Archna Sagdeo

Observation of large dielectric permittivity and dielectric relaxation phenomenon in Mn-doped lanthanum gallate

Polycrystalline LaGa1−xMnxO3 (x = 0, 0.05, 0.1, 0.15, 0.2 and 0.3) samples were prepared via the solid-state reaction method. These samples were characterized using synchrotron-based X-ray diffraction (XRD) and the X-ray absorption near edge structure (XANES). XRD studies confirm the orthorhombic structure for the prepared samples whereas XANES analysis reveals the co-existence of Mn3+ and Mn4+ in all Mn-doped samples. Dielectric relaxation is observed for all Mn-doped samples whereas a large dielectric constant (e′) is perceived in samples with higher Mn doping (x = 0.2 and x = 0.3). Occurrence of a large e′ is attributed to the huge decrease in impedance with increasing Mn doping which is governed by the hopping charge transport and extrinsic interface effects, whereas at high frequencies, this effect is observed possibly due to dipolar effects associated with the possible off-centrosymmetry of the MnO6 octahedron which is indicated by the pre-edge feature (Mn K-edge) in XANES and validated through P–E measurements. The appearance of dielectric relaxation was credited to the dipolar effects associated with the flipping of the Mn3+/Mn4+ dipole i.e., with the hopping of charge carriers between Mn3+ and Mn4+ under an external electric field. The value of activation energy (Ea = 0.36 eV), extracted from temperature-dependent dielectric data, reveals the polaron hopping mechanism.

Room temperature magnetodielectric studies on Mn-doped LaGaO3

Polycrystalline samples of LaGa1−xMnxO3 (0 ≤ x ≤ 0.3) were prepared by the solid-state reaction route. The phase purity of these samples was confirmed by powder x-ray diffraction experiments carried out on BL-12 at the Indus-2 synchrotron radiation source. The sample with x = 0.2 shows significant change in the value of capacitance with the application of a magnetic field. The observed results were understood by systematically analyzing magnetocapacitance (MC), magnetoresistance (MR), and dielectric loss as a function of frequency. Our results and analysis suggest that the observed magnetodielectric (MD) coupling may be due to the MR effect of the Maxwell–Wagner type and/or field-induced dipolar relaxation. Further, it is observed that oxygen stoichiometry plays a very crucial role in the observed MD coupling.

Electronic and optical properties of BaTiO3 across tetragonal to cubic phase transition: An experimental and theoretical investigation

Temperature dependent diffuse reflectance spectroscopy measurements were carried out on polycrystalline samples of BaTiO3 across the tetragonal to cubic structural phase transition temperature (TP). The values of various optical parameters such as band gap (Eg), Urbach energy (Eu), and Urbach focus (E0) were estimated in the temperature range of 300 K to 480 K. It was observed that with increasing temperature, Eg decreases and shows a sharp anomaly at TP. First principle studies were employed in order to understand the observed change in Eg due to the structural phase transition. Near TP, there exist two values of E0, suggesting the presence of electronic heterogeneity. Further, near TP, Eu shows metastability, i.e., the value of Eu at temperature T is not constant but is a function of time (t). Interestingly, it is observed that the ratio of Eu (t=0)/Eu (t = tm), almost remains constant at 300 K (pure tetragonal phase) and at 450 K (pure cubic phase), whereas this ratio decreases close to the transition temperature, which confirms the presence of electronic metastability in the pure BaTiO3. The time dependence of Eu, which also shows an influence of the observed metastability can be fitted with the stretched exponential function, suggesting the presence of a dynamic heterogeneous electronic disorder in the sample across TP. First principle studies suggest that the observed phase coexistence may be due to a very small difference between the total cohesive energy of the tetragonal and the cubic structure of BaTiO3. The present work implies that the optical studies may be a sensitive probe of disorder/heterogeneity in the sample.

Fe doped LaGaO3: good white light emitters

A polycrystalline sample of LaGa1−xFexO3 has been prepared by solid state reaction. The structural, optical and electronic structure of Fe doped LaGaO3 have been investigated. It is observed that with Fe doping the lattice parameter of the prepared samples systematically increases whereas the optical band gap decreases. First-principles density functional theory (DFT) calculations were carried out to understand the effect of Fe doping on the electronic structure and to understand the origin of systematic decrease in optical band gap. DFT studies suggests that there exists an indirect band gap to direct band gap transition with Fe doping which is further confirmed using photoluminescence measurements. Photoluminescence studies suggest that Fe doped LaGaO3 are potential candidates as white light emitting materials.

