Search

Multiferroic materials have undergone extensive research in the past two decades in an effort to produce a sizable room-temperature magneto-electric (ME) effect in either exclusive or composite materials for use in a variety of electronic or spintronic devices. These studies have looked into the ME effect by switching the electric polarization by the magnetic field or switching the magnetism by the electric field. Here, an innovative way is developed to knot the functional properties based on the tremendous modulation of electronics and magnetization by the electric field of the topotactic phase transitions (TPT) in heterostructures composed of metallic-magnet/TPT-material. It is divulged that application of a nominal potential difference of 2-3 Volts induces gigantic changes in magnetization by 100-250% leading to colossal Voltomagnetic effect, which would be tremendously beneficial for low-power consumption applications in spintronics. Switching electronics and magnetism by inducing TPT through applying an electric field requires much less energy, making such TPT-based systems promising for energy-efficient memory and logic applications as well as opening a plethora of tremendous opportunities for applications in different domains.

In recent years, photosensitive materials have been in huge demand because of their fascinating ability to convert absorbed photon energy to generate strain and henceforth tuning the physical properties. In this report we detect the photosensitive activity of SrMnO3. Using the power dependent and temperature dependent Raman study with different laser sources having wavelengths across the optical band gap of SrMnO3, we divulge the photosensitive character of SrMnO3 thin films. Upon laser light illumination, Raman modes soften and softening further increases with increase in laser power. Similar kind of mode variation is observed with increasing temperature at fixed laser power. XAS in presence of laser illumination, reveals the change in crystal field splitting associated with mode softening., Comment: 28,13

Na2IrO3 (NIO) is known to be a spin-orbit (SO) driven j=1/2 pseudo-spin Mott-Hubbard (M-H) insulator. However, the microscopic origin of the pseudo-spin state and the role of local structural distortions have not been clearly understood. Using a combination of theoretical calculations and x-ray spectroscopy, we show that the energetics in the vicinity of Fermi level (EF) is governed by SO interactions, electron correlation and local octahedral distortions. Contrary to the earlier understanding, here we show that the j=3/2 and 1/2 pseudo-spin states have admixture of both t2g and eg characters due to local structural distortion. Reduction of local octahedral symmetry also enables Ir 5d- O2p hybridization around the EF resulting in a M-H insulator with enhanced charge transfer character. The possibility of Slater insulator phase is also ruled out by a combination of absence of room temperature DoS in valence band spectra, calculated moments and temperature dependent magnetization measurements., Comment: 18 pages, 8 figures

Exchange bias (EB) shifts are commonly reported for the ferromagnetic (FM)/antiferromagnetic (AFM) bilayer systems. While stoichiometric ordered Sm$_{2}$NiMnO$_{6}$ (SNMO) and BaTiO$_{3}$ (BTO) are known to possesses FM and diamagnetic orderings respectively, here we have demonstrated the cooling field dependent EB and training effects in epitaxial SNMO/BTO/SNMO (SBS) heterostructure thin films. The polarized Raman spectroscopy and magnetometric studies reveal the presence of anti-site cation disorders in background of ordered lattice in SNMO layers, which introduces Ni-O-Ni or Mn-O-Mn local AFM interactions in long range Ni-O-Mn FM ordered host matrix. We have also presented growth direction manipulation of the degree of cation disorders in the SNMO system. Polarization dependent X-ray absorption measurements, duly combined with configuration interaction simulations suggest charge transfer from Ni/Mn 3\textit{d} to Ti 3\textit{d} orbitals through O 2\textit{p} orbitals across the SNMO/BTO (SB) interfaces, which can induce magnetism in the BTO spacer layer. The observed exchange bias in SBS heterostructures is discussed considering the pinning of moments due to exchange coupling at SB (or BTO/SNMO) sandwich interface., Comment: 11 pages, 3 figures

Coupling between different interactions allows to control physical aspects in multifunctional materials by perturbing any of the degrees of freedom. Here, we aim to probe the correlation among structural, electronic and magnetic observables of Sm$ _{2} $NiMnO$ _{6} $ ferromagnetic insulator double perovskite. Our employed methodology includes thermal evolution of synchrotron X-ray diffraction, near edge and extended edge hard X-ray absorption spectroscopy and bulk magnetometry. The magnetic ordering in SNMO adopts two transitions, at T$ _{C} $=159.6K due to ferromagnetic arrangement of Ni-Mn sublattice and at T$ _{d} $=34.1K because of anti-parallel alignment of polarized Sm paramagnetic moments with respect to Ni-Mn network. The global as well as local crystal structure of SNMO undergoes isostructural transitions across T$ _{C} $ and T$ _{d} $, observed by means of temperature dependent variation in Ni/Mn-O, Ni-Mn bonding characters and super exchange angle in Ni-O-Mn linkage. Hybridization between Ni, Mn 3\textit{d}, O 2\textit{p} electronic states is also modified in the vicinity of magnetic transition. On the other hand, the signature of Ni/Mn anti-site disorders are evidenced from local structure and magnetization analysis. The change in crystal environments governs the magnetic response by imposing alteration in metal - ligand orbital overlap. Utilizing these complimentary probes we have found that structural, electronic and magnetic states are inter-coupled in SNMO which makes it a potential platform for technological usage., Comment: 14 pages, 10 figures

