Browsing by Author "V.P.S. Awana"

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Disorder Induced Magnetic Behavior of Non-Stoichiometric Co0.75Mn0.5Fe0.75Si Full-Heusler Alloy
(Springer, 2022) Bal Govind; Purnima Bharti; Sahiba Bano; Ashish Kumar; Satyendra Singh; V.P.S. Awana
Many stoichiometric Heusler alloys have been extensively investigated in recent years due to their wide range of magnetic properties. However, a lack of investigation has been found for non-stoichiometric Heusler alloys. This report endeavored to grow few non-stoichiometric compositions but only succeeded in growing Co0.75Mn0.5Fe0.75Si composition. It is crystallized in LiMgPdSn prototype (space group-F43m) structure having lattice parameter of 5.66 Å, and transmission electron microscopy reveals a single phase of Co0.75Mn0.5Fe0.75Si quaternary Heusler alloy. Magnetization variation with applied magnetic field reveals slightly deviated from the magnetic moments (~ 3.53–3.70 μ B/ f. u.) of stoichiometric CoFeMnSi alloys. Interestingly, the magnetic moment of Co0.75Mn0.5Fe0.75Si is found to be 3.80 μ B/ f. u. at room temperature, which is greater than the magnetic moments (3.44 μ B/ f. u.) at 5 K temperature. Moreover, a significant splitting between zero-field cooled (ZFC) and field cooled (FC) curve points towards the co-existence of the ferromagnetic (FM) and antiferromagnetic (AFM) state in the Co0.75Mn0.5Fe0.75Si sample, which is associated due to disorder present in a cubic structure. Thus, the presence of exchange biasing associated with the FM and AFM eventually shows extreme importance in magneto-resistive device and sensors, which is not evidenced in stoichiometric Heusler alloys. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
Effect of Ca doping for Y on structural/microstructural and superconducting properties of YBa2Cu3O7-δ
(Elsevier, 2005) Rajiv Giri; V.P.S. Awana; H.K. Singh; R.S. Tiwari; O.N. Srivastava; Anurag Gupta; B.V. Kumaraswamy; H. Kishan
We report systematic studies of structural, microstructural and transport properties of Y1-xCaxBa2Cu3O 7-δ bulk samples with 0.20 ≥ x ≥ 0.0. The partial substitution of Ca2+ at Y3+ site in YBa2Cu 3O7-δ leads towards slightly overdoped regime, which along with disorder in CuO2 planes is responsible for decrease in superconducting transition temperature (Tc) of the doped system. The microstructural variants being studied by transmission electron microscopy (TEM) technique in imaging and selected area diffraction modes revealed an increase in the density of twins with increase in Ca concentration despite the fact that there is slight decrease in the orthorhombicity. This is against the general conviction of decrease in twin density with decreasing orthorhombicity. An increase in twin density results in sharpening of twin boundaries with increasing Ca concentration. These sharpened twin boundaries may work as effective pinning centers. A possible correlation between microstructural features and superconducting properties has been put forward and discussed in the present investigation. © 2005 Elsevier B.V. All rights reserved.
