Showing papers by "Mukul Gupta published in 2019"

3D Hierarchical Boron-Doped Diamond-Multilayered Graphene Nanowalls as an Efficient Supercapacitor Electrode

Abstract: Synthesis of stable hybrid carbon nanostructure for high-performance supercapacitor electrode with long life-cycle for electronic and energy storage devices is a real challenge. Here, we present a ...

Rigid-band electronic structure of scandium nitride across the n -type to p -type carrier transition regime

Abstract: Intentional doping and unintentional impurities in intrinsic semiconductors generate carriers that enable device operations. Fundamental to the electronic activity of dopants and impurities is the introduction of defect states inside the forbidden energy gap of semiconductors having shallow and/or deep characteristics, which fundamentally define the ability to engineer its physical properties and associated device performance. Here we demonstrate that unintentional electron doping by oxygen (${\mathrm{O}}_{\mathrm{N}}$) impurities and intentional magnesium hole doping ($\mathrm{M}{\mathrm{g}}_{\mathrm{Sc}}$) in scandium nitride (ScN) do not introduce any defect states inside its fundamental bandgap and that the rigid-band electronic structure remains unchanged. Employing a combination of spectroscopic techniques as well as first-principles density functional theory analysis, we show that the ${\mathrm{O}}_{\mathrm{N}}$ and $\mathrm{M}{\mathrm{g}}_{\mathrm{Sc}}$ defect states in ScN are located inside the bands, which leaves behind the virgin ScN bandgap as well as the valence and conduction band edges that are important for electronic transport. The rigid-band electronic structure of ScN with respect to the electron and hole doping results in high electron and hole concentrations due to the free movement of Fermi level and results in tunable electronic and thermoelectric properties necessary for device applications.

Structural and magnetic properties of stoichiometric Co 4 N epitaxial thin films

Abstract: In this work, we measured N self-diffusion in the Co-N system and found an unexpected result that N diffuses out almost completely around 500 K, leaving behind fcc Co irrespective of the amount of N used to deposit Co-N. On the other hand, in previous attempts the ${\mathrm{Co}}_{4}\mathrm{N}$ phase has always been grown at 550 K or above. In view of our finding, it appears that fcc Co could have been mistaken for ${\mathrm{Co}}_{4}\mathrm{N}$, probably due to the closeness of their lattice parameters (LP; fcc Co = 3.54 $\AA{}$, ${\mathrm{Co}}_{4}\mathrm{N}$= 3.74 $\AA{}$). Therefore, ${\mathrm{Co}}_{4}\mathrm{N}$ -- an interesting material for its high spin-polarization ratio and high magnetic moment remained unexplored. By bringing down the growth temperature, we report the growth of stoichiometric ${\mathrm{Co}}_{4}\mathrm{N}$ epitaxial thin films. Films were grown using a direct current reactive magnetron sputtering process on ${\mathrm{LaAlO}}_{3}$ (LAO mismatch $1.4%$) and MgO (mismatch $11.3%$) substrates and their structural and magnetic properties were studied. Precise magnetic moment (${M}_{s}$) of ${\mathrm{Co}}_{4}\mathrm{N}$ samples were measured using polarized neutron reflectivity and compared with bulk magnetization results. We found that the ${M}_{s}$ of ${\mathrm{Co}}_{4}\mathrm{N}$ is higher due to a magnetovolume effect. Unlike previous findings, we observed that substrates induce misfit strain and strain inhomogeneity is the cause of modifications in magnetic ensemble such as coercivity, saturation magnetization, and magnetic anisotropy. A consequence of incoherent strain present in our samples is also reflected in the magnetic anisotropy leading to a superposition of strong fourfold and a small fraction of uniaxial magnetic anisotropy. Obtained results are presented and discussed in this work.

