Sankaranarayanan V

The dielectric relaxation behavior of the fluorite structured nanocrystalline Ce0.8Gd0.1Pr0.1 O 2-δ compound was studied in the temperature range of 200-550 °C. Two different types of relaxation processes were observed corresponding to (1) defect pairs such as (Pr′Ce - VO) and (Gd′Ce - VO) and (2) the trimers such as (Pr′Ce - VO - Gd′Ce). The correlation between ionic conduction and the dielectric properties of the nanocrystalline material is discussed. Very low values of the migration energy and association energy of the oxygen vacancies are observed, which are 0.42 and 0.03 eV, respectively. The obtained value of association energy agrees well with the theoretical prediction on doubly doped ceria. © 2009 American Institute of Physics.

The influence of Mm substitution (Mm = mischmetal) on structural, transport and magnetic properties of (Dy1-xMmx)Co2 (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys has been investigated by means of X-ray diffraction (XRD), temperature dependent electrical resistivity (ρ(T)), ac susceptibility (χ(T)) and thermopower (S(T)). XRD patterns show the formation of solid solutions crystallizing with cubic Laves (C15) type structure at room temperature. The pronounced discontinuities in the resistivity and thermopower at Curie temperature (TC) are explained based on the suppression of the spin-fluctuation contribution. The gradual decrease in TC and sharpness of discontinuities in ρ(T) and S(T) with increasing Mm substitution has been discussed. © 2008 Elsevier Ltd. All rights reserved.

Neutron diffraction measurement on the spin glass double perovskite Sr2FeCoO6 reveals site disorder as well as Co3+ intermediate spin state. In addition, multiple valence states of Fe and Co are confirmed through M̈ossbauer and X-ray photoelectron spectroscopy. The structural disorder and multiple valence lead to competing ferromagnetic and antiferromagnetic interactions and subsequently to a spin glass state, which is reflected in the form of an additional T-linear contribution at low temperatures in specific heat. A clear evidence of Jahn-Teller distortion at the Co3+-O6 complex is observed and incorporating the physics of Jahn-Teller effect, the presence of localized magnetic moment is shown. A large, negative and anomalous magnetoresistance of ≈63% at 14 K in 12 T applied field is observed for Sr2FeCoO6. The observed magnetoresistance could be explained by applying a semi-empirical fit consisting of a negative and a positive contribution and show that the negative magnetoresistance is due to spin scattering of carriers by localized magnetic moments in the spin glass phase. © Springer-Verlag 2012.

Structural, magnetic and magnetocaloric properties of sol-gel prepared, nanocrystalline oxides Pr 1-xA xMn1-yCo yO 3 (A = Ca, Sr) (x = 0.3; y = 0.5) (cubic, space group Fm3̄m) have been studied. From the X-ray data, the crystallite size of Pr 0.7Ca 0.3Mn 0.5Co 0.5O 3 and Pr 0.7Sr 0.3Mn 0.5Co 0.5O 3 samples is found to be ∼24 nm and ∼15 nm respectively. High resolution transmission electron microscopy image shows average particle size of ∼34 nm and ∼20 nm. Magnetization measurements indicate a Curie temperature of ∼153 K and ∼172 K in applied magnetic field of 100 Oe for Pr 0.7Ca 0.3Mn 0.5Co 0.5O 3 and Pr 0.7Sr 0.3Mn 0.5Co 0.5O 3 compounds. The magnetization versus applied magnetic field curves obtained at temperatures below 150 K show significant hysteresis and magnetization is not saturated even in a field of 7 T. The magnetocaloric effect is calculated from M versus H data obtained at various temperatures. Magnetic entropy change shows a maximum near T C for both the samples and is of the order ∼2.5 J/kg/K. © 2011 American Scientific Publishers.

The magnetic and electrical transport properties of Ce doped LaMnO3 bulk samples have already been reported for various Ce concentrations. However there are few reports on the magnetotransport properties of these compounds. In this paper, we report the magnetic and magnetotransport properties of nanocrystalline La0.8Ce0.2MnO3 and La0.7Ce0.3MnO3 samples with different particle size. The nanocrystalline samples with different particle size were prepared through citrate-complex method by calcining the precursor at different temperatures. The transmission electron microscopy analysis of both the compounds reveal that the particle size increases with calcination temperature. Temperature dependence of magnetization of all the samples shows ferro- to paramagnetic transition. The zero field cooled and field cooled magnetization curves show irreversibility just below the magnetic transition temperature. The temperature dependence of resistivity of all the samples exhibits a metal to insulator transition without an impurity peak around 250 K, which is generally observed in multiphase samples. The results indicate that magnetoresistance (MR) increases with decreasing particle size. The observed MR will be discussed by spin dependent tunneling and enhancement of double exchange mechanism upon application of magnetic field. The physical properties suggest that the samples are single phase in nature. © 2006 Elsevier B.V. All rights reserved.

