arXiv subjects

Mukul Gupta

Publications and source records attributed to Mukul Gupta.

At least 37 records · Page 2Linked to original sources

arXiv2019

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

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 Mössbauer 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.

cond-mat.mtrl-sci↗

arXiv2019

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

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.

cond-mat.mtrl-sci↗

arXiv2019

Interface Sharpening in Miscible and Isotopic Multilayers: Role of Short-Circuit Diffusion

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 inter-diffusivities 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 [Z. Erdélyi et al., Science 306, 1913 (2004).]. General nature of the effect is conclusively demonstrated using isotopic multilayers of 57Fe/naturalFe, 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 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.

cond-mat.mtrl-sci↗

arXiv2019

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

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.

cond-mat.mtrl-sci↗

arXiv2019

Finding pathways for stoichiometric Co4N thin films

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 ($ϕ$3\,inch). By reducing the target size to $ϕ$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.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure by X-ray absorption spectroscopy and observation offield induced unusually slowspin relaxation from magnetic properties in pyrochlore Eu$_{2-x}$Fe$_x$Ti$_2$O$_7$

X-ray absorption spectroscopy (XAS) as well as x-ray magnetic circular dichroism (XMCD) and magnetization of hybrid pyrochlore Eu2-xFexTi2O7 were investigated, where the rare earth Eu (4f) was replaced with transition metal Fe (3d) to introduce competing 4f-3d interactions. It is confirmed that the valence states of Eu and Fe ions are formally trivalent while that of Ti ions are tetravalent (3d0). The analysis yielded that the tetravalent Ti ions occupy octahedral sites with distorted Oh symmetry which is triggered by the presence of vacant 8a anionic site adjacent to TiO6 octahedra. Further study with Fe doping revealed that it essentially reduces the octahedral distortion by introducing anionic disorder (migration of 48f oxygen ions to 8a site). Analysis of O K edge XAS spectra further confirmed the Fe substitution causing the systematic change in the ligand (O2-) coordination of the Ti4+ cations. On the other hand, a new field induced transition (with Fe doping) at low temperature T* (4 K<T*< 8 K) in ac susceptibility with unusually slow spin relaxation was observed. The transition shifted towards higher temperatures both with increasing applied field and Fe concentration. However, the single ion spin freezing (Tf ~35 K) appears to be suppressed with Fe substitution. Interestingly, small amount of Fe3+ ion substitution showed significant enhancement in the dc magnetization at lower temperatures (<100 K). Analysis further indicated rise of dipolar FM exchange interaction with Fe doping.

cond-mat.mtrl-sci↗

arXiv2018

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

Interfacial reaction between iron and MgO has been studied in situ during deposition of iron on MgO surface, using soft x-ray absorption spectroscopy. High sensitivity of the technique which is capable of detecting even a sub monolayer of a given phase, combined with in situ measurements as a function of iron layer thickness, allowed one to make a quantitative estimate of interfacial phases. Two different substrates namely, MgO (001) 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 is 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 (001) surface, FeO is formed, suggesting that the reaction is limited by the availability of oxygen atoms. Magnetic behaviour of the FeO layer gets modified significantly due to proximity effect of the bulk ferromagnetic iron layer.

cond-mat.mtrl-sci↗

arXiv2018

Density and Microstructure of Amorphous Carbon Thin Films

In this work, we studied amorphous carbon ($a$-C) thin films deposited using direct current (dc) and high power impulse magnetron sputtering (HiPIMS) techniques. The microstructure and electronic properties reveal subtle differences in $a$-C thin films deposited by two techniques. While, films deposited with dcMS have a smooth texture typically found in $a$-C thin films, those deposited with HiPIMS consist of dense hillocks surrounded by a porous microstructure. The density of $a$-C thin films is a decisive parameter to judge their quality. Often, x-ray reflectivity (XRR) has been used to measure the density of carbon thin films. From the present work, we find that determination of density of carbon thin films, specially those with a thickness of few tens of nm, may not be accurate with XRR due to a poor scattering contrast between the film and substrate. By utilizing neutron reflectivity (NR) in the time of flight mode, a technique not commonly used for carbon thin films, we could accurately measure differences in the densities of $a$-C thin films deposited using dcMS and HiPIMS.

cond-mat.mtrl-sci↗

arXiv2017

Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory

The electronic structure of double perovskite Pr2MnNiO6 is studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 2+ and 4+ states respectively. Using charge transfer multiplet analysis of Ni and Mn 2p XPS spectra, we find charge transfer energies Δ of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni2+ and Mn4+ reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the atomic values of 8.00 and 3.00 respectively thereby indicating the covalent nature of the system. The O 1s edge absorption spectra reveal a band gap of 0.9 eV which is comparable to the value obtained from first principle calculations for U-J >= 2 eV. The density of states clearly reveal a strong p-d type charge transfer character of the system, with band gap proportional to average charge transfer energy of Ni2+ and Mn4+ ions.