Effect of interfacial interdiffusion on magnetism in epitaxial Fe 4 N films on LaAlO 3 substrates

Abstract: Epitaxial ${\mathrm{Fe}}_{4}\mathrm{N}$ thin films grown on ${\mathrm{LaAlO}}_{3}$ (LAO) substrate using sputtering and molecular beam epitaxy techniques have been studied in this work. Within the sputtering process, films were grown with conventional direct current magnetron sputtering (dcMS) and using a high power impulse magnetron sputtering (HiPIMS) process. Surface morphology and depth profile studies on these samples reveal that HiPIMS deposited film has the lowest roughness, the highest packing density, and the sharpest interface. We found that the substrate-film interface and the microstructure play a vital role in affecting the electronic hybridization and magnetic properties of ${\mathrm{Fe}}_{4}\mathrm{N}$ films. La from the LAO substrate and Fe from the film interdiffuse and form an undesired interface. The magnetic moment (${M}_{s}$) was compared using bulk, element-specific and magnetic depth profiling techniques. We found that ${M}_{s}$ was the highest when the thickness of the interdiffused layer was lowest and such conditions can only be achieved in the HiPIMS grown samples. The presence of a small moment at the N site was also evidenced by element-specific x-ray circular dichroism measurement in the HiPIMS grown sample. A large variation in the ${M}_{s}$ values of ${\mathrm{Fe}}_{4}\mathrm{N}$ films found in the experimental works carried out so far could be due to such an interdiffused layer which is generally not expected to form in otherwise stable oxide substrate at a low substrate temperature $\ensuremath{\approx}675\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In addition, a consequence of substrate-film interdiffusion and microstructure resulted in the different kinds of magnetic anisotropies in ${\mathrm{Fe}}_{4}\mathrm{N}$ films grown using different techniques. A detailed investigation of the substrate-film interface and microstructure on the magnetization of ${\mathrm{Fe}}_{4}\mathrm{N}$ film is presented and discussed in this work.

In situ soft x-ray absorption spectroscopic study of polycrystalline Fe/MgO interfaces.

Abstract: Interfacial interactions between a layer of iron and MgO hold the key to various phenomena like tunnel magnetoresistance, perpendicular magnetic anisotropy, interlayer exchange coupling, observed in the system. Interface structure has been studied in situ during deposition of iron on MgO surface, using soft x-ray absorption spectroscopy (SXAS). Sub-monolayer sensitivity of SXAS, combined with in situ measurements as a function of iron layer thickness, allowed one to study the evolution of interface with film thickness. Two different substrates namely MgO (0 0 1) single crystal, and a polycrystalline MgO film on Si substrate have been used in order to elucidate the role of the state of MgO surface in controlling the interface structure. It is found that at the interface of iron and MgO film, about two monolayers of Fe3O4 are formed. Fe3O4 being the oxide of iron with the highest heat of formation, the reaction appears to be controlled thermodynamically. On the other hand, on the interface with MgO (0 0 1) surface, FeO is formed, suggesting that the reaction is limited by the availability of oxygen atoms. Magnetic behavior of the FeO layer gets modified significantly due to proximity effect of the bulk ferromagnetic iron layer.

3D Hierarchical Boron-Doped Diamond-Multilayered Graphene Nanowalls as an Efficient Supercapacitor Electrode

Abstract: Synthesis of stable hybrid carbon nanostructure for high-performance supercapacitor electrode with long life-cycle for electronic and energy storage devices is a real challenge. Here, we present a one-step synthesis method to produce conductive boron-doped hybrid carbon nanowalls (HCNWs), where sp²-bonded graphene has been integrated with and over a three-dimensional curved wall-like network of sp³-bonded diamond. The spectroscopic studies such as X-ray absorption, Raman, and X-ray photoelectrons clearly reveal the coexistence of diamond and graphene in these nanowalls, while the detailed transmission electron microscopy studies confirm the unique microstructure where a diamond nanowall is encased by a multilayered graphene. Interestingly, these HCNWs yield a high double layer capacitance value of 0.43 mF cm–² and electrode retention of 98% over 10 000 cycles of charging/discharging in 1 M Na₂SO₄ electrolyte. The remarkable supercapacitive performance can be attributed to the 3D interconnected network of diamond nanowalls surrounded by highly conducting graphene.

