Books by Jitendra Kumar Tripathi
Ions Induced Granular Metallic Magnetic Silicide: For Potential Spintronic Devices
Papers by Jitendra Kumar Tripathi
In this paper, we report on the temperature dependent surface modifications in molybdenum (Mo) sa... more In this paper, we report on the temperature dependent surface modifications in molybdenum (Mo) samples due to 100 eV He + ion irradiation in extreme conditions as a potential candidate to plasma-facing components in fusion devices alternative to tungsten. The Mo samples were irradiated at normal incidence, using an ion fluence of 2.6 Â 10 24 ions m À2 (with a flux of 7.2 Â 10 20 ions m À2 s À1 ). Surface modifications have been studied using high-resolution field emission scanning electron-(SEM) and atomic force (AFM) microscopy. At 773 K target temperature homogeneous evolution of molybdenum nanograins on the entire Mo surface were observed. However, at 823 K target temperature appearance of nano-pores and pin-holes nearby the grain boundaries, and Mo fuzz in patches were observed. The fuzz density increases significantly with target temperatures and continued until 973 K. However, at target temperatures beyond 973 K, counterintuitively, a sequential reduction in the fuzz density has been seen till 1073 K temperatures. At 1173 K and above temperatures, only molybdenum nano structures were observed. Our temperature dependent studies confirm a clear temperature widow, 823-1073 K, for Mo fuzz formation. Ex-situ high resolution X-ray photoelectron spectroscopy studies on Mo fuzzy samples show the evidence of MoO 3 3d doublets. This elucidates that almost all the Mo fuzz were oxidized during open air exposure and are thick enough as well. Likewise the microscopy studies, the optical reflectivity measurements also show a sequential reduction in the reflectivity values (i.e., enhancement in the fuzz density) up to 973 K and after then a sequential enhancement in the reflectivity values (i.e., reduction in the fuzz density) with target temperatures. This is in well agreement with microscopy studies where we observed clear temperature window for Mo fuzz growth.
Self-ordered magnetic alpha-FeSi2 nano-stripes on Si(111)
Appl. Phys. Lett. 102, 251604 (2013)
We fabricated self-assembled one-dimensional alpha-FeSi2 magnetic nano-stripes, using solid phase... more We fabricated self-assembled one-dimensional alpha-FeSi2 magnetic nano-stripes, using solid phase epitaxy of 1 monolayer Fe on vicinal Si(111) surfaces followed by a 550 C annealing. Structural and magnetic properties were investigated by scanning tunneling microscopy and superconducting quantum interference device magnetometry, respectively. The nano-stripes show superparamagnetism at room-temperature and at 4K coercivity about 3 times higher along the long nano-stripe (11-2) direction than along the perpendicular (-110) one. The calculated magnetic moment at 4K was 3.3 Boharmagneton/Fe atom, implying a significant ferromagnetic coupling of the Fe atoms in the system. Such nano-stripes may be useful in high-density magnetic memory storage and spintronics devices.
The effect of Fe-coverage on the structure, morphology and magnetic properties of α-FeSi2 nanoislands
Nanotechnology 23, 495603 (2012)
Self-assembled α-FeSi2 nanoislands were formed using solid-phase epitaxy of low (~1.2 ML) and hig... more Self-assembled α-FeSi2 nanoislands were formed using solid-phase epitaxy of low (~1.2 ML) and high (~21 ML) Fe coverages onto vicinal Si(111) surfaces followed by thermal annealing. At a resulting low Fe-covered Si(111) surface, we observed in situ, by real-time scanning tunneling microscopy and surface electron diffraction, the entire sequence of Fe–silicide formation and transformation from the initially two-dimensional (2 × 2)-reconstructed layer at 300 °C into (2 × 2)-reconstructed nanoislands decorating the vicinal step-bunch edges in a self-ordered fashion at higher temperatures. In contrast, the silicide nanoislands at a high Fe-covered surface were noticeably larger, more three-dimensional, and randomly distributed all over the surface. Ex situ x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy indicated the formation of an α-FeSi2 island phase, in an α-FeSi2{112} ∥ Si{111} orientation. Superconducting quantum interference device magnetometry showed considerable superparamagnetism, with ~1.9 μB/Fe atom at 4 K for the low Fe-coverage, indicating stronger ferromagnetic coupling of individual magnetic moments, as compared to high Fe-coverage, where the calculated moments were only ~0.8 μB/Fe atom. Such anomalous magnetic behavior, particularly for the low Fe-coverage case, is radically different from the non-magnetic bulk α-FeSi2 phase, and may open new pathways to high-density magnetic memory storage devices.