Observation of room temperature magnetodielectric effect in Mn-doped lanthanum gallate and study of its magnetic properties

Polycrystalline samples of Mn-doped LaGa1−xMnxO3 (LGMO) with 0 ≤ x ≤ 0.2 have been prepared via solid-state reaction method. The structural phase purity of all these samples was confirmed by powder X-ray diffraction experiments carried out at the BL-12 beamline of the Indus-2 synchrotron radiation source. Room-temperature (RT) dielectric measurements were performed in the absence and presence of a magnetic field. A noticeable magnetodielectric (MD) effect, i.e., a change in the value of the dielectric constant owing to the application of a low magnetic field, was observed in the LGMO sample with x = 0.2 (LG8M2O). In order to separate the intrinsic and resistive contributions present in the observed RT MD effect, magnetoresistance impedance spectroscopy (MRIS) was performed at RT. The present MRIS analysis suggests that at frequencies corresponding to the grain contribution (≥105 Hz for the present samples), the observed MD phenomenon appears to be an intrinsic property of the presently studied samples, whereas at lower probing frequencies (<105 Hz) the observed change appears to be dominated by MR (considering frequency-dependent resistance), which was possibly due to the coexistence of Mn3+ and Mn4+. The coexistence of Mn3+ and Mn4+ was revealed by XANES (Mn K-edge) spectroscopy. Moreover, RT and low-temperature magnetization–magnetic field (M–H) measurements, along with M–T measurements in FC and ZFC modes, were performed to investigate the state of magnetic ordering. The appearance of a narrow M–H loop indicates the presence of some magnetic ordering at RT. Furthermore, a ferromagnetic (FM) transition observed around 36 K and a normal M–H loop with saturated magnetization recorded at 5 K confirm FM ordering at low temperatures, whereas a bifurcation in FC-ZFC curves indicates competing FM and antiferromagnetic (AFM) interactions at low temperatures.

Effect of oxygen partial pressure on properties of asymmetric bipolar pulse dc magnetron sputtered TiO 2 thin films

A set of titanium dioxide thin films have been deposited on fused silica substrates by the novel asymmetric bipolar pulsed dc reactive sputtering technique using different oxygen partial pressures in the sputtering ambient in the range of 0%–21%. For investigating long-range structural properties of the samples, grazing incidence x-ray diffraction (GIXRD) measurements and for probing local structure surrounding Ti sites, extended x-ray absorption fine structure (EXAFS) measurements have been carried out. Optical properties of the films have been investigated by transmission spectrophotometry in UV–visible–near IR range and it has been observed that as oxygen partial pressure in the sputtering ambient is increased, refractive index of the films varies in a nonlinear fashion. Microscopically, it has been found that this nonlinear variation can be explained by the local structure tool EXAFS, while GIXRD which works on average long-range order fails to explain this. Such a variation of optical properties with increase in oxygen partial pressure during deposition of the films is attributed to the competition between the two processes, viz., improvement in the stoichiometry of the films and reduction in the mobility of the adatoms on the surface of the growing films.

Disappearance of dielectric anomaly in spite of presence of structural phase transition in reduced BaTiO3: Effect of defect states within the bandgap