The structural, electronic and magnetic properties of anti-site disordered Sm$ _{2} $NiMnO$ _{6} $ double perovskite has been studied. RE$_{2}$NiMnO$_{6}$ (RE: rare-earth) ordered double perovskite is commonly believed to show two distinct magnetic phase transitions viz, paramagnetic to ferromagnetic (FM) transition at T = T$ _{C} $ due to Ni-O-Mn super exchange interaction and another transition at T = T$ _{d} $ due to coupling of RE spins with Ni-Mn network. In our present study, we have observed that the presence of intrinsic B-site disorder results in an additional antiferromagnetic (AFM) coupling, mediated via Ni-O-Ni and Mn-O-Mn local bond pairs. As a consequence, the magnetic behavior of SNMO comprises of co-existing FM-AFM phases, which are respectively governed by the anti-site ordered and disordered structures. Field dependent inverted cusp like trend in M(T) and two step reversible loop behavior in M(H) measurements indicate the presence of competing FM-AFM phases over a wide range of temperature values (T$ _{d} < $ T $ < $ T$ _{C} $)., Comment: 8 pages, 3 figures

Ferromagnetic insulators (FM-Is) are the materials of interest for new generation quantum electronic applications. Here, we have investigated the physical observables depicting FM-I ground states in epitaxial Sm$ _{2} $NiMnO$ _{6} $ (SNMO) double perovskite thin films fabricated under different conditions to realize different level of Ni/Mn anti-site disorders (ASDs). The presence of ASDs immensely influence the characteristic magnetic and anisotropy behaviors in SNMO system by introducing short scale antiferromagnetic interactions in predominant long range FM ordered host matrix. Charge disproportion between cation sites in form of $ Ni^{2+}+Mn^{4+} \longrightarrow Ni^{3+}+Mn^{3+} $, causes mixed valency in both Ni and Mn species, which is found insensitive to ASD concentrations. Temperature dependent photo emission, photo absorption measurements duly combined with cluster model configuration interaction simulations, suggest that the eigenstates of Ni and Mn cations can be satisfactorily described as a linear combination of the unscreened $ d^{n} $ and screened $ d^{n+1} \underline{L} $ ($ \underline{L} $: O 2\textit{p} hole) states. The electronic structure across the Fermi level (E$ _{F} $) exhibits closely spaced Ni $ 3d $, Mn $ 3d $ and O $ 2p $ states. From occupied and unoccupied bands, estimated values of the Coulomb repulsion energy ($ U $) and ligand to metal charge transfer energy ($ \Delta $), indicate charge transfer insulating nature, where remarkable modification in Ni/Mn $ 3d $ - O $ 2p $ hybridization takes place across the FM transition temperature. Existence of ASD broadens the Ni, Mn $ 3d $ spectral features, whereas spectral positions are found to be unaltered. Hereby, present work demonstrates SNMO thin film as a FM-I system, where FM state can be tuned by manipulating ASD in the crystal structure, while I state remains intact., Comment: 21 pages, 9 figures, and supplemental information