Effect of Sn substitution at Sb site on the magnetic properties of Mn2NiSb full-Heusler alloy
(Elsevier Ltd, 2022) Bal Govind; Purnima Bharti; Ashish Kumar; Sahiba Bano; Satyendra Singh; V.P.S. Awana
Alignment of spin of Mn-Mn atoms in Heusler structure mainly depends upon the distance between them which can be easily manipulated by substituting the main group Z-elements in the Mn2-based full-Heusler alloys. In this article, we report the effect of Sn doping at the Sb site in the Mn2NiSb1−XSnX (0.02≤x≤0.1) full-Heusler alloys. We have performed the X-Ray diffraction (XRD) measurement of each sample to obtain the structural information of the synthesized full-Heusler phase. Rietveld refinement performed on the each XRD data of Mn2NiSb1−XSnX (0.02≤x≤0.1) revealed that all the samples were crystallized in the Hg2CuTi prototype (Space group-Fm3m) structure. Analysis of selected area diffraction (SAED) pattern obtained from transmission electron microscopy (TEM) measurement revealed that a single phase of full-Heusler was obtained for the highest Sn content (x = 0.1) sample. Magnetization variation with applied magnetic field plot exhibited the ferromagnetic ground state of each sample as evidenced from hysteresis loop and saturation was obtained up to 1.0 Tesla field. Zero field cooled (ZFC) and field cooled (FC) measurements were also investigated and splitting between ZFC and FC were found to be increasing with an increase in the Sn content. This increment in ZFC and FC splitting with Sn content was attributed to the coupling between the ferrimagnetic (FIM) nature of Mn2NiSn phase with ferromagnetic (FM) nature of Mn2NiSb phase at the interfaces. Small shifting of hysteresis curve observed in the virgin M-H loop around the origin also indicated the spontaneous exchange biasing due to the FM/FIM interaction. Compensation of magnetization followed by exchange biasing with two magnetic phase (FM/FIM) governed by the controlled doping of Sn in Mn2NiSb have been alluringly elaborated. Direct interaction between different known magnetic phases/sub-lattices are rarely investigated which can be used to make suitable materials for spintronic application such as magneto resistive read heads and sensors. © 2022 Elsevier B.V.
Enhancement in magnetic and dielectric properties of La and Pr co substituted BiFeO3
(Elsevier Ltd, 2013) Amit Srivastava; H.K. Singh; V.P.S. Awana; O.N. Srivastava
This report underlines the systematic studies of crystalline structure, magnetic and ferroelectric properties of polycrystalline Bi 1-x yLaxPryFeO3 ceramic samples, in which × changes continuously from 0 to 0.2 for y = 0 and y from 0 to 0.2 for × = 0.2. X-ray diffraction (XRD) patterns revealed that La and Pr substitution at Bi site in the ceramic eliminates the usual impurity phases completely. Rietveld refinement of the XRD patterns shows that the crystal structure changes gradually from Rhombohedral (R3c) to Orthorhombic (pbnm) with increasing La and Pr concentration. This transition has significant effects on the multiferroic properties of Bi1 × yLaxPr yFeO3 ceramics. Substantial enhancement in magnetization of Bi1 × yLaxPryFeO3 has been observed and this is found to be correlated with the evolution of structural phase change with doping of Pr in samples having lanthanum concentration of × = 0.2. This leads to the suppression of helical spin order. However, the enhancement in magnetic behavior also takes place due to nanocrystallite nature of Bi1 × yLaxPryFeO3 (x = 0.2, y = 0.05-0.2). For the nanocrystallites having sizes lower than 62 nm, which is the period of spin cycloid, this spin configuration will get destroyed resulting in the enhancement of magnetization. The studies of microstructures employing SEM and TEM revealed that Bi1 × yLa xPryFeO3 consists of nano size grained microstructures. It is also found that dielectric constant and dielectric loss get improved by La and Pr co-substitution. The dielectric constant for x = 0.0, y = 0.0 is 81 which changes to 354 for × = 0.2, y = 0.15 at 100 Hz. Dielectric losses are strongly diminished from 0.960 for × = 0.0, y = 0.0, 0.321 for × = 0.2, y = 0.0 and to 0.20 for × = 0.2, y = 0.05 at 100 Hz. This improvement in dielectric properties is likely due to the changes of lattice parameters and suppression of oxygen vacancies caused by La and Pr co substitution. © 2012 Elsevier B.V. All rights reserved.