Finding pathways for stoichiometric Co4N thin films

Abstract: In this work, we studied the pathways for formation of stoichiometric \tcn~thin films. Polycrystalline and epitaxial \tcn~films were prepared using reactive direct current magnetron (dcMS) sputtering technique. A systematic variation in the substrate temperature (\Ts) during the dcMS process reveals that the lattice parameter (LP) decreases as \Ts~increases. We found that nearly stoichiometric \tcn~films can be obtained when \Ts~= 300\,K. However, they emerge from the transient state of Co target ($\phi$3\,inch). By reducing the target size to $\phi$1\,inch, now the \tcn~phase formation takes place from the metallic state of Co target. In this case, LP of \tcn~film comes out to be $\sim$99\p~of the value expected for \tcn. This is the largest value of LP found so far for \tcn. The pathways achieved for formation of polycrystalline \tcn~were adopted to grow an epitaxial \tcn~film, which shows four fold magnetic anisotropy in magneto-optic Kerr effect measurements. Detailed characterization using secondary ion mass spectroscopy indicates that N diffuses out when \Ts~is raised even to 400\,K. Measurement of electronic structure using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy further confirms it. Magnetization measurements using bulk magnetization and polarized neutron reflectivity show that the saturation magnetization of stoichiometric \tcn~film is even larger than pure Co. Since all our measurements indicated that N could be diffusing out, when \tcn~films are grown at high \Ts, we did actual N self-diffusion measurements in a CoN sample and found that N self-diffusion was indeed substantially higher. The outcome of this work clearly shows that the \tcn~films grown prior to this work were always N deficient and the pathways for formation of a stoichiometric \tcn~have been achieved.

In-situ growth of iron mononitride thin films studied using x-ray absorption spectroscopy and nuclear resonant scattering

Abstract: We studied the structural and magnetic properties of in-situ grown iron mononitride (FeN) thin films. Initial stages of film growth were trapped utilizing synchrotron based soft x-ray absorption near edge spectroscopy (XANES) at the N K-edge and nuclear resonant scattering (NRS). Films were grown using dc-magnetron sputtering, separately at the experimental stations of SXAS beamline (BL01, Indus 2) and NRS beamline (P01, Petra III). It was found that the initial stages of film growth differs from the bulk of it. Ultrathin FeN films, exhibited larger energy separation between the t2g and eg features and an intense eg feature in the N K-edge pattern. This indicates that a structural transition is taking place from the rock-salt (RS)-type FeN to zinc-blende (ZB)-type FeN when the thickness of films increases beyond 5nm. The behavior of such N K-edge features correlates very well with the emergence of a magnetic component appearing in the NRS pattern at 100K in ultrathin FeN films. Combining the in-situ XANES and NRS measurements, it appears that initial FeN layers grow in a RS-type structure having a magnetic ground state. Subsequently, the structure changes to ZB-type which is known to be non-magnetic. Observed results help in resolving the long standing debate about the structure and the magnetic ground state of FeN.

Depth-resolved compositional analysis of W/B4C multilayers using resonant soft X-ray reflectivity.

Abstract: W/B4C multilayers (MLs) consisting of ten layer pairs with varying boron carbide layer thicknesses have been investigated. The ML structures were characterized using grazing-incidence hard X-ray reflectivity (GIXR), resonant soft X-ray reflectivity (RSXR), hard X-ray photoelectron spectroscopy (HAXPES) and X-ray absorption near-edge spectroscopy (XANES). Depth-resolved spectroscopic information on the boron carbide layer in W/B4C MLs was extracted with sub-nanometre resolution using reflectivity performed in the vicinity of the B K-edge. Interestingly, these results show that the composition of boron carbide films is strongly dependent on layer thicknesses. HAXPES measurements suggest that most of the boron is in the chemical state of B4C in the multilayer structures. XANES measurements suggest an increase in boron content and C-B-C bonding with increase in boron carbide layer thickness.