cond-mat.str-el↗

arXiv2017

On the origin of Blue Luminescence in Mg doped GaN

We uncover the origin of blue luminescence (BL) peak in Mg doped GaN thin film using a combination of experimental X-ray absorption near edge spectroscopy (XANES), first-principles calculations based on density functional theory and full multiple scattering theoretical analysis of various possible defect complexes and their XANES signatures. We demonstrate that a defect complex composed of Mg substituted at Ga site (MgGa) and Mg at interstitial site (Mgi) is primarily responsible for the observed BL by Donor-Acceptor Pair transition (DAP) associated with a deep donor state in the gap. It correlates with a higher (lower) oxidation state of N (Ga) in heavily Mg doped GaN than in its pristine structure, evident in our experiments as well as calculations. Physical and chemical mechanisms identified here point out a route to achieving efficient p-type GaN.

cond-mat.mtrl-sci↗

arXiv2017

Structure and Magnetization of Co4N Thin Film

In this work, we studied the local structure and the magnetization of Co4N thin films deposited by a reactive dc magnetron sputtering process. The interstitial incorporation of N atoms in a fcc Co lattice is expected to expand the structure and such expansion yields interesting magnetic properties characterized by a larger than Co magnetic moment and a very high value of spin polarization ratio in Co4N. By optimizing the growth conditions, we prepared Co4N film having lattice parameter close to its theoretically predicted value. The N concentration was measured using secondary ion mass spectroscopy. Detailed magnetization measurements using bulk magnetization method and polarized neutron reflectivity confirm that the magnetic moment of Co in Co4N is higher than that of Co.

cond-mat.mtrl-sci↗

arXiv2016

GaN Nanowall Network: A new possible route to obtain efficient p-GaN and enhanced light extraction

We demonstrate that GaN formed in a Nanowall Network (NwN) morphology can overcome fundamental limitations in optoelectronic devices, and enable high light extraction and effective Mg incorporation for efficient p-GaN. We report the growth of Mg doped GaN Nanowall network (NwN) by plasma assisted molecular beam epitaxy (PA-MBE) that is characterized by Photoluminescence (PL) spectroscopy, Raman spectroscopy, high-resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS) and Secondary ion mass spectroscopy (SIMS). We record a photo-luminescence enhancement ($ \approx $3.2 times) in lightly doped GaN as compared to that of undoped NwN. Two distinct (and broad) blue luminescence peaks appears at 2.95 and 2.7 eV for the heavily doped GaN (Mg $>10^{20}$ atoms $cm^{-3}$), of which the 2.95 eV peak is sensitive to annealing is observed. XPS and SIMS measurements estimate the incorporated Mg concentration to be $10^{20}$ atoms $cm^{-3}$ in GaN NwN morphology, while retaining its band edge emission at $\approx$ 3.4 eV. A higher Mg accumulation towards the GaN/Al$_2$O$_3$ interface as compared to the surface was observed from SIMS measurements.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic instability and $f-d$ hybridization in CeFe$_2$ on substituting Cr, Ag and Au for Fe

Hybridization between Ce $f$ and conduction $d$ states has been speculatively known to be one of the mechanisms responsible for magnetic instability of the ferromagnetic ground state of CeFe$_2$. Substituting Fe by small amounts of certain elements stabilizes it to an antiferromagnetic state below the Curie temperature via a first-order second phase transition. In the present work, we seek any direct relation between the $f-d$ hybridization and the second transition by measuring primarily dc magnetization and Ce M$_{4,5}$ edge x-ray absorption spectra of Ce(Fe$_{1-x}$M$_x$)$_2$ pseudobinaries, with M = Cr, Ag and Au. X-ray diffraction and x-ray photoelectron spectroscopy measurements are also performed essentially to monitor the quality of the samples. Whereas Cr impurity is found to cause the second transition, Ag and Au apparently do not induce any. In the former, the Curie and second transition temperatures vary systematically, but differently, with $x$. Our results imply that there is a definite proportionality between the $x$ dependences of the second transition temperature and the $f-d$ hybridization strength estimated qualitatively from the absorption spectra.

cond-mat.str-el↗

arXiv2016

Structural and magnetic properties of Co-N thin films deposited using magnetron sputtering at 523 K

