NiO film on pSi substrate with significant magnetic field sensitivity. Magnetic field induced enhanced current has been observed for NiO/nSi structure which is related to the magnetic field response of magnetic grains (of nickel silicide phases) or isolated magnetic clusters favouring the paths for spin alignment or spin-dependent scattering resulting in a negative magnetoresistance of ~50 %. © 2016, Springer Science+Business Media New York.
Preface
(Springer Singapore, 2019) Karuna Singh; Neelabh Srivastava
[No abstract available]
Realizing NiO nanocrystals from a simple chemical method
(Indian Academy of Sciences, 2010) Neelabh Srivastava; P.C. Srivastava
Nanocrystalline NiO has been prepared successfully by a simple chemical route using NiCl2-6H2O and NaOH aqueous solution at a temperature of 70°C. The prepared material has been characterized from XRD, SEM, and M-H characteristics. It has been found that NiO nanocrystals have been formed which shows a superparamagnetic/superantiferromagnetic behaviour. © Indian Academy of Sciences.
Recent trends in human and animal mycology
(Springer Singapore, 2019) Karuna Singh; Neelabh Srivastava
Fungal pathogens pose an on-going and serious threat for poikilotherms and homeotherms, and can cause a broad spectrum of diseases ranging from innocuous to life-threatening. In addition, long-term exposure to some mycotoxigenic moulds can lead to mycotoxicoses in human and animals. Given the expanding population of immune compromised hosts, the list of fungal opportunists grows longer every year. Moreover, antifungal resistance, drug-related toxicity and our limited arsenal of antifungals have exacerbated the situation. To address these problems, strategies such as the identification of novel targets, use of the structure-activity relationship in rational drug design, development of new formulations, modification of existing antifungals to combat resistance, and bioavailability enhancement are called for. For the reader’s convenience, this book has been divided into three sections. The first six chapters of Section I provide a timely review of mycoses, from endemic to cosmopolitan and from generalized to specific, while both chapters of Section II focus on risks associated with mycotoxins. In closing, the two chapters of Section III describe potential antifungal leads and drug candidates based on phytochemicals and coumarin scaffold. © Springer Nature Singapore Pte Ltd. 2019.
SCAPS study on the effect of various hole transport layer on highly efficient 31.86% eco-friendly CZTS based solar cell
(Nature Research, 2023) Rahutosh Ranjan; Nikhil Anand; Manish Nath Tripathi; Neelabh Srivastava; Arvind Kumar Sharma; Masamichi Yoshimura; Li Chang; Rajanish N. Tiwari
Copper Zinc Tin Sulphide (CZTS) is a propitious semiconductor for active absorber material in thin-film solar cells (SCs). Here, SC architecture comprising FTO/ZnS/CZTS/variable HTLs/Au is discussed. Fluorine-doped tin oxide (FTO) and gold (Au) are used as front and back contacts, respectively. Zinc sulphide (ZnS) is used as an active electron transport layer (ETL), while different Cu-based materials (Cu2O, CuO, CuI, and CuSCN) are used as hole transport layers (HTL). A one-dimensional solar cell capacitance simulator (SCAPS-1D) is utilized to simulate the SC structure. Among different Cu-based HTLs, Cu2O is preferred as a potential candidate for high cell performance of CZTS-based SC. The effects of various layer parameters such as thickness, doping density, and carrier concentrations, electron affinity of HTL and absorber, respectively, are also discussed. After optimization of the device, variation of operating temperature and the effect of series and shunt resistance are also taken into consideration. The optimized results of thickness and acceptor concentration (NA) of absorber material are 1.5 µm and approx. 1.0 × 1019 cm−3, respectively. In addition, the function of HTL (with and without) in the designed SC structure is also studied. Capacitance–voltage (C–V) characteristics are also discussed to get an insight of built-in potential. We have achieved cell performances viz. efficiency = 31.86%, short circuit current density = 32.05 mA/cm2, open circuit voltage = 1.19 V, and fill factor = 83.37%. © 2023, The Author(s).
Synthesis and characterization of (single- and poly-) crystalline NiO nanorods by a simple chemical route
(2010) Neelabh Srivastava; P.C. Srivastava
Crystalline cubic NiO rods with diameter ranging from nanometer to few hundred nanometers and lengths up to 10 μm have been realized from a simple chemical route. The chemical reaction, of aqueous solutions of nickel chloride and sodium hydroxide with different molar ratios of NiCl2 and NaOH, formed the nickel hydroxide precursor which on thermal dehydration resulted in NiO nanocrystals with rod like morphology. The synthesized material has been characterized from XRD, SEM, TEM, EDAX and UVvis absorption spectroscopy. The TEM micrographs show the rod like morphology of the formed crystals and the selected area electron diffraction pattern confirms the single-crystalline and polycrystalline nature of the observed rod like nanocrystals. The optical absorption band gap of NiO nanorods has been found to be of ∼3.62 eV. © 2010 Elsevier B.V.
Synthesis of wurtzite ZnS nanocrystals at low temperature
(2013) Prayas Chandra Patel; Neelabh Srivastava; P.C. Srivastava
Nanocrystallites of wurtzite hexagonal ZnS have been successfully synthesized without using any capping agent by simple chemical precipitation method at a low calcination temperature of 150 C. It has been found that the size of the synthesized ZnS nanocrystallites decreases as Zn2+:S 2- ratio is decreased. The synthesized nanoparticles have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis absorption spectroscopy and M-H characteristics. The XRD patterns have confirmed that the prepared ZnS nanoparticles are of wurtzite hexagonal phase. XRD, SEM and TEM studies have shown the decrease in the particle size with the increase in S2- source. TEM images have clearly shown that size distribution of the particles lie in the range of 5-30 nm. The optical absorption bandgap of the synthesized nanocrystals has been found to be in the range of 3.69-3.74 eV. Magnetization study has shown the 'diamagnetic' behavior of synthesized ZnS nanocrystallites with a weak ferromagnetic behavior in the low field regime. The observed weak ferromagnetism has been understood due to the presence of defects in the synthesized ZnS nanoparticles. © 2013 Springer Science+Business Media New York.