Effect of Low energy He+ - ion irradiation on structural and magnetic properties of thin Pt/Cr/Co Multilayers
In this work, we report on the changes in structural and magnetic properties of [Pt (0.7 nm)/ Cr ... more In this work, we report on the changes in structural and magnetic properties of [Pt (0.7 nm)/ Cr (x nm)/ Co (0.5 nm)]x15/Si, x = 0.1 and 0.2 nm, due to 10 keV He+-ion irradiation at room temperature in the fluence range of 2E15 to 5E16 ions-cm-2. Enhancement in the coercivity values with a fairly square magnetization reversal loop (for both the multilayers), upon irradiation to the fluence of 5E16 ions-cm-2 was observed. Above finding is discussed in the realm of ion beam mixing, leading to the CoCrPt ternary alloy phase formation, after low-energy He+-ion irradiation.
Ion erosion induced nanostructured semiconductor surfaces
We consider nanostructures formed on semiconductor surfaces of Si(100), InP(100) and GaAs using m... more We consider nanostructures formed on semiconductor surfaces of Si(100), InP(100) and GaAs using medium energy (50–100 keV) Ar+-ion beam sputtering. The issues of dependence of nanostructure formation on these semiconductor substrates on ion-energy, -fluence, -flux, angle of incidence, and crystallographic orientation are addressed. The threshold fluence for formation of nano-islands on Si(100) implanted with normally incident 50 keV Ar+-ions was found to be 2 × 1017 ions/cm2. For InP(100) implanted with 100 keV Ar+-ions an increase in angle of incidence results in decrease of surface roughness. Surfaces of GaAs(100) and GaAs(111) implanted with normally incident 50 keV Ar+-ions show nanopits of density 3–4 × 109/cm2. The existing theories are applied to explain the formation of observed surface nanostructures.
Coverage-dependent self-organized ordering of Co- and Ti-silicide nanoislands along step-bunch edges of vicinal Si(111)
We report on self-organized solid-phase epitaxially grown Ti- and Co-silicide nanoislands ordered... more We report on self-organized solid-phase epitaxially grown Ti- and Co-silicide nanoislands ordered along step-bunch edges of a vicinal Si(111) surface, as a function of the initial metal (Ti,Co) coverage. While the degree of self-ordering (in terms of step decoration) for Co-silicide islands was higher than for Ti-silicide islands, both types of island had an inverse dependence of the order parameter on coverage. In addition, the positions of the island sites were found to be correlated, and affected by the step-island and island-island interactions, leading to their alignment along the step-bunch ⟨110⟩ directions, even for the terrace islands. Aberration-corrected high-resolution transmission electron microscopy was used to characterize the islands’ crystal structure and growth orientation. The above findings can be understood by the use of coarse-grain modeling.
Investigation of swift heavy ion induced mixing in metal/polymer systems
We have investigated swift heavy ion induced mixing in Ni/Teflon and Au/Teflon systems using 120 ... more We have investigated swift heavy ion induced mixing in Ni/Teflon and Au/Teflon systems using 120 MeV Au ions. Our earlier studies showed that the Ni/Teflon system exhibits strong interface mixing as determined by Rutherford backscattering spectrometry (RBS). In the present work, Ni/Teflon system is further studied by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Moreover, RBS studies are extended to demonstrate interface mixing in Au/Teflon system. The observed mixing is explained on the basis of chemical activeness (or reactivity) of the metal/polymer systems leading to chemical reaction. Chemical reaction occurs in the hot zone around the ions path between the Ni and reactive polymer species. Thermodynamic property bond dissociation energy has been taken into account for the reactivity of the systems.
Evidence of electronic growth in titanium- and cobalt-silicide islands
A possibility of growing highly mismatched and yet two-dimensional heteroepitaxial deposits, is b... more A possibility of growing highly mismatched and yet two-dimensional heteroepitaxial deposits, is both fascinating in terms of basic science and technologically important. So far, stabilization of flat morphology by a reduction of the overall electron energy in a quantum well (hence termed ‘‘electronic growth’’), has been observed exclusively in heteroepitaxy of simple metals, lead, and silver. This work shows, that a broader class of functional materials can be grown ‘‘electronically,’’ such as titaniumand cobalt-silicide nano-islands on Si(111), despite their more complex electronic structure.