Impact of sintering temperature on room temperature magneto-resistive and magneto-caloric properties of Pr2/3Sr1/3MnO3
(Elsevier Ltd, 2013) Ramesh Chandra Bhatt; Shiva Kumar Singh; P.C. Srivastava; S.K. Agarwal; V.P.S. Awana
Magneto-resistive and magneto-caloric properties of polycrystalline Pr 2/3Sr1/3MnO3 have been studied as a function of sintering temperature (Ts) between 1260 and 1450 °C. Reitveld refinement of their X-ray diffraction (XRD) patterns confirms their single phase crystalline structure with orthorhombic Pbnm space group. The point of maximum value of temperature coefficient of resistance (TCRmax) and Curie temperature (Tc) decreased slightly with Ts. Magneto-resistance (MR) and magnetic entropy (DSM) increased markedly with sintering temperature. This could be attributed to the observed sharpness of both the magnetic and resistive transitions due to better grain connectivity. Optimum results are obtained for the sample with Ts = 1400 °C. MR at Tc of the same is found to be as large as 32% at 1T and 58% at 5T magnetic fields. The maximum entropy change (DSMmax) near its Tc is 2.3 J kg-1 K-1 and 7.8 J kg--1 K-1 upon 1T and 5T fields change respectively. These characteristics [MR (32% 1T, 58% 5T) and reasonable change in magnetic entropy (7.8 J kg-1 K-1, 5T)] generate possibility that the optimized compound can be used as a potential magnetic refrigerant close to room temperature. © 2013 Elsevier B.V. All rights reserved.
Impact of Zn substitution on phase formation and superconductivity of Bi1.6Pb0.4Sr2Ca2Cu 3-xZnxO10-δ with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12
(2005) R.B. Saxena; Rajiv Giri; V.P.S. Awana; H.K. Singh; M.A. Ansari; B.V. Kumaraswamy; Anurag Gupta; Rashmi Nigam; K.P. Singh; H. Kishan; O.N. Srivastava
Samples of series Bi1.6Pb0.4Sr2Ca 2Cu3-xZnxO10-δ with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12 are synthesized by solid-state reaction route. All the samples crystallize in tetragonal structure with majority (> 90%) of Bi-2223 (Bi2Sr2Ca2Cu3O10) phase (c-lattice parameter ∼ 36 Å). The proportion of Bi-2223 phase decreases slightly with an increase in x. The lattice parameters a and c of main phase (Bi-2223) do not change significantly with increasing x. Superconducting critical transition temperature (Tc) decreases with x as evidenced by both resistivity [ρ(T)] and ac magnetic susceptibility [χ(T)] measurements. Interestingly the decrement of Tc is not monotonic and the same saturates at around 96 K for x > 0.06. In fact Tc decreases fast (∼ 10 K/at%) for x = 0.015 and 0.03 samples and later nearly saturates for higher x values. Present results of Zn doping in Bi-2223 system are compared with other Zn-doped HTSC (high temperature superconducting) systems, namely the RE-123 (REBa2Cu3O7) and La-214 ((La, Sr)2CuO4). © World Scientific Publishing Company.
Magneto-resistance, magneto-caloric and bolometric properties in Pr2/3Sr1/3MnO3:PdO composites near room temperature
(Institute of Physics Publishing, 2014) Ramesh Chandra Bhatt; P.C. Srivastava; V.P.S. Awana; H. Kishan; S.K. Agarwal
Temperature coefficient of resistance (TCR), magneto-resistance (MR) and magneto-caloric properties of polycrystalline pristine Pr2/3Sr1/3MnO3 (PSMO) and its composites with PdO (10-30 mole %) (PSMO:Pd) have been studied. Rietveld refinement of their x-ray diffraction (XRD) patterns confirms their single phase crystalline structure with orthorhombic Pbnm space group and Pd appears in pure metallic phase. Of the various composites investigated, PSMO: Pd30 composite exhibits significant TCR of 9.7%K-1 (at 281 K) and low field MR value of 33.5% (H= 1 T). Magnetic entropy change (ΔSM) has been found to be 3.5 J kg-1K-1 (1 T), 6.3 J kg-1K-1 (2 T) and 8.7 J kg-1K-1 (3 T) at 283K for PSMO:Pd10 sample which show a decreasing trend with increasing Pd content. Present investigations reveal excellent characteristics for bolometric, magnetic sensing and magnetic refrigerating applications near room temperature by PSMO:Pd composites. © 2014 IOP Publishing Ltd.
