Mohindar S. Seehra

Semiconducting and ferromagnetic behavior of sputtered Co-doped TiO2 thin films above room temperature

We have investigated Co-doped TiO2 thin films grown by reactive co-sputtering. X-ray diffraction showed a single phase polycrystalline rutile structure, without any segregation of Co into particulates within the instrumental resolution limit. The atomic content of Co ranged from 1% to 12%. The temperature dependence of resistivity showed an extrinsic semiconducting behavior. From optical absorption measurements, the band gap Eg≈3.25±0.05 eV was found, independent of the Co concentration, and in agreement with a literature value. Room temperature M-H loops showed a ferromagnetic behavior for Co content higher than 3%. The magnetic moment per Co atom was estimated to be about 0.94 μB, suggesting a low spin configuration of Co ions. The temperature dependence of remanent magnetization revealed a Curie temperature higher than 400 K for Co content of 12%.

Structural and multiferroic properties of La-modified BiFeO3 ceramics

The coexistence of the magnetic and the electrical properties in lanthanum (La)-modified bismuth ferrite (Bi1−xLaxFeO3, x=0.05, 0.1, 0.15, and 0.2) ceramics was studied and compared with those of bismuth ferrite (BiFeO3). The presence of a small secondary phase of BiFeO3 (arises due to excess Bi2O3) was removed on La substitution at the Bi site, as observed in x-ray diffraction (XRD). The effect of La substitution on dielectric constant, loss tangent, and remnant polarization of the samples was studied in a wide range of temperature (77–400K) and frequency (1kHz–1MHz). The variation of magnetization, coercive field, and exchange bias with temperature (2–300K) and La concentration were investigated. These changes in the magnetic parameters with La doping along with those of the electron magnetic resonance parameters measured at 300K and 9.28GHz are understood in terms of increase in the magnetic anisotropy and magnetization. These results also show that stabilization of crystal structure and nonuniformity ...

A comparative study of the magnetic properties of bulk and nanocrystalline Co3O4

A comparative study of the magnetic and electron paramagnetic resonance (EPR) parameters of bulk and Co3O4 nanoparticles (NP), synthesized by a sol–gel process, is presented. Both samples possess the cubic phase with a slightly lower (by 0.34%) lattice parameter for the Co3O4 NP. The average crystallite size D = 17 nm determined by x-ray diffraction (XRD) for the Co3O4 NP is quite consistent with the electron microscopic observations. The bulk Co3O4 has particle size in the 1–2 µm range. A Neel temperature of TN = 30 K (lower than the 40 K usually quoted in the literature) is determined from the analysis of the magnetic susceptibility versus temperature data for bulk Co3O4. This TN = 30 K is in excellent agreement with the TN = 29.92 K reported from specific heat measurements. The Co3O4 NP powder exhibits a still lower TN = 26 K, possibly due to the associated finite size effects. The values of coercivity, Hc = 250 Oe, and exchange bias, He = −350 Oe, together with the training effect have been observed in the Co3O4 NP sample (cooled in 20 kOe). Both Hc and He approach zero as . For T>TN, the χ versus T data for both samples fit the modified Curie–Weiss law (χ = χ0+C/(T+θ)). The magnitudes of C, θ and TN are used to determine the following: exchange constants J1ex = 11.7 K, J2ex = 2.3 K, and magnetic moment per Co2+ ion μ = 4.27 μB for bulk Co3O4; and J1ex = 11.5 K, J2ex = 2.3 K and μ = 4.09 μB for Co3O4 NP. EPR yields a single Lorentzian line near g = 2.18 in both samples but with a linewidth ΔH that is larger for the Co3O4 NP. Details of the temperature dependence of ΔH, line intensity I0, and disappearance of the EPR on approach to TN are different for the two samples. These effects are discussed in terms of spin–phonon interaction and additional surface anisotropy present in Co3O4 NP.

Conversion of methanol to olefins over cobalt-, manganese- and nickel-incorporated SAPO-34 molecular sieves

Silicoaluminophosphate (SAPO)-34 molecular sieves modified with transition metals Co, Mn, and Ni were investigated for their activity, selectivity, and lifetime in the methanol to olefins (MTO) reaction and compared with that of unmodified SAPO-34. The electronic state and location of the metal ions in these materials were determined by magnetic measurements (temperature and magnetic field variations of magnetization). These magnetic studies show that for MnSAPO-34 and CoSAPO-34 metal ions are incorporated into the SAPO framework with concentrations ≃0.12 and 0.17 wt.%, respectively. Nickel modified SAPO-34 samples were prepared by two different procedures. For Ni-SAPO-34 prepared by one procedure (Method I), the magnetic studies show the presence of nanoparticles (Ni or NiO) outside the SAPO framework. For NiSAPO-34 prepared by a second procedure (Method II), these studies show that the nickel ions are incorporated into the SAPO framework. Although the total C2–C4 olefins activity on these materials was comparable to that of unmodified SAPO-34, significant variations in the deactivation behavior were observed among various metal-modified catalysts. MnSAPO-34 was found to be the best catalyst based on catalyst lifetime. In the case of Ni-SAPO-34 (Method I), unusually high amounts of methane were observed in the products due to the presence of nanoparticles outside the SAPO framework.