Effects of thermal annealing on structural and magnetic properties of thin Pt/Cr/Co multilayers
Thermal stability of thin Pt/Cr/Co multilayers and the subsequent changes in their structural, ma... more Thermal stability of thin Pt/Cr/Co multilayers and the subsequent changes in their structural, magnetic, and magneto-optical properties are reported. We observe CoCrPt ternary alloy phase formation due to annealing at temperatures about 773 K, which is accompanied by enhancement in the coercivity value. In addition, 360 degree domain wall superimposed on a monodomain like background has been observed in the pristine multilayer, which changes into a multidomain upon annealing at 873 K.
Ion Beam Modification of Magnetic Thin Films Grown by Physical Vapor Deposition Techniques
This review reports on the changes in structural and magnetic properties of Pt/Cr/Co multilayers ... more This review reports on the changes in structural and magnetic properties of Pt/Cr/Co multilayers and Co/Pt bi- and multi-layers modified under wide range of ion-energy, -species, and -fluences. We observe irradiation induced CoCrPt ternary alloy phase formation and CoPt ordered/disordered phase formation for Pt/Cr/Co and Co/Pt system, respectively. Phase formation is accompanied by an enhancement in the coercivity. These findings are explained in the light of ion beam induced recoil mixing and ionization events using TRIDYN_FZD and Monte Carlo SRIM simulations.
Phase formation and changes in magnetic property of Co/Cr/ Pt multilayers
We report on CoCrPt ternary alloy phase formation and changes in magnetic properties of Co/Cr/Pt ... more We report on CoCrPt ternary alloy phase formation and changes in magnetic properties of Co/Cr/Pt multilayers due to 1 MeV N+ ion irradiation at room temperature. Irradiation induced changes occurring across the Co/Cr/Pt interfaces are studied by X-ray reflectivity, glancing angle X-ray diffraction, and magneto optical Kerr effect measurements. Irradiation induced mixing across the interface shows an enhancement in the coercivity, sufficient coercivity squareness and magnetic anisotropy. The results are discussed in the realm of ion beam induced recoil mixing across the interfaces.
Modifications in structural and optical properties of Mn-ion implanted CdS thin films
In this paper, we report on modifications in structural and optical properties of CdS thin films ... more In this paper, we report on modifications in structural and optical properties of CdS thin films due to 190 keV Mn-ion implantation at 573 K. Mn-ion implantation induces disorder in the lattice, but does not lead to the formation of any secondary phase, either in the form of metallic clusters or impurity complexes. The optical band gap was found to decrease with increasing ion fluence. This is explained on the basis of band tailing due to the creation of localized energy states generated by structural disorder. Enhancement in the Raman scattering intensity has been attributed to the enhancement in the surface roughness due to increasing ion fluence. Mn-doped samples exhibit a new band in their photoluminescence spectra at 2.22 eV, which originates from the d–d (4T1 → 6A1) transition of tetrahedrally coordinated Mn2+ ions.
Ion induced modification in structural and magnetic properties of Pt/Cr/Co multilayers
This paper reports on the change in the magnetic and the structural properties of Pt/Cr/Co multil... more This paper reports on the change in the magnetic and the structural properties of Pt/Cr/Co multilayers due to 1 MeVN+ -ion irradiation at room temperature.Weobserve irradiation induced formation of the CoCrPt ternary alloy phase at a fluence of 1E16 ions/ cm2. Phase formation is accompanied by an enhancement in the coercivity. The enhancement in the coercivity is attributed to inhomogeneous alloying and possible
mixing-induced strain. These findings are explained in the light of ion beam induced recoil mixing and ionization events.
Structural and optical properties of Mn-doped CdS thin films prepared by ion implantation
We report on structural and optical properties of Mn-doped CdS thin films prepared by 190 keV Mn-... more We report on structural and optical properties of Mn-doped CdS thin films prepared by 190 keV Mn-ion implantation at different temperatures. Mn-ion implantation in the fluence range of 1E13–1E16 ions cm−2 does not lead to the formation of any secondary phase. However, it induces structural disorder, causing a decrease in the optical band gap. This is addressed on the basis of band tailing due to creation of localized energy states and Urbach energy calculations. Mn-doped samples exhibit a new band in their photoluminescence spectra at 2.22 eV, which originates from the d-d (4T1→6A1) transition of tetrahedrally coordinated Mn2+ ions.