Interface sharpening in miscible and isotopic multilayers: Role of short-circuit diffusion

Abstract: Atomic diffusion at nanometer length scale may differ significantly from bulk diffusion, and may sometimes even exhibit counterintuitive behavior. In the present work, taking Cu/Ni as a model system, a general phenomenon is reported which results in sharpening of interfaces upon thermal annealing, even in miscible systems. Anomalous x-ray reflectivity from a Cu/Ni multilayer has been used to study the evolution of interfaces with thermal annealing. Annealing at 423 K results in sharpening of interfaces by about 38%. This is the temperature at which no asymmetry exists in the interdiffusivities of Ni and Cu. Thus, the effect is very general in nature, and is different from the one reported in the literature, which requires a large asymmetry in the diffusivities of the two constituents [Erd\'elyi et al., Science 306, 1913 (2004)]. The general nature of the effect is conclusively demonstrated using isotopic multilayers of $^{57}\mathrm{Fe}/^{\mathrm{natural}}\mathrm{Fe}$, in which evolution of isotopic interfaces has been observed using nuclear resonance reflectivity. It is found that annealing at suitably low temperature (e.g., 523 K) results in the sharpening of the isotopic interfaces. Since chemically it is a single Fe layer, any effect associated with concentration dependent diffusivity can be ruled out. The results can be understood in terms of fast diffusion along short-circuit paths like triple junctions, which results in an effective sharpening of the interfaces at relatively low temperatures.

Structural and magnetic properties of CoNi surface alloys

Abstract: The Co 100 − x Ni x ( x = 20 , 40 & 60 ) surface alloys were deposited on Si (100) substrate by dc-magnetron sputtering technique and were investigated using Grazing Incidence X-ray Diffraction (GIXRD), X-ray reflectivity (XRR), Energy dispersive X-ray analysis (EDAX) and Magneto-Optical Kerr Effect (MOKE) measurements. GIXRD measurements suggested a compositional induced structural transition from hcp to fcc structure at x = 40. MOKE measurements indicate that the prepared alloys are soft ferromagnet with the coercive field in the range of few mT. The MOKE microscopy domain image suggests that magnetization reversal occurring through domain wall motion and coherent rotation. The angle-dependent coercivity (Hc) is numerically fitted with a two-phase model of magnetization reversal. Large anisotropy dispersion is observed for hcp Co–Ni alloys compared to fcc Co–Ni alloys. The hcp Co80Ni20 alloy has highest value of coercivity at 0° but for fcc Co60Ni40 and Co40Ni60 alloys, the coercivity became highest at 120°. The magnetization reversal along hard axis occurs through coherent rotation for fcc Co60Ni40 and Co40Ni60 alloys whereas in hcp Co80Ni20 alloy magnetization reversal is governed by formation of fine domains.

Structural and magnetic properties of FeN thin films grown on TiN

Abstract: We studied the growth of iron mononitride (FeN) thin film on a titanium mononitride (TiN) underlayer. FeN samples with a zinc blende (ZB)-type structure were deposited using reactive magnetron sputtering with and without the TiN underlayer on an amorphous quartz substrate. Combining x-ray diffraction, conversion electron Mossbauer spectroscopy, nuclear forward scattering and N K-edge x-ray absorption near edge spectroscopy measurements, we probed the long range and the local structure in our samples. FeN films are known to exhibit structural and magnetic imperfections. With the TiN underlayer such imperfections get reduced to a large extent. By studying films of different thicknesses, the origin of imperfections present in FeN films has turned out to be correlated with the film growth process. Obtained results are presented and discussed in this work.

Influence of interface and microstructure on magnetization of epitaxial Fe4N thin film

Abstract: Epitaxial Fe4N thin films grown on lattice-matched LaAlO3 (LAO) substrate using sputtering and molecular beam epitaxy techniques have been studied in this work. Within the sputtering process, films were grown with conventional direct current magnetron sputtering (dcMS) and for the first time, using a high power impulse magnetron sputtering (HiPIMS) process. Surface morphology and depth profile reveal that HiPIMS deposited film has the lowest roughness, the highest packing density and the sharpest interface. La from the LAO substrate and Fe from the film interdiffuse and forms an undesired interface spreading to an extent of about 10-20 nm. In the HiPIMS process, layer by layer type growth leads to a globular microstructure which restricts the extent of the interdiffused interface. Such substrate-film interactions and microstructure play a vital role in affecting the electronic hybridization and magnetic properties of Fe4N films. The magnetic moment (Ms) was compared using bulk, element-specific and magnetic depth profiling techniques. We found that Ms was the highest when the thickness of the interdiffused layer was lowest and only be achieved in the HiPIMS grown samples. Presence of small moment at the N site was also evidenced by element-specific x-ray circular dichroism measurement in HiPIMS grown sample. A large variation in the Ms values of Fe4N found in the experimental works carried out so far could be due to such interdiffused layer which is generally not expected to form in otherwise stable oxide substrate. In addition, a consequence of substrate-film interdiffusion and microstructure results in different kinds of different kind of magnetic anisotropies in films grown using different techniques.