The functional characteristics of double perovskites with unique ferromagnetic-insulator ground state have been controversial due to the unavoidable presence of anti-site disorders (ASDs). Here, we aim to investigate the origin of magnetic ordering on local and global scales in Sm$_{2}$NiMnO$_{6}$ (SNMO) double perovskite system. Different calcination routes are exploited to generate different cation arrangements in SNMO and the corresponding magnetic configurations are examined using the high energy (E $ \sim $0.3 eV) `hot neutrons', which has helped to overcome Sm absorption as well as to record total (Bragg's+diffuse) scattering profiles with high momentum transfer (Q$ _{max} \sim $24 angstrom$ ^{-1} $). We have observed that the Ni-Mn sublattice adopts long range collinear ferromagnetic $ F_{x}F_{z} $ structure with commensurate $k$=(0, 0, 0) propagation vector, below ordering temperature T $ \lesssim $ 160 K, irrespective of variable ASD concentrations. In addition, the signatures indicating the antiparallel polarization of Sm paramagnetic moments with respect to Ni-Mn network, are noticed in the vicinity of anomalous magnetic transitions at T $ \lesssim $ 35 K. The real space pair distribution function calculations have provided a direct visualization of ASDs by means of broadening in Ni/Mn-Mn/Ni linkage. Employing the Reverse Monte Carlo approach on diffuse magnetic scattering profiles, we have observed the negative spin-spin correlation function which suggests the Ni-Ni antiferromagnetic exchange interactions ranging up to first nearest neighbor distance. These results confirm that the existence of ASDs in cation ordered host matrix leads to competing ferromagnetic-antiferromagnetic phases in a broad temperature range, which quantitatively governs the temperature dependent bulk magnetic observables of SNMO system., Comment: 14 pages, 8 figures, and Supplementary Information

The room temperature (300 K) electronic structure of pulsed laser deposited LaNi_{x}V_{1-x}O_{3} thin films have been demonstrated. The substitution of early-transition metal (TM) V in LaVO_{3} thin films with late-TM Ni leads to the decreasing in out-of-plane lattice parameter. Doping of Ni does not alter the formal valence state of Ni and V in LaNi_{x}V_{1-x}O_{3} thin films, divulging the absence of carrier doping into the system. The valence band spectrum is observed to comprise of incoherent structure owing to the localized V 3d band along with the coherent structure at Fermi level. With increase in Ni concentration, the weight of the coherent feature increases, which divulges its origin to the Ni 3d-O 2p hybridized band. The shift of Ni 3d-O 2p hybridized band towards higher energy in Ni doped LaVO_{3} films compared to the LaNiO_{3} film endorses the modification in ligand to metal charge transfer (CT) energy. The Ni doping in Mott-Hubbard insulator LaVO_{3} leads to the closure of Mott-Hubbard gap by building of spectral weight that provides the delocalized electrons for conduction. A transition from bandwidth control Mott-Hubbard insulator LaVO_{3} to negative CT metallicity character in LaNiO_{3} film is observed. The study reveals that unlike in Mott-Hubbard insulators where the strong Coulomb interaction between the 3d electrons decides the electronic structure of the system, CT energy can deliver an additional degree of freedom to optimize material properties in Ni doped LaVO_{3} films., Comment: 30 pages, 8 figures

The predictability of any characteristic functional aspect in a double perovskite system has always been compromised by its strong dependence over the inevitably present anti-site disorders (ASD). Here, we aim to precisely map the quantitative and qualitative nature of ASD with the corresponding modifications in observables describing the magnetic and electronic state in epitaxial Sm$ _{2} $NiMnO$ _{6} $ (SNMO) double perovskite thin films. The concentration and distribution patterns of ASD are effectively controlled by optimizing growth conditions and estimated on both local and global scales utilizing extended X-ray absorption fine structure and bulk magnetometry. Depending upon the defect densities, the nature of disorder distribution can vary from homogeneous to partially segregated patches. Primarily, the effect of varying B-site cationic arrangement in SNMO is reflected as the competition of long range ferromagnetic (FM) and short scale antiferromagnetic (AFM) interactions originated from ordered Ni-O-Mn and disordered Ni-O-Ni or Mn-O-Mn bonds, respectively, which leads to systematic shift in magnetic transition temperature and drastic drop in saturation magnetization. In addition, we have observed that the gradual increment in density of ASD leads to significant deviation from uniaxial anisotropy character, reduction in anisotropy energy and enhancement of moment pinning efficiency. However, the observed signatures of $ Ni^{2+}+Mn^{4+} \longrightarrow Ni^{3+}+Mn^{3+} $ charge disproportionation is found to be independent of cation disorder densities. This work serves as a basic route-map to tune the characteristic magnetic anisotropy, magnetic phase transitions, and magnetization reversal mechanism by controlling ASD in a general double perovskite system., Comment: 25 pages, 7 figures and supplementary information

