Subhash C. Kashyap

Room temperature ferromagnetism in Mn doped TiO2 thin films: Electronic structure and Raman investigations

In this work dc-magnetization, electronic structural, and Raman investigations of Ti1−xMnxO2 (x = 0.00, 0.05, 0.10, and 0.15) thin films deposited on fused-quartz substrate by a simple and cost effective spray pyrolysis technique have been reported. X-ray diffraction revealed the formation of pure anatase TiO2 phase devoid of elemental Mn clusters in all the Mn incorporated TiO2 films. It is established by x-ray photoelectron spectroscopic (XPS) measurements that Ti ions substituted by Mn ions in both divalent and trivalent states in the TiO2 matrix. No peak corresponding to Mn+4 could be evidenced by XPS. The Raman study has further established the formation of TiO2 in anatase structure in both pure TiO2 and Mn-doped TiO2 films. The Ti1−xMnxO2 films with x ≥ 0.05 exhibit ferromagnetic ordering at room temperature which arises most likely due to formation of bound magnetic polarons.

High temperature ferromagnetism in Mn-doped SnO2 nanocrystalline thin films

It has been possible to induce room temperature ferromagnetism, exhibiting high transition temperature, in tin oxide thin films by introducing manganese in a SnO2 lattice. The observed temperature dependence of the magnetization predicts a Curie temperature exceeding 550 K. A maximum saturation magnetic moment of 0.18±0.04 μB per Mn ion has been estimated for spray pyrolized Sn1−xMnxO2−δ thin films, with x=0.10. For Mn concentration (x) higher than 0.10, the films show linear behavior. The magnetization-versus-field studies indicate that the origin of ferromagnetism lies neither in ferromagnetic metal clusters nor in the presence of metastable phases. The structure factor calculations reveal that Mn has been incorporated in the SnO2 lattice. Also, the electron transport investigation indicates that there is a change of Mn occupancy from substitutional to interstitial sites of the SnO2 lattice when the Mn concentration exceeds 7.5 at. %. These films do not exhibit anomalous Hall effects at room temperature...

Cobalt-substituted SnO2 thin films: A transparent ferromagnetic semiconductor

A maximum room temperature ferromagnetic moment of 0.47μB∕Co ion has been observed in Sn0.9Co0.1O2−δ films grown by spray pyrolysis. The films have high conductivity (∼150Ω−1cm−1) and ∼70% transmittance in the visible region. A systematic variation of saturation magnetization, carrier concentration, electrical conductivity, and optical transmission edge in Sn1−xCoxO2−δ(0.05⩽x⩽0.15) films is correlated with the change in Co concentration, and a carrier mediated Ruderman-Kittel-Kasuya-Yoshida interaction has been proposed as the most probable mechanism for the ferromagnetic ordering. The maximum blueshift in the transmission edge by ∼215meV (at x=0.10) is attributed to the extra carriers generated by Co substitution.

Intrinsic and extrinsic origin of room temperature ferromagnetism in ZnO:Co (5at.%)

The high temperature magnetization behavior of an air sintered nanocrystalline ZnO:Co (5at.%) sample while it is subjected to thermal cycles in vacuum/air ambient is reported in the present paper. The pristine air sintered sample exhibits intrinsic room temperature ferromagnetism. Upon vacuum annealing, the magnetization of this sample increases significantly and the sample remains ferromagnetic up to 630°C. This has been attributed to the formation of Co clusters, and thus, the ferromagnetic ordering is extrinsic in nature. The subsequent air annealing of the vacuum annealed ZnO:Co (5at.%) sample, however, reverts back the magnetization to a value that was observed in the pristine air sintered sample. This is due to complete oxidation of the cobalt clusters. Furthermore, the sample’s magnetization at room temperature and also its temperature dependence up to 630°C remain unaffected by further annealing in vacuum or air, a feature required of a material for its device applications. The Curie temperature o...

Observation of room temperature ferromagnetism in nanocrystalline ZnO?:?Co system

Nanocrystalline ZnO?:?Co samples (having 5?at.% Co) exhibiting room temperature ferromagnetism (RTFM) have been synthesized from acetate precursors. The effect of sequential sintering of the samples at higher temperatures for prolonged duration in air ambient has been investigated for their phase-purity and magnetic characteristics. The nanocrystalline ZnO?:?Co obtained after calcinations at 600??C exhibits RTFM. The study further indicates that the Co3O4 phase progressively forms with increasing sintering temperature, and partially transforms into CoO at and above 800??C. Irrespective of the sintering temperature and duration, the samples sintered at and above 800??C exhibited identical magnetization behaviour with regard to the saturation moment and field required for technical saturation. The ferromagnetism persisting even at higher temperatures (?800??C) arises intrinsically in the insulating Zn1?xCoxO phase. The recently proposed bound magnetic polaron model appears to account for the observed ferromagnetic ordering in this carrier deficient oxide system. A paramagnetic contribution to the overall magnetization is also observed, which is attributed to the paramagnetic Co3O4 and CoO and possibly to isolated polarons and/or non-interacting Co2+ ions.