Dynamics of reactive sputtering affecting phase formation of Co–N thin films

Abstract: Target poisoning is one of the major drawbacks of reactive sputtering (RS) affecting the stoichiometry of a compound film depositing onto a substrate. The dynamics of RS has been understood in terms of the well-known ‘Berg’s Model’ (Berg et al. in Thin Solid Films 565:186, 2014; Berg and Nyberg in Thin Solid Films 476(2):215, 2005) which demonstrates that by reducing the size of a sputter target, the drawbacks of RS can be minimized. In the present work, we compare the evolution of cobalt–nitride (Co–N) phases in an RS process from two different sized magnetron sources having diameter of 1 and 3 in. Samples were studied for their long-range ordering and local structure using X-ray diffraction and X-ray absorption spectroscopy. Quantification of N at.% was carried out using secondary ion mass spectroscopy. Polarized neutron reflectivity and bulk magnetization measurements were carried out to study the magnetic properties of Co–N thin film samples. We found that the N incorporation in Co–N films can be accelerated utilizing the smaller source. In addition, by reducing the target size from 3 to 1 in., the range of metallic state can be extended. The $${\mathrm {Co}}_{4}{\mathrm {N}}$$ phase emerging from the metallic state of the smaller target has a lattice parameter more closer to its theoretical value. An insight has been presented on the RS process and it can be readily seen from this work that the poisoning of the target can be minimized for the growth of the $${\mathrm {Co}}_{4}{\mathrm {N}}$$ film from the smaller target and can be understood in accordance with the available theoretical models.

Size induced structural changes in maricite-NaFePO4: an in-depth study by experiment and simulations

Abstract: Rechargeable batteries based on the most abundant elements, such as sodium and iron, have a great potential in the development of cost effective sodium ion batteries for large scale energy storage devices. We report, for the first time, crystallite size dependent structural investigations on maricite-NaFePO4 through X-ray diffraction, X-ray absorption spectroscopy and theoretical simulations. Rietveld refinement analysis on the X-ray diffraction data reveals that a decrease in the unit cell parameters leads to volume contraction upon reduction in the crystallite size. Further, the atomic multiplet simulations on X-ray absorption spectra provide unequivocally a change in the site symmetry of transition metal ions. The high resolution oxygen K-edge spectra reveal a substantial change in the bonding character with the reduction of crystallite size, which is the fundamental cause for the change in the unit cell parameters of maricite-NaFePO4. In parallel, we performed first-principles density functional theory (DFT) calculations on maricite-NaFePO4 with different sodium ion vacancy concentrations. The obtained structural parameters are in excellent agreement with the experimental observations on the mesostructured maricite-NaFePO4. The volumetric changes with respect to crystallite size are related to the compressive strain, resulting in the improvement in the electronic diffusivity. The nano-crystalline maricite-NaFePO4 with improved kinetics will open a new avenue for its usage as a cathode material in sodium ion batteries.