Electronic structure of pulsed laser deposited epitaxial LaVO3 (LVO) thin film grown on LaAlO3 (001) (LAO) substrate, has been studied at room temperature and at 130 K, which is below the structural (141 K) and magnetic transition temperatures (143 K) of single crystal LVO. Significant modification in spectral intensity, largely around the V 3d - O 2p hybridized region, is observed in valence band spectrum (VBS) of LVO film at 130 K. Resonant photoemission study at 130 K confirms the presence of charge transfer screened 3dnL (L: hole in the O 2p) final state along with the dominant 3dn-1 final state at 1.5 eV binding energy in the valence band. On the contrary, in the room temperature VBS dominant V 3d sates with only 3dn-1 kind final state is accentuated. To understand this difference, density functional theory (DFT) calculations are employed. The changes in crystal structure from room temperature orthorhombic (O-LVO) to low temperature monoclinic (M-LVO) symmetry leads to remarkable change in the electronic structure around the Fermi level, including transition from direct to indirect nature of band gap. Our calculations also confirm an enhanced O 2p character in the valence band edge of M-LVO that is hybridized with V 3d. These results are further corroborated with octahedral distortions associated with the structural transition., Comment: 26 pages, 6 figures

This study aims towards a systematic reciprocity of the tunable synthesis parameters - partial pressure of N$_2$ gas, ion energy (\Ei) and Ti interface in TiN thin film samples deposited using ion beam sputtering at ambient temperature (300\,K). At the optimum partial pressure of N$_2$ gas, samples were prepared with or without Ti interface at \Ei~=~1.0 or 0.5\,keV. They were characterized using x-ray reflectivity (XRR) to deduce thickness, roughness and density. The roughness of TiN thin films was found to be below 1\,nm, when deposited at the lower \Ei~of 0.5\,keV and when interfaced with a layer of Ti. Under these conditions, the density of TiN sample reaches to 5.80($\pm$0.03)\,g~cm$^{-3}$, a value highest hitherto for any TiN sample. X-ray diffraction and electrical resistivity measurements were performed. It was found that the cumulative effect of the reduction in \Ei~from 1.0 to 0.5\,keV and the addition of Ti interface favors (111) oriented growth leading to dense and smooth TiN films and a substantial reduction in the electrical resistivity. The reduction in \Ei~has been attributed to the surface kinetics mechanism (simulated using SRIM) where the available energy of the sputtered species (\Esp) leaving the target at \Ei~= 0.5\,keV is the optimum value favoring the growth of defects free homogeneously distributed films. The electronic structure of samples was probed using N K-edge absorption spectroscopy and the information about the crystal field and spin-orbit splitting confirmed TiN phase formation. In essence, through this work, we demonstrate the role of \Esp~and Ti interface in achieving highly dense and smooth TiN thin films with low resistivity without the need of a high temperature or substrate biasing during the thin film deposition process., Comment: 14 pages; 6 figures

The crystal structure of hexagonal-Sr0.6Ba0.4MnO3 allows various competing superexchange interactions, leading to intriguing magnetic properties. Local structural changes modify overlapping between Mn and oxygen ions with temperature. Calculations based on our model spin-Hamiltonian reveal that the dominant linear antiferromagnetic superexchange interaction between the oxygen-linked Mn4+ ions results in short range correlations (SRC), manifesting a smooth drop in magnetization below 325K. Dominance of superexchange interaction changes its allegiance towards the non-linear oxygen-linked Mn-O-Mn interactions, onsetting long-range correlations (LRC) below 225K. Below the SRC-LRC crossover temperature, electrical response arising from the interacting dipoles exhibits power-law divergent behaviour of relaxation time, upon cooling. Non-ergodic character of the dipole-cluster glass state is examined via the indispensable aging and rejuvenation effects, similar to the spin glasses. Competitive-frustration among spin-exchange and local-strain is reckoned as responsible for the electrical glass origin., Comment: 20 pages, 6 figures, 32 references. arXiv admin note: substantial text overlap with arXiv:1807.01303

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., Comment: 8 pages, 8 figures, Supplementary Material

SrCoO2.5 (SCO) is a charge transfer insulator with 3d6 ground state configuration leading to antiferromagnetic nature. It is observed that substrate induced strain engineering modifies the ground state of SCO thin film with 3d7L (L:O-2p hole) configuration causing negative charge transfer energy.The consequent strong hybridization between O-2p and Co-3d bands causes a hole in O-2p band leading to hole mediated unconventional ferromagnetic ordering in SrCoO2.5 thin film. This opens up a new avenue to tune the electronic structure vis a vis magnetic property via strain engineering., Comment: 15 pages, 3 figures

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., Comment: 9 pages, 5 figures

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., Comment: 12 pages, 18 figures

Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed the high spin state for the enhancement. Our magnetization results show high magnetic moment of 3.3 Bohr-magnetron/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 Bohr-magnetron/Ru ion observed in bulk compound. Contrary to the expectation the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin-orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment. Strain disorder is shown to be an efficient mean to control the spin-orbit coupling., Comment: 10 pages, 6 figures