Modification of the magnetic and the structural properties of Pt/Cr/Co multilayers by He+ ion irradiation
We report on the effects of 2 MeV He+-ion irradiation on the magnetic and structural properties o... more We report on the effects of 2 MeV He+-ion irradiation on the magnetic and structural properties of Pt/Cr/Co multilayers. We observe He+ -ion irradiation leads to mixing across the interfaces [Pt (2.5 nm)/Cr(0.8 nm)/Co (3.0 nm)] x 6/Si multilayers. In addition, we observe Co–Cr–Pt phase formation at the highest fluence of 5.5 E16 ions / cm2. This is accompanied by an enhancement in the coercivity. Such enhancement in the coercivity is attributed to inhomogeneous alloying and a possible mixing-induced strain. High-resolution transmission electron microscopy confirms the formation of CoCrPt ternary alloy phase. These findings are explained in the light of ion beam induced recoil mixing and ionization events.
Ion irradiation induced nano granular magnetic Fe5Si3 silicide phase formation in Fe/Si structures
In this work we report on swift heavy ion irradiation induced nano granular magnetic metallic Fe5... more In this work we report on swift heavy ion irradiation induced nano granular magnetic metallic Fe5Si3 silicide phase formation due to strong interfacial intermixing in Fe/Si bilayer samples. The bilayer samples have been irradiated with swift (100 MeV) heavy (56Fe7+) ions having a fluence of 1 × 1014 ions cm−2. It has been found that swift heavy ion causes strong irradiation induced interfacial intermixing and forms nano granular magnetic metallic Fe5Si3 silicide in Fe/Si bilayer samples, which shows strong uniform magnetic interaction and interacting magnetic particle behaviour at room temperature (RT).
Effect of ion beam irradiation on magnetic and structural properties of Pt/Cr/Co multilayers
This paper discusses the effect of ion beam irradiation on the magnetic and structural properties... more This paper discusses the effect of ion beam irradiation on the magnetic and structural properties of Pt/Cr/Co multilayers. We observe Co–Cr–Pt ternary alloy phase formation in 1 MeV N+ ion irradiated [Pt (2.5 nm)/Cr (0.8 nm)/Co (3.0 nm)]×6/Si multilayers for a fluence of 1 × 1016 ions cm−2 and beyond. The observed phase formation is accompanied by an enhancement in the average grain size, surface roughness and coercivity. Monte Carlo simulation has been performed to study ion-induced defect evolution and atomic displacements to correlate the above observed effects.
Tuning magnetic properties of Co/Pt thin films using energetic ions
In this paper, we present the changes occurring in Co/Pt bi- and multi-layer thin films modified ... more In this paper, we present the changes occurring in Co/Pt bi- and multi-layer thin films modified under wide range of ion energy and species and fluence. We have shown the possibility of achieving controlled tuning of magnetic properties of the Co/Pt thin film system. Monte Carlo simulation results for ion-induced atomic displacements were used to explain the observed effects of ion-irradiation interface mixing across the Co/Pt interfaces. Phase formation has been explained in the light of heat of formation rule. On the other hand, we propose that ion induced point defect clustering governs the changes occurring in the structural and the magnetic properties.
MAGNETIC BEHAVIOR OF NANOGRANULAR SILICIDE PHASE (FORMED DUE TO SWIFT HEAVY Fe7+ ION IRRADIATION-INDUCED INTERMIXING)
Fe films (of ∼50 nm) on p-silicon substrates have been deposited by electron beam evaporation tec... more Fe films (of ∼50 nm) on p-silicon substrates have been deposited by electron beam evaporation technique to realize Fe/pSi devices. The devices have been irradiated from 100 MeV swift heavy ions of Fe7+ with a fluence of 1014 ions-cm−2. The morphological and structural characterization have been done from SEM, AFM and XRD facilities. Magnetic behavior has been studied from vibrating sample magnetometer (VSM) and SQUID facilities. SEM and AFM studies show the formation of a nanogranular structure. Further, XRD data has shown the formation of intermixed magnetic nanogranular silicide phase (Fe5Si3) having average grain size of 25 nm. M–H
and ZFC (zero field-cooled)/FC (field-cooled) studies show the magnetic behavior of interacting magnetic particles. The observed results have been understood as a typical behavior of
interacting magnetic nanograins.
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Books by Jitendra Kumar Tripathi
Papers by Jitendra Kumar Tripathi
mixing-induced strain. These findings are explained in the light of ion beam induced recoil mixing and ionization events.
and ZFC (zero field-cooled)/FC (field-cooled) studies show the magnetic behavior of interacting magnetic particles. The observed results have been understood as a typical behavior of
interacting magnetic nanograins.