Direct synthesis of electrowettable nanostructured hybrid diamond

Abstract: The possibilities of precise control over the wetting characteristics of carbon-based hybrid nanostructures consisting of both sp2 and sp3 hybridized carbons using the electrowetting technique were demonstrated. An excellent polarity-dependent electrowetting behavior in the presence of an electrolyte followed an abrupt transition from the highly hydrophobic (contact angle ∼ 142°) Cassie–Baxter states to a hydrophilic (∼30°) Wenzel state, where diamond films acted as the anode. In addition, we also reported a remarkable transition from weakly hydrophobic to nearly superhydrophobic diamond nanostructures by chemical and morphological manipulations. The unique structural properties with precisely tailored morphology and surface roughness enabled such transitions on the nanostructured surface. This approach of preparing environmental stable hydrophobic surfaces with polarity-dependent wetting and precise control of the wetting mode transition could be used in numerous applications such as the electrochemical transport of liquids, supercapacitors, and low-friction microfluidics.

Magnetism and structure of in-situ grown FeN films studied using N K-edge XAS and nuclear resonance scattering

Abstract: We studied the structural and magnetic properties of \textit{in-situ} grown iron mononitride (FeN) thin films. Initial stages of film growth were trapped utilizing synchrotron based soft x-ray absorption near edge spectroscopy (XANES) at the N $K$-edge and nuclear resonant scattering (NRS). Films were grown using dc-magnetron sputtering, separately at the experimental stations of SXAS beamline (BL01, Indus 2) and NRS beamline (P01, Petra III). It was found that the initial stages of film growth differs from the bulk of it. Ultrathin FeN films, exhibited larger energy separation between the t$_{2g}$ and e$_g$ features and an intense e$_g$ feature in the N $K$-edge pattern. This indicates that a structural transition is taking place from the rock-slat (RS)-type FeN to zinc-blende(ZB)-type FeN when the thickness of films increases beyond 5\,nm. The behavior of such N $K$-edge features correlates very well with the emergence of a magnetic component appearing in the NRS pattern at 100\,K in ultrathin FeN films. Combining the \textit{in-situ} XANES and NRS measurements, it appears that initial FeN layers grow in RS-type structure having a magnetic ground state. Subsequently, the structure changes to ZB-type which is known to be non-magnetic. Observed results help in resolving the long standing debate about the structure and the magnetic ground state of FeN.

Influence of annealing on spin pumping in sputtered deposited Co/Pt bilayer thin films

Abstract: We report on the ferromagnetic resonance study of the Co/Pt thin films as a function of the post growth annealing. We found a significant increase in the spin mixing conductance, Gilbert damping parameter and inhomogeneous linewidth broadening with increase in annealing temperature. This behavior is correlated to the structural degradation of Co layer and in particular formation of CoxPt1−x alloy at the interface, which gives rise to increase in dissipation of spin current at the interface. The results are important for understanding of spin-orbit torque in Co/Pt bilayer structures.

Structural and magnetic properties of co-sputtered Fe0.8C0.2 thin films

Abstract: We studied the structural and magnetic properties of \FeC~thin films deposited by co-sputtering of Fe and C targets in a direct current magnetron sputtering (dcMS) process at a substrate temperature (\Ts) of 300, 523 and 773\,K. The structure and morphology was measured using x-ray diffraction (XRD), x-ray absorption near edge spectroscopy (XANES) at Fe $L$ and C $K$-edges and atomic/magnetic force microscopy (AFM, MFM), respectively. An ultrathin (3\,nm) $^{57}$\FeC~layer, placed between relatively thick \FeC~layers was used to estimate Fe self-diffusion taking place during growth at different \Ts~using depth profiling measurements. Such $^{57}$\FeC~layer was also used for $^{57}$Fe conversion electron Mossbauer spectroscopy (CEMS) and nuclear resonance scattering (NRS) measurements, yielding the magnetic structure of this ultrathin layer. We found from XRD measurements that the structure formed at low \Ts~(300\,K) is analogous to Fe-based amorphous alloy and at high \Ts~(773\,K), pre-dominantly a \tifc~phase has been formed. Interestingly, at an intermediate \Ts~(523\,K), a clear presence of \tefc~(along with \tifc~and Fe) can be seen from the NRS spectra. The microstructure obtained from AFM images was found to be in agreement with XRD results. MFM images also agrees well with NRS results as the presence of multi-magnetic components can be clearly seen in the sample grown at \Ts~= 523\,K. The information about the hybridization between Fe and C, obtained from Fe $L$ and C $K$-edges XANES also supports the results obtained from other measurements. In essence, from this work, experimental realization of \tefc~has been demonstrated. It can be anticipated that by further fine-tuning the deposition conditions, even single phase \tefc~phase can be realized which hitherto remains an experimental challenge.