Zero field cooled spontaneous exchange bias (SEB) is observed in epitaxial La0.7Sr0.3MnO3 (LSMO):NiO self-assembled nanocomposite thin films grown on (001) SrTiO3 single crystal substrate by pulsed laser deposition. SEB is displayed by the novel asymmetry in the hysteresis loop of the composite film along with the field cooled conventional exchange bias (CEB) effect. The training effect shows that exchange bias relaxation is disorder mediated. It is revealed from DC magnetization results that such nanocomposite film divulges spin glass like behaviour, which is arising due to competing ferromagnetic and antiferromagnetic inteactions at the assorted interfaces of ferromagnetic LSMO and antiferromagnetic NiO. Also corroborated by x-ray magnetic circular dichroism (XMCD) measurements, we conclude that SEB is mainly originating due to ferromagnetic coupling of unstable interfacial antiferromagnetic spin due to NiO with the ferromagnetic LSMO at the disordered interface. These results of self-assembled thin films provide a useful input to realize and understand microscopic origin of SEB for device application., Comment: 26 Pages, 9 Figures

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

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., Comment: 6 figures

Hexagonal Sr0.6Ba0.4MnO3 (SBMO) follows P63/mmc symmetry where MnO6 octahedra are both face-shared (Mn2O9 bi-octahedra) and corner-shared via oxygen anion. It undergoes ferroelectric (FE) and antiferromagnetic (AFM) orderings close to the room temperature. Magnetic properties appear to be governed by intricate exchange interactions among Mn4+ ions within and in adjacent Mn2O9 bi-octahedra, contingent upon the local structural changes. Calculations based on our model spin-Hamiltonian reveal that the dominant linear AFM fluctuations between the Mn4+ ions of two oxygen-linked bi-octahedra result in short range correlations, manifest as a smooth drop in magnetization below 325 K. Competition between spin-exchange and local-strain is reckoned as responsible for the atypical magneto-electricity, obtained near the room temperature., Comment: 15 pages, 4 figures, 1 table, and 28 references

We elaborate here why the antiferromagnetically ordered GdCrO$_3$ responds in a diamagnetic way under certain conditions, by monitoring the evolution of the microscopic global and local magnetic phases. Using high energy $\sim$ 0.3 eV neutrons, the magnetic ordering is shown to adopt three distinct magnetic phases at different temperatures: G$_x^{Cr}$,A$_y^{Cr}$,F$_z^{Cr}$ below N\'eel temperature = 171 K; (F$_x^{Cr}$, C$_y^{Cr}$, G$_z^{Cr}$)$\bullet$( F$_x$$^{Gd}$,C$_y$$^{Gd}$) below 7 K and an intermediate phase for 7 K $ \le T \le$ 20 K in the vicinity of spin-reorientation phase transition. Although, bulk magnetometry reveals a huge negative magnetization (NM) in the terms of both magnitude and temperature range ( $M_{- max}$ ( 18 K)$\sim$ 35 $\times M_{+ max}$ (161 K), $\Delta T \sim 110$ K in presence of $\mu_0H$ = 0.01 T); the long-range magnetic structure and derived ordered moments are unable to explain the NM. Real-space analysis of the total (Bragg's + diffuse) scattering reveals significant magnetic correlations extending up to $\sim$ 9 $\AA$. Accounting for these short-range correlations with a spin model reveals spin frustration in the S= 3 ground state, comprising competing first, second and third next nearest exchange interactions with values J$_1$ = 2.3 K, J$_2$ = -1.66 K and J$_3$ = 2.19 K in presence of internal field, governs the observance of NM in GdCrO$_{3}$., Comment: 11 pages, 8 figures

We have studied the electronic structure of Mo rich Mo$_{1-x}$Re$_x$ alloys (0$\leq$ x $\leq$0.4) using valence band photoemission spectroscopy in the photon energy range 23-70 eV and density of states calculations. Comparison of the photoemission spectra with the density of states calculations suggests that with respect to the Fermi level $E_F$, the $d$ states lie mostly in the range 0 to -6 eV binding energy whereas $s$ states lie in the range -4 to -10 eV binding energy. We have observed two resonances in the photoemission spectra of each sample, one at about 35 eV photon energy and other at about 45 eV photon energy. Our analysis suggest that the resonance at 35 eV photon energy is related to the Mo $4p$-$5s$ transition and the resonance at 45 eV photon energy is related to the contribution from both the Mo $4p$-$4d$ transition (threshold: 42 eV) and Re $5p$-$5d$ transition (threshold: 46 eV). In the CIS plot, the resonance at 35 eV incident photon energy for binding energy features in the range of $E_F$ (B.E. = 0) to -5 eV becomes progressively less prominent with the increasing Re concentration $x$ and vanishes for $x>$ 0.2. The difference plots obtained by subtracting the valence band photoemission spectrum of Mo from that of Mo$_{1-x}$Re$_x$ alloys, measured at 47 eV photon energy, reveal that the Re $d$ like states appear near $E_F$ when Re is alloyed with Mo. These results indicate that interband $s$-$d$ interaction, which is weak in Mo, increases with the increasing $x$ and influences the nature of superconductivity in the alloys with higher $x$., Comment: 15 pages, 7 figures