We report the structural, optical, ferroelectric, and dielectric properties of reduced BaTiO3 samples. For this purpose, oxygen vacancies in BaTiO3 are created by heating these samples with a Ti metal in a vacuum environment at different temperatures. It is observed that with an increase in oxygen deficiencies, the c/a ratio decreases as compared to that of the oxygen treated sample. The ferroelectric properties of the oxygen deficient samples are visibly different as compared to those of the oxygen treated sample. The disappearance of the P-E loop and the anomaly in the temperature variation of the dielectric constant have been observed; however, the structural phase transition corresponding to ferroelectric phase transitions still persists. Thus, it appears that the anomaly in dielectric data and the presence of the P-E loop are getting masked possibly by the Maxwell-Wagner effect. The presence of Ti+3 states in the prepared samples has been confirmed by X-ray absorption near edge structure measurements. The Kubelka-Munk optical absorption shows the presence of extra states below fundamental transition, indicating the emergence of new electronic states within the bandgap, which might be due to Ti+3 states. These new states appear at different energy positions, and with different intensities for different samples, which are reduced in the presence of Ti. These new states within the bandgap appear to modify the electronic structure, thereby reducing the overall bandgap, and hence, they seem to modify the ferroelectric and dielectric properties of the samples. Our results may be treated as experimental evidence for theoretically proposed defect states in oxygen deficient or reduced BaTiO3.We report the structural, optical, ferroelectric, and dielectric properties of reduced BaTiO3 samples. For this purpose, oxygen vacancies in BaTiO3 are created by heating these samples with a Ti metal in a vacuum environment at different temperatures. It is observed that with an increase in oxygen deficiencies, the c/a ratio decreases as compared to that of the oxygen treated sample. The ferroelectric properties of the oxygen deficient samples are visibly different as compared to those of the oxygen treated sample. The disappearance of the P-E loop and the anomaly in the temperature variation of the dielectric constant have been observed; however, the structural phase transition corresponding to ferroelectric phase transitions still persists. Thus, it appears that the anomaly in dielectric data and the presence of the P-E loop are getting masked possibly by the Maxwell-Wagner effect. The presence of Ti+3 states in the prepared samples has been confirmed by X-ray absorption near edge structure measurements...

Observation of Kondo behavior in the single crystals of Mn-doped Bi2Se3 topological insulator

The Bi2Se3 and Mn doped Bi2Se3 (i.e. Bi1.5Mn0.5Se3, referred as BiMnSe) single crystals were grown using a melt growth method and characterized for their structural, electrical and magnetic properties. These crystals were found to be of single phase having rhombohedral structure with the space group (R3¯m). The magnetoresistance measurements have been carried out in the temperature range 5-50 K, under magnetic fields up to 8 T. The variation of resistivity with temperature shows the Metallic behavior in case of Bi2Se3, and up-turn at low temperatures in the BiMnSe. Metallic behavior was observed up to T > 40 K, whereas below 40K, Kondo effect has been observed. A saturating resistance upturn at low temperatures is observed in the BiMnSe, indicating the presence of Kondo effect due to the existence of localized impurity spins. While the Bi2Se3 is diamagnetic, the BiMnSe is in ferromagnetic state at 5 K. This study opens up a new direction to investigate the physics and device applications of magnetically tunable topological insulators.The Bi2Se3 and Mn doped Bi2Se3 (i.e. Bi1.5Mn0.5Se3, referred as BiMnSe) single crystals were grown using a melt growth method and characterized for their structural, electrical and magnetic properties. These crystals were found to be of single phase having rhombohedral structure with the space group (R3¯m). The magnetoresistance measurements have been carried out in the temperature range 5-50 K, under magnetic fields up to 8 T. The variation of resistivity with temperature shows the Metallic behavior in case of Bi2Se3, and up-turn at low temperatures in the BiMnSe. Metallic behavior was observed up to T > 40 K, whereas below 40K, Kondo effect has been observed. A saturating resistance upturn at low temperatures is observed in the BiMnSe, indicating the presence of Kondo effect due to the existence of localized impurity spins. While the Bi2Se3 is diamagnetic, the BiMnSe is in ferromagnetic state at 5 K. This study opens up a new direction to investigate the physics and device applications of magnetically t...

Phase transformation of [Co/Ti]x10 multilayer under swift heavy ion irradiation

Swift heavy ions induced structural and magnetic modifications of Co/Ti multilayers have been studied with 120 MeV Ag9+ ions at different ion fluences. The decrease in 1st Bragg peak intensity of X-ray reflectivity spectra confirms an increase in Co-Ti mixing at the interfaces, whereas X-ray diffraction (XRD) measurements reveal amorphization of the crystalline Co layer. XRD also indicates a shift in peak position towards a lower angle, confirming the development of tensile stress upon irradiation whereas an increase in the peak width suggests the reduction in the grain size. Vibrating sample magnetometer measurements show a reduction in Mr/Ms due to Co-Ti interface mixing and formed bcc-Co3Ti metastable phase. The X-ray absorption fine structure (XAFS) technique has been utilized to obtain variation in Co-Co and Co-Ti bond distances as a function of ion fluences. Quantitative estimation of Co3Ti phase generated due to ion irradiation has also been obtained using Co K-edge XAFS fitting.