Realization of diverse functionalities by tuning magnetic interactions in rare-earth perovskite oxide thin films opens up exciting technological prospects. Strain-induced tuning of magnetic interactions in rare-earth cobaltates and nickelates is of central importance due to their versatility in electronic transport properties. Here we report the spin reorientation induced switching of anisotropic magnetoresistance (AMR) and its tunability with strain in epitaxial LaCo0.5Ni0.5O3-δ thin films across the ferromagnetic transition. Moreover, with strain tuning, we observe a twofold to fourfold symmetry crossover in AMR across the magnetic transition temperature. The magnetization measurements reveal a ferromagnetic transition around 50 K. At temperatures below this transition, there is a subtle change in the magnetization dynamics, which reduces the ferromagnetic long-range ordering in the system. X-ray absorption and x-ray magnetic circular dichroism spectroscopy measurements at the Co and Ni L edges reveal a Co spin state transition below 50 K, leading to the AMR switching and also the presence of Ni2+ and Co4+ ions evidencing the charge transfer from Ni to Co ions. Our work demonstrates the tunability of magnetic interactions mediated electronic transport in cobaltate-nickelate thin films, which is relevant in understanding Ni-Co interactions in oxides for their technological applications such as in AMR sensors.

The optical switching properties of a solid state device based on Ho0.6 Mm0.4 Co2 (HMC) alloy thin film as a switching active layer, water pretreated Nafion membrane as a solid electrolyte, and a transparent conducting indium tin oxide (ITO) as a counterelectrode are investigated. The device is simple and has a reduced layer sequence of HMC/Pd/Nafion/ITO. The reversible optical switching of this device has been studied during electrochemical galvanostatic charging-discharging as well as cyclic voltammetric measurements. Further, the optical switching durability of the device has been tested by repeated electrochemical hydrogenation-dehydrogenation, and the variations in the optical switching properties are discussed. The special characteristic of the device is that it can reversibly switch between a metallic reflecting state and a semiconducting transparent state by a small reversible applied current/voltage indicating the potential substitution for conventional electrochromic devices. © 2008 American Institute of Physics.

Novel foam composites comprising functionalized graphene (f-G) and polyvinylidene fluoride (PVDF) were prepared and electrical conductivity and electromagnetic interference (EMI) shielding efficiency of the composites with different mass fractions of f-G have been investigated. The electrical conductivity increases with the increase in concentration of f-G in insulating PVDF matrix. A dramatic change in the conductivity is observed from 10 -16 S · m-1 for insulating PVDF to 10-4 S · m-1 for 0.5 wt.% f-G reinforced PVDF composite, which can be attributed to high-aspect-ratio and highly conducting nature of f-G nanofiller, which forms a conductive network in the polymer. An EMI shielding effectiveness of ≈20 dB is obtained in X-band (8-12 GHz) region and 18 dB in broadband (1-8 GHz) region for 5 wt.% of f-G in foam composite. The application of conductive graphene foam composites as lightweight EMI shielding materials for X-band and broadband shielding has been demonstrated and the mechanism of EMI shielding in f-G/PVDF foam composites has been discussed. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The effect of rare earth ion (RE = Er3+, Tb3+, Ho3+) substitution in Pr0.74Sr0.26MnO 3 (PSMO(0.26)) on the magnetic and transport properties is reported. It is found that even for a small concentration of substitution of Pr 3+ ions by Er3+, Tb3+, or Ho3+ ions there is a significant change in the transport and magnetic properties compared to the parent compound Pr0.74Sr0.26MnO3. The observed changes are attributed to the increase in the average A-site disorder upon substituting smaller ions (ionic radius of Er3+ = 1.062 Å, Ho3+ = 1.072 Å, Tb3+ = 1.095 Å) in place of bigger Pr3+ ion (ionic radius = 1.179 Å). The Ho3+ substituted sample was found to behave anomalously when compared to the other two rare earth ion-substituted (Tb3+ and Er3+) samples. Such anomalous behaviour could most likely be caused by the magnetic coupling of the Ho and Mn ions.

A very effective method of bandgap engineering in perovskite ferroelectrics by successive multiple doping (SMD) is proposed, tested, and compared with conventional incremental solitary doping (ISD). The application of SMD of La, Nb, and Fe (2% each) in Pb(Zr,Ti)O3 (PZT) tailors the bandgap to 2.74 from 3.53 eV (bandgap of pure PZT). This finds potential applications in ferroelectrics-based photovoltaics and optoelectronics. In contrast, only La doping even up to 10% curtails the bandgap to 3.10 eV. The X-ray photoelectron spectroscopy study confirms the presence of all elements doped and their chemical states. Analysis of photoluminescence shows that the emissions corresponding to the incorporations of La, Nb, and Fe are in the forbidden region of the energy band. In the case of only La doping, the emission corresponding to La gets broadened as its concentration increases. These observations are summarized and the mechanism of a larger bandgap reduction by SMD compared with solitary doping is explained in a systematic way. Comparing the ferroelectric hysteresis, true switchable polarization, and fatigue behaviors, it is found that ferroelectricity is affected marginally by SMD of La, Nb, and Fe compared with that by the ISD of La up to10%.