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., Comment: 6 pages, 5 figures, 2 tables

Rashba spin-orbit splitting in the magnetic materials opens up a new perspective in the field of spintronics. Here, we report a giant Rashba-type spin-orbit effect on PrGe [010] surface in the paramagnetic phase with Rashba coefficient {\alpha}_R=5 eV{\AA}. Significant changes in the electronic band structure has been observed across the phase transitions from paramagnetic to antiferromagnetic (44 K) and from antiferromagnetic to the ferromagnetic ground state (41.5 K). We find that Pr 4f states in PrGe is strongly hybridized with the Pr 5d and Ge 4s-4p states near the Fermi level. The behavior of Rashba effect is found to be different in the k_x and the k_y directions showing electron-like and the hole-like bands, respectively. The possible origin of Rashba effect in the paramagnetic phase is related to the anti-parallel spin polarization present in this system. First-principles density functional calculations of Pr terminated surface with the anti-parallel spins shows a fair agreement with the experimental results. We find that the anti-parallel spins are strongly coupled to the lattice such that the PrGe system behaves like weak ferromagnetic system. Analysis of the energy dispersion curves at different magnetic phases showed that there is a competition between the Dzyaloshinsky-Moriya interaction and the exchange interaction which gives rise to the magnetic ordering in PrGe. Supporting evidences of the presence of Dzyaloshinsky-Moriya interaction are observed as anisotropic magnetoresistance with respect to field direction and first-order type hysteresis in the X-ray diffraction measurements. A giant negative magnetoresistance of 43% in the antiferromagnetic phase and tunable Rashba parameter with temperature across the magnetic transitions makes this material a suitable candidate for technological application in the antiferromagnetic spintronic devices., Comment: 4 figures, supplementary information

Resonant photoemission spectroscopy has been used to investigate the character of Fe 3d states in FeAl alloy. Fe 3d states have two different character, first is of itinerant nature located very close to the Fermi level, and second, is of less itinerant (relatively localized character), located beyond 2 eV below the Fermi level. These distinct states are clearly distinguishable in the resonant photoemission data. Comparison between the results obtained from experiments and first principle based electronic structure calculation show that the origin of the itinerant character of the Fe 3d states is due to the ordered B2 structure, whereas the relatively less itinerant (localized) Fe 3d states are from the disorders present in the sample. The exchange splitting of the Fe 3s core level peak confirms the presence of local moment in this system. It is found that the itinerant electrons arise due to the hybridization between Fe 3d and Al 3s-3p states. Presence of hybridization is observed as a shift in the Al 2p core-level spectra as well as in the X-ray near edge absorption spectra towards lower binding energy. Our photoemission results are thus explained by the co-existence of ordered and disordered phases in the system., Comment: 7 figures

Si-SiO2 multilayer nanocomposite (NCp) films, grown using pulsed laser deposition with varying Si deposition time are investigated using Raman spectroscopy/mapping for studying the variation of Si phonon frequency observed in these NCps. The lower frequency (LF) phonons (~ 495 - 510 cm-1) and higher frequency (HF) phonons (~ 515 - 519 cm-1) observed in Raman mapping data (Fig. 1A) in all samples studied are attributed to have originated from surface (Si-SiO2 interface) and core of Si nanocrystals, respectively. The consistent picture of this understanding is developed using Raman spectroscopy monitored laser heating/annealing and cooling (LHC) experiment at the site of a desired frequency chosen with the help of Raman mapping, which brings out clear difference between core and surface (interface) phonons of Si nanocrystals. In order to further support our attribution of LF being surface (interface) phonons, Raman spectra calculations for Si41 cluster with oxygen termination are performed which shows strong Si phonon frequency at 512 cm-1 corresponding to the surface Si atoms. This can be considered analogous to the observed phonon frequencies in the range 495 - 510 cm-1 originating at the Si-SiO2 interface (extended). These results along with XPS data show that nature of interface (oxygen bonding) in turn depends on the size of nanocrystals and thus LF phonons originate at the surface of smaller Si nanocrystals. The understanding developed can be extended to explain large variation observed in Si phonon frequencies of Si-SiO2 nanocomposites reported in the literature, especially lower frequencies.

Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectra (XPS) of Nd doped phosphate glasses have been studied before and after gamma irradiation in order to find the behavior of chemical bonds, which decide the structural changes in the glass samples. IR absorption spectra of these glasses are found dominated mainly by the characteristics phosphate groups, water (OH) present in the glass network as well as on the composition of glass matrix. The effects of gamma irradiation are observed in the form of bond breaking and possible re-arrangement of the bonding in the glass. Energy dispersive X-ray spectroscopy (EDX) and XPS measurements show changes in the relative concentration of elements; particularly decrease in the concentration of oxygen in the glass samples after gamma irradiation, a possible source of oxygen vacancies. The decrease in the asymmetry in O 1s spectra after gamma irradiation indicates towards decrease in the concentration of bridging oxygen as a result of P-O-P bond breaking. Asymmetric profile of Nd 3d5/2peak after gamma irradiation is found to be due to conversion of Nd3+ to Nd2+ in the glass matrix.

Valence band onset (Ev), valence band tail (VBT) and valence plasmons (VPs) have been studied as a function of sputtering of SnO2 and In2-xSnxO3 (ITO) thin films, using ultraviolet photoemission spectroscopy (UPS). Decrease in Ev with respect to the Fermi level and increase in the density of energy levels of VBT have been observed after 5 minutes of sputtering using Ar+ ions (500V). Bulk and surface components of VPs of Sn, SnO and SnO2 in sputtered SnO2 thin films have been observed in UPS spectra. Similarly, bulk and surface components of VPs of In, Sn and oxygen deficient ITO in sputtered ITO thin films have been observed in UPS spectra. Possible roles of Ev and increase in the density of energy levels of VBT are discussed in the mechanisms of current transport through heterojunctions of SnO2 with semiconductors., Comment: To be published (2014)

For a detailed study on the semiconductor to metal transition (SMT) in ZnO thin films doped with Al in the concentration range from 0.02 to 2%, we grew these films on (0001) sapphire substrates using sequential pulsed laser deposition. It was found that the Al concentration in the films increased monotonically with the ratio of ablation durations of the Alumina and ZnO targets used during the deposition. Using X-ray photo electron spectroscopy it was found that while most of the Al atoms occupy the Zn sites in the ZnO lattice, a small fraction of the Al also gets into the grain boundaries present in the films. The observed SMT temperature decreased from ~ 270 to ~ 50 K with increase in the Al concentration from 0.02 to 0.25 %. In the Al concentration range of ~ 0.5 to 2 % these doped ZnO films showed metallic behavior at all the temperatures without undergoing any SMT. A theoretical model based on thermal activation of electrons and electron scatterings due to the grain boundaries, ionic impurities and phonons has been developed to explain the observed concentration and temperature dependent SMT., Comment: 5 figures, 24 pages

Thin films of Fe3O4 have been deposited on single crystal MgO (100) and Si (100) substrates using pulsed laser deposition. Films grown on MgO substrate are epitaxial with c-axis orientation whereas, films on Si substrate are highly <111> oriented. Film thicknesses are 150 nm. These films have been irradiated with 200 MeV Ag ions. We study the effect of the irradiation on structural and electrical transport properties of these films. The fluence value of irradiation has been varied in the range of 5 x 1010 ions/cm2 to1 x 1012 ions/cm2. We compare the irradiation induced modifications on various physical properties between the c-axis oriented epitaxial film and non epitaxial but <111> oriented film. The pristine film on Si substrate shows Verwey transition (TV) close to 125 K, which is higher than generally observed in single crystals (121 K). After the irradiation with the 5 x 1010 ions/cm2 fluence value, TV shifts to 122 K, closer to the single crystal value. However, with the higher fluence (1 x 1012 ions/cm2) irradiation, TV again shifts to 125 K., Comment: 11 pages, 3 figurs and 1 table

Thin films of magnetite (Fe3O4) are grown on single crystal GaAs (100) substrate by pulsed laser deposition. X ray diffraction (XRD) result shows the (111) preferred orientation of the Fe3O4 film and x-ray photoelectron spectroscopy confirm the presence of single phase Fe3O4 in the film. The electrical transport property of the film shows the characteristic Verwey transition at 122 K and below 110 K, the transport follows variable range hopping type conduction mechanism. The film shows room temperature magnetization hysteresis loop suggesting the ferrimagnetic behavior of the film with saturation magnetization value close to 470 emu/cc., Comment: 16 pages, 5 figurs