In this work, we studied cobalt nitride (Co-N) thin films deposited using a dc magnetron sputtering method at a substrate temperature (\Ts) of 523\,K. We find that independent of the reactive gas flow (\pn) used during sputtering, the phases of Co-N formed at this temperature seems to be identical having N \pat~$\sim$5. This is contrary to Co-N phases formed at lower \Ts. For \Ts$\sim$300\,K, an evolution of Co-N phases starting from Co(N)$\rightarrow$\tcn$\rightarrow$Co$_3$N$\rightarrow$CoN can be seen as \pn increases to 100\p, whereas when the substrate temperature increases to 523\,K, the phase formed is a mixture of Co and \tcn, independent of the {\pn} used during sputtering. We used x-ray diffraction (XRD) to probe long range ordering, x-ray absorption spectroscopy (XAS) at Co absorption edge for the local structure, Magneto-optical Kerr e ffect (MOKE) and polarized neutron reflectivity (PNR) to measure the magnetization of samples. Quantification of N \pat~was done using secondary ion mass spectroscopy (SIMS). Measurements suggest that the magnetic moment of Co-N samples deposited at 523\,K is slightly higher than the bulk Co moment and does not get affected with the \pn~used for reactive sputtering. Our results provide an important insight about the phase formation of Co-N thin films which is discussed in this work.

cond-mat.mtrl-sci↗

arXiv2015

Phase formation, thermal stability and magnetic moment of cobalt nitride thin films

Cobalt nitride (Co-N) thin films prepared using a reactive magnetron sputtering process by varying the relative nitrogen gas flow (\pn) are studied in this work. As \pn~increases, Co(N), \tcn, Co$_3$N and CoN phases are formed. An incremental increase in \pn, after emergence of \tcn~phase at \pn=10\p, results in a continuous expansion in the lattice constant ($a$) of \tcn. For \pn=30\p, $a$ maximizes and becomes comparable to its theoretical value. An expansion in $a$ of \tcn, results in an enhancement of magnetic moment, to the extent that it becomes even larger than pure Co. Though such higher (than pure metal) magnetic moment for Fe$_4$N thin films have been theoretically predicted and evidenced experimentally, higher (than pure Co) magnetic moment are evidenced in this work and explained in terms of large-volume high-moment model for tetra metal nitrides.

cond-mat.mtrl-sci↗

arXiv2015

Effect of Al doping on phase formation and thermal stability of iron nitride thin films

In the present work, we systematically studied the effect of Al doping on the phase formation of iron nitride (Fe-N) thin films. Fe-N thin films with different concentration of Al (Al=0, 2, 3, 6, and 12 at.%) were deposited using dc magnetron sputtering by varying the nitrogen partial pressure between 0 to 100%. The structural and magnetic properties of the films were studied using X-ray diffraction and polarized neutron reflectivity. It was observed that at the lowest doping level (2 at.% of Al), nitrogen rich non-magnetic Fe-N phase gets formed at a lower nitrogen partial pressure as compared to the un-doped sample. Interestingly, we observed that as Al doping is increased beyond 3at.%, nitrogen rich non-magnetic Fe-N phase appears at higher nitrogen partial pressure as compared to un-doped sample. The thermal stability of films were also investigated. Un-doped Fe-N films deposited at 10% nitrogen partial pressure possess poor thermal stability. Doping of Al at 2at.% improves it marginally, whereas, for 3, 6 and 12at.% Al doping, it shows significant improvement. The obtained results have been explained in terms of thermodynamics of Fe-N and Al-N.

cond-mat.mtrl-sci↗

arXiv2015

The origin of anomalous diffusion in iron mononitride thin films

We have studied the origin of a counter intuitive diffusion behavior of Fe and N atoms in a iron mononitride (FeN) thin film. It was observed that in-spite of a larger atomic size, Fe tend to diffuse more rapidly than smaller N atoms. This only happens in the N-rich region of Fe-N phase diagram, in the N-poor regions, N diffusion coefficient is orders of magnitude larger than Fe. Detailed self-diffusion measurements performed in FeN thin films reveal that the diffusion mechanism of Fe and N is different - Fe atoms diffuse through a complex process, which in addition to a volume diffusion, pre-dominantly controlled by a fast grain boundary diffusion. On the other hand N atoms diffuse through a classical volume-type diffusion process. Observed results have been explained in terms of stronger Fe-N (than Fe-Fe) bonds generally predicted theoretically for mononitride compositions of transition metals.

cond-mat.mtrl-sci↗

arXiv2014

Study of magnetic iron nitride thin films deposited by high power impulse magnetron sputtering

In this work, we studied phase formation, structural and magnetic properties of iron-nitride (Fe-N) thin films deposited using high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (dc-MS). The nitrogen partial pressure during deposition was systematically varied both in HiPIMS and dc-MS. Resulting Fe-N films were characterized for their microstructure, magnetic properties and nitrogen concentration. We found that HiPIMS deposited Fe-N films show a globular nanocrystalline microstructure and improved soft magnetic properties. In addition, it was found that the nitrogen reactivity impedes in HiPIMS as compared to dc-MS. Obtained results can be understood in terms of distinct plasma properties of HiPIMS.

cond-mat.mtrl-sci↗