Structure and transport properties of amorphous Ge1−xCx:H thin films obtained by activated reactive evaporation

Abstract Hydrpgenated amorphous germanium-carbon (a-Ge1−xCx:H) thin films have been prepared by an Activated Reactive Evaporation (ARE) technique. Germanium was evaporated from a resistively heated tungsten basket through an acetylene plasma. The films thus obtained were characterized for the compositional, structural, electrical and optical properties. The effects of substrate temperature, role of hydrogen and variation of optical energy gap with carbon content were also investigated. Typically, films deposited at 200 °C showed a sharp absorption edge corresponding to an optical gap of about 1.3 eV and thermally activated electrical conduction over a wide range of temperatures.

Preparation of Cu-Co Alloy Thin Films on n-Si by Galvanostatic DC Electrodeposition

Thin films of Cu-Co alloy exhibiting excellent metallic luster were galvanostatically electrodeposited directly on Si substrates in a single citrate bath containing sulfates of the metal ions. The direct electrodeposition onto n-Si substrates eliminated a processing step, of depositing a conducting layer, if an insulator were to be used as a substrate. In addition, the use of silicon as a substrate is promising in integrating this inexpensive technique with semiconductor microelectronic device fabrication technology. The as-deposited films exhibited giant magnetoresistance (GMR) of 1% (T = 300 K, H = 10 kOe) and 5% (T = 10 K and H = 50 kOe). The annealed films showed an increase in GMR from I to 2.7%. The X-ray diffraction studies of as-deposited metastable films indicate solid solution like behavior following Vegards law. However, our magnetic studies evidenced the presence of very fine nanograins of Co in these films. A systematic study of the effect of various processing parameters like deposition current density, bath temperature, and pH on the composition, structure, and microstructure of thin films was carried out to understand the electrodeposition process. Deposition current density and bath temperature have significant control over the composition and microstructure. The pH of the electrolyte seems to affect the topography most. Like physical vapor deposition (PVD), the electrodeposition process yields thin films via a nucleation-controlled growth mechanism. The superiority of the electrodeposition technique over the PVD technique is emphasized.

Growth of ZnO Whiskers, Platelets, and Dendrites

Zinc oxide whiskers, platelets, and dendrites have been grown on the ZnO substrate by a simplified and modified vapor phase technique which involves the production of zinc vapor at the growth temperatures itself. By this method, whiskers‐a few mm in length and a fraction of a mm in diameter‐have been obtained at growth temperatures ranging from 960 to 980°C. Change in growth habit from whiskers to platelets has also been observed. Various growth mechanisms have been discussed. It appears that the growth of these crystals takes place at supersaturations higher than those usually required for whisker growth. The dendrites have been observed to grow at about 930°C.Zinc oxide whiskers, platelets, and dendrites have been grown on the ZnO substrate by a simplified and modified vapor phase technique which involves the production of zinc vapor at the growth temperatures itself. By this method, whiskers‐a few mm in length and a fraction of a mm in diameter‐have been obtained at growth temperatures ranging from 960 to 980°C. Change in growth habit from whiskers to platelets has also been observed. Various growth mechanisms have been discussed. It appears that the growth of these crystals takes place at supersaturations higher than those usually required for whisker growth. The dendrites have been observed to grow at about 930°C.

Complex Permittivity and Permeability of Co

We measured dielectric, magnetic, and microwave properties of Co 2U hexaferrite (Ba4Co2Fe36O 60) polycrystalline bulk and composite thick film samples, and studied the effect of annealing temperature on phase formation and microstructure. We synthesized the bulk samples from a precursor prepared by the citrate method. The values of dielectric constant at radio frequencies (50 Hz-1 MHz) of the bulk Co2U12 sample sintered at 1200degC are much higher, and the resistivities are lower, compared to M-type barium hexaferrites. Coercivity is also low, having a value of 5 G for the Co2U12 sample, whereas the saturation magnetization value is 59 emu/g, which is comparable to that of M-type hexaferrites. We also measured the complex permittivity and permeability for Co2U12 samples at microwave frequencies and found the values high compared to M-type barium hexaferrite at these frequencies. Thick composite films were prepared from a ferrite-polymer mix, and all the above properties were studied for these films. We observed that these thick films have lower values of dielectric and magnetic parameters both at low and microwave frequencies. We measured microwave-absorbing properties for the ferrite-polymer sample (ferrite to polymer ratio 70/30) which showed a maximum reflection loss of -37.5 dB at 11.5 GHz for the 3.5-mm-thick sample

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Room‐temperature studies of I‐V characteristics and variation of both the dielectric constant and dielectric loss with frequency are presented for ZnO‐Bi2O3 samples having different compositions and prepared by sintering at different temperatures. The nonlinearity parameter, dc resistivity, dielectric constant, and dielectric loss depend on the composition and sintering temperature. These parameters have been optimized for obtaining maximum nonlinearity and dielectric constant. The occurrence of a loss peak is, however, seen to be independent of the composition and sintering temperature. The present study, supported by a scanning electron microscope analysis, confirms the necessity of a two‐phase structure in the composite for its nonlinearity and high dielectric constant. The variation in the dielectric constant and nonlinearity parameter with sintering temperature for samples of different compositions has been explained on the basis of change in barrier height of Schottky barriers; the presence of these...