We report the growth of undoped and Fe (2 and 5 at. percentage) doped molybdenum oxide thin films on c-plane of sapphire substrate using pulsed laser ablation. X-ray diffraction results show that the films are oriented in (100) direction and have monoclinic structure based on MoO2 phase as also supported by Raman spectroscopy. The x-ray photoelectron spectroscopy reveals chemical state of Fe is +2, which favors the substitutional occupancy of Fe ion in the MoO2 matrix. The room temperature resistivity of all the films are very low (approx.100 microohm-cm). The Fe doped samples show ferromagnetic behavior at room temperature., Comment: 18 pages, 8 figurs and 2 tables

The electronic structure of pulsed laser deposited Mo1-xFexO2 (x=0, 0.02 and 0.05) thin films has been investigated using resonant photoemission spectroscopy near Mo-4p absorption edge. In all the samples a broad Fano- like resonance peak at ~46 eV is observed in the whole area of the valence band which indicates the contribution of the Mo-4d states in the entire valence band region. The doping of Fe in these films leads to decrease in Mo 4d states contributing to electronic states at lower binding energy region. In addition to this, we also observe a shoulder at 4.9 eV in the valence band spectra of doped samples. It is proposed that the origin of this shoulder is due to the Fe hybridised states., Comment: 16 pages, 4 figures ,. J. Phys. Cond. Mat. (accepted)

The effect of substrate strain on the electronic valence band structure of La0.7Ca0.3MnO3 thin films has been investigated. For this purpose La0.7Ca0.3MnO3 thin films have been simultaneously grown on SrTiO3 and LaAlO3 substrates using pulsed laser deposition technique. These samples were characterized by x-ray diffraction, x-ray reflectivity, core level x-ray photoelectron spectroscopy, and scanning electron microscopy measurements. Our experiments confirmed that all these samples have same chemical composition but different strain configuration. The electronic structure of these samples is probed through valence band spectroscopy measurements on Indus-1 synchrotron x-ray source. We observe that strain has large effect on the valence band electronic structure of La0.7Ca0.3MnO3. The results are explained on the basis of strain induced orbital occupancy and change in the crystal field splitting due to Mn-O bond length.

The resistivity and magnetoresistance measurements were carried out on thin film of La0.7Ca0.3MnO3 to investigate the possible origin of low temperature resistivity minimum observed in these samples. We observed large hysteresis in the magnetoresistance at low temperature; 5K and the sample current I has large effect on resistivity minima temperature. The observation of hysteresis at low temperatures suggests the presence of inhomogeneity at low temperatures. These in-homogeneities consist of regions of different resistive phases. It appears that the high resistive phase prevents the tunneling of charge carriers between two low resistive regions and thus giving rise to the resistivity minimum in these samples., Comment: Figure-3 and figure-4 are important new results

Resonance Raman (RR) peaks of and stretching modes and their higher harmonics have been observed superimposed on photoluminescence (PL) spectrum of thin films. Commercial fluorine doped thin films deposited by sputtering on glass and thin films deposited on Si by laser ablation have been studied. The dispersions of CO and OH stretching RR modes are ~ 600 cm-1/eV and 800 cm-1 respectively. The dispersion of the third harmonic of CO stretching mode is ~ 2000 cm-1/eV. Similar dispersion of RR peak of stretching modes and higher harmonics superimposed on PL spectra has been observed in Mo1-xFexO2 thin films and SiO2 bulk glass. Large dispersion of RR peaks seems to be a common property of oxides with impurities of and ., Comment: 13 pages including three figures

The effects of ion beam induced atomic mixing and subsequent thermal treatment in Si/C multilayer structures are investigated by use of the technique of grazing incidence X-ray diffraction (GIXRD) and Raman spectroscopy. The [Si (3.0 nm) / C (2.5 nm)]x10 /Si multilayer films were prepared by electron beam evaporation under ultra high vacuum (UHV) environment. The layer thicknesses were measured using in-situ quartz crystal oscillator. These multilayer films were subjected to 40 keV Ar+ ion irradiation with fluences 5E-16 (low fluence) and 1E-17 ions / cm2 (high fluence).The as-prepared and irradiated multilayer samples were annealed at 773 K for one hour. The GIXRD and Raman spectroscopy results reveal the formation of different phases of SiC in these multilayer structures. Deposition induced reactions at the nano-structured interface and subsequent room temperature Ar ion irradiation at low fluence result in formation of the hexagonal SiC phase. High fluence Ar+ ion irradiation and subsequent annealing at 773 K for one hour leads to precipitation of the cubic-SiC phase., Comment: 16 pages, 3 figures. to be appear soon in NIM B (article in press)