Role of growth parameters on structural and magnetic properties of Fe4N thin films grown by reactive magnetron sputtering

Abstract: We studied the effect of growth parameters on the phase formation of Fe4N in a reactive sputtering process. The Fe4N phase formation was found to be extremely sensitive to N2 gas flow and sputter rate but not so much on the substrate temperature. But post-deposition annealing (PDA) time was found to be a critical parameter to get single phase Fe4N. As PDA time increases, the anti-bonding states characteristic of Fe4N start to diminish due to N out-diffusion. Signatures of such N diffusion process can also be seen from the Fe and N depth profiles. We found that the residual resistivity ratio was maximum for the optimized Fe4N films. Magnetization measurements were performed using bulk magnetization as well as polarized neutron reflectivity. We found that the magnetic moment of optimized Fe4N film was about 2.3 μB/atom at 15 K. Magnetic anisotropy measurements carried out using magneto optical-Kerr effect exhibit a rotatable anisotropy.

Magneto-optical Kerr effect and nuclear resonant scattering study of uni-directional anisotropy in hard-soft magnetic bilayers

Abstract: The present work reports the unconventional exchange bias (EB) phenomena in an exchange-coupled hard and soft magnetic bilayer system and the tunability of EB. The EB phenomena, i.e., shifting of the hysteresis loop of the soft (Fe) layer is observed when the hard magnetic ( L 1 0 FePt) layer is under the remanent state indicating the development of unidirectional anisotropy. The nuclear resonant scattering measurements clearly reveal the development of unidirectional anisotropy in the soft magnetic (Fe) layer, when the hard magnetic layer is under the remanent state. The magnetization reversal process is investigated by measuring two in-plane orthogonal components of magnetization, i.e., parallel ( M ∥) and perpendicular ( M ⊥) to the applied field using the magneto-optical Kerr effect (MOKE). When the magnetic field is applied parallel (antiparallel) to the biasing field direction, ( H SAT) magnetization reversal is nonuniform, and on the other hand, the rotation of magnetization is observed when the magnetic field is applied away from the H SAT direction. In addition, the sign inversion of the M ⊥ component is observed when the magnetic field is applied at the same angle on either side of the H SAT direction, which clearly imply the change in handedness of the chirality of spin structure during the magnetization reversal of the soft layer. Further, it is observed that the EB decreases with the increase of soft magnetic (Fe) layer thickness, demonstrating the tunable nature of EB phenomena even in these unconventional systems.The present work reports the unconventional exchange bias (EB) phenomena in an exchange-coupled hard and soft magnetic bilayer system and the tunability of EB. The EB phenomena, i.e., shifting of the hysteresis loop of the soft (Fe) layer is observed when the hard magnetic ( L 1 0 FePt) layer is under the remanent state indicating the development of unidirectional anisotropy. The nuclear resonant scattering measurements clearly reveal the development of unidirectional anisotropy in the soft magnetic (Fe) layer, when the hard magnetic layer is under the remanent state. The magnetization reversal process is investigated by measuring two in-plane orthogonal components of magnetization, i.e., parallel ( M ∥) and perpendicular ( M ⊥) to the applied field using the magneto-optical Kerr effect (MOKE). When the magnetic field is applied parallel (antiparallel) to the biasing field direction, ( H SAT) magnetization reversal is nonuniform, and on the other hand, the rotation of magnetization is obse...

Chemical analysis and non-linear optical properties of TiO2 thin films

Abstract: In the present work, TiO2 thin films were deposited on quartz substrate by ion beam sputtering method at varying oxygen partial pressure. The film deposition on the substrate was confirmed by x-ray photoelectron spectroscopy technique. From the results of closed aperture z-scan performed using He-Ne laser, the value of non-linear refractive indices were found decreasing with increase in partial pressure. Thermo-optic coefficient also decreased with increase in oxygen partial pressure, operated during the process of deposition.