Modeling of disorder and X-ray diffraction in coal-based graphitic carbons

Abstract In this work, X-ray diffractograms of about 20 coal-based graphites and carbons, with different degrees of graphitic order, have been modeled using the recent computational model of Shi et al. This model relates the crystalline parameters c (2d002) and a and the crystallite sizes Lc and La, to the parameters of the disorder present in carbons. These disorder parameters include the probability P for a random displacement δ between adjacent layers, the probability Pt for finding a 3R stacking fault, the strain parameters, and the preferred orientation. Plots among d002, Lc, and La lead us to conclude that a step transition exists in graphites near d 002 - = 3.37 A where a two-fold increase in Lc is observed for a fixed La. This observation of a step at 3.37 A is consistent with the recent observation of Bragg et al. in a graphite where disorder was produced by successive grinding, thermal treatments and neutron irradiation. A theoretical interpretation of this step-transition is however not yet available. Plots of d002 against 〈δ2〉, 2P, (2P + Pt) are discussed in terms of the disorder models proposed by Franklin and by Ruland. The preferred orientation evaluated from the fit varies systematically with the ratio L c L a and the experimental X-ray anisotropy ratio.

Dielectric and magnetic properties of sol-gel-derived lead iron niobate ceramics

In this work, we report the synthesis of sol-gel-derived lead iron niobate [Pb1.10(Fe0.5Nb0.5)O3] (PFN) powders and sintered ceramics. The PFN powders were calcined at (Ta), 973, 1073, 1173, 1273, and 1373 K for 3 h in air. As envisaged from x-ray-diffraction analyses, PFN powder calcined at 1173 K was crystallized into pure monoclinic perovskite phase whereas powders calcined at all other temperatures had varied amounts of retained pyrochlore (Pb3Nb4O13) phase coexisted with dominant monoclinic perovskite phase. The PFN pellet (prepared using the phase pure powder calcined at 1173 K) sintered at 1373 K for 4 h in air also had a minute quantity of retained pyrochlore phase coexisting with desired perovskite phase. From the temperature dependence of measured capacitance and loss tangent at different frequencies, the ferroelectric to paraelectric phase-transition temperature of PFN ceramics was observed at Tc≈370K. The diffused nature of this transition and high dielectric constant of PFN is related to the ...

On the room temperature ferromagnetism in Co-doped TiO2 films

We report detailed studies of magnetization (M) as a function of magnetic field H (up to 50 kOe) and temperature T (5–380 K) in x% Co/TiO2 (rutile) films for x=1, 4, 6, 10, and 12. These films, prepared by sputtering, show room temperature ferromagnetism (RTFM). Measurements of M vs T at H=50 Oe reported here in zero-field-cooled (ZFC) and field-cooled (FC) cases show M peaking at Tp≃30 K for 1% Co to Tp≃125 K for 12% Co doped films whereas bifurcation of the FC and ZFC data is observed near 300 K. For T<Tp, the coercivity Hc rises sharply. These observations and the electron magnetic resonance data suggest that the observed RTFM is at least partly due to magnetic nanoparticles, most likely of Co, that are undetected in standard X-ray diffraction (XRD) for lightly doped samples. XRD studies show that a fraction of Co does substitute for Ti in CoxTi1−xO2. However, because of interference from Co nanoparticles, the RTFM of CoxTi1−xO2 in these samples could not be established.

Microstructure, dangling bonds and impurities in activated carbons

Wide-angle X-ray diffraction (WAXD), magnetometry, electron spin resonance (ESR) and Raman spectroscopy are used to investigate three activated PICA carbons (viz. GX203, P1400, Med 50) with surface areas of 1000, 1150 and 2000 m2/g, respectively. Although WAXD showed the absence of any minerals in these carbons, temperature and magnetic field variations of the magnetization shows significant magnetic impurities with concentrations in the order GX203>Med 50>P1400. Analysis of WAXD and Raman spectra shows these carbons to consist of graphitic crystallites with La≃30 A and Lc≃10 A. All three carbons give an ESR line due to dangling bonds with g=2.0028(3) but the linewidth ΔH and the spin concentration Ns are strongly affected by air/oxygen. The linewidths are reduced from ΔH∼4 Oe for the as-received samples to ΔH∼1.2 Oe for the evacuated samples. The Ns values are enhanced upon evacuation, with the enhancement factors of about 40, 130 and 670 for GX203, P1400 and Med 50 respectively, correlating well with the increase in the relative surface areas of these carbons. Magnetization studies using a SQUID magnetometer show that below about 30 K, all three carbons yield Curie-law variations with Ns∼1019/gram, in good agreement with the Ns values obtained from the room temperature ESR studies on evacuated samples.

Multiferroic properties of Pb(Zr,Ti)O3∕CoFe2O4 composite thin films

In the present work we report multiferroic behavior in lead zirconate titanate (PZT)–cobalt iron oxide (CFO) composite thin films. It is found that upon annealing, the multilayered structures are intermixed at least partially, and CFO is phase separated into PZT matrix to form a composite film. The phase separation behavior has been characterized by x-ray photoelectron spectroscopy depth profiling of the constituent elements in conjunction with dielectric spectroscopy measurements. The composite films exhibited ferroelectric as well as ferromagnetic characteristics at room temperature. The coupling between the ferroelectric and the ferromagnetic order parameters has been demonstrated through the reduction of ferroelectric polarization when measured under an applied magnetic field.

Magnetism of Co-doped titania thin films prepared by spray pyrolysis

Co-doped TiO2 (anatase) films prepared by spray pyrolysis at 500 °C are investigated for the possible existence of room temperature ferromagnetism (RTF). These films were characterized by x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy at room temperature, followed by studies of the temperature and magnetic field dependence of the magnetization by squid magnetometry. For Co doping of upto 10 at. %, no phase other than anatase could be detected, and there is no RTF in these films. Instead, the magnetic susceptibility χ fits the modified Curie–Weiss law χ=χ0+C/(T+θ), with θ≃5 K and magnitude of C consistent with Co2+ paramagnetism in Ti1−xCoxO2−x. Only for T⩽5 K, a hysteresis loop with coercivity Hc=200 Oe is observed. These observations suggest Co2+ paramagnetism, with exchange interaction becoming important for T⩽5 K in the case of 10% Co/TiO2 films.