Magnetotransport and thermal properties characterization of 55 K superconductor SmFeAsO0.85F0.15
(American Institute of Physics Inc., 2013) Amit Srivastava; Anand Pal; Saurabh Singh; C. Shekhar; H.K. Singh; V.P.S. Awana; O.N. Srivastava
This report fairly underlines the magneto-transport, thermal properties characterization and bulk superconductivity in the FeAs-based SmFeAsO 0.85F0.1. The phase formation and structure are confirmed by Rietveld analysis of room temperature powder X-ray diffraction (XRD) data. Electron microscopy was employed to unravel the micro structural details, such as perfection of the lattice and the grain morphology including size and boundaries. The electrical and magnetic measurements have been carried out to confirm the bulk superconductivity and understand the nature of electrical transport in the normal and superconducting state. The intra-grain critical current density (Jc) with applied magnetic field is calculated from isothermal DC magnetization (MH) plots using conventional Bean critical state model. Superconductivity is observed at transition temperature (Tc) above 55 K without HPHT (high pressure high temperature) synthesis route. The value of Jc is found to be around 5.26 × 104 A/cm2 at 5 K in zero field. The dependence of thermally activated flux flow energy (U/kB) on the applied magnetic field has been observed. AC susceptibility measurements were performed for 55 K superconducting SmFeAsO0.85F0.15 sample at various amplitude of applied AC drive field and its granular nature is confirmed. The parent compound SmFeAsO is found to be magnetic with Fe spin density wave (SDW) like order below 150 K, on the other hand the F doped SmFeAsO0.85F0.15 sample is bulk superconducting at below 55 K. Both Fe (SDW) at 150 K for SmFeAsO and 55 K superconductivity in case of SmFeAsO0.85F0.15 sample has confirmed by Specific heat [Cp(T)] measurement too. Further Sm orders anti-ferro-magnetically at 4.5 K for non-superconducting and at 3.5 K for superconducting samples, also the entropy change is reduced significantly for the later than the former. Summarily complete physical property characterization for both non-superconducting SmFeAsO and 55 K superconductor SmFeAsO 0.85F0.15 samples is provided and discussed in the current article. © 2013 © 2013 Author(s).
Near room temperature magneto-transport (TCR & MR) and magnetocaloric effect in Pr2/3Sr1/3MnO3:Ag2O composite
(Elsevier Ltd, 2015) Ramesh Chandra Bhatt; V.P.S. Awana; H. Kishan; P.C. Srivastava
The magneto-transport and magnetocaloric effect in Pr2/3Sr1/3MnO3:Ag2O (PSMO:Ag) with Ag2O (10-30 mol%) have been studied by resistivity and magnetization measurements. Temperature coefficient of resistance (TCR), magneto-resistance (MR) and magnetocaloric properties of the composite have shown different responses to Ag addition. Rietveld refinement confirms single phase crystalline structure with orthorhombic Pbnm space group and presence of silver metal as well. TCR changes randomly with Ag, increasing initially to 7.8% K-1for PSMO:Ag10 composite which then decreases marginally with increasing Ag content. MR has been found to reach a maximum value of 30.5% at 1 T applied field for PSMO:Ag10 composite. Magnetic entropy change (ΔSM) increases with increasing Ag content and is found to be 3.5 J kg-1K-1(1 T), 6.1 J kg-1K-1(2 T) and 8.4 J kg-1K-1(3 T) for PSMO:Ag30 composite at 285 K. The maximum refrigeration capacity of 39.5 J kg-1(1 T), 91.0 J kg-1(2 T) and 107.7 J kg-1(3 T) has been found for the PSMO:Ag20 composite. PSMO:Ag composite exhibits potential for bolometric, magnetic sensing and magnetic refrigerating applications near room temperature. © 2014 Elsevier B.V. All rights reserved.
Observance of improved magneto-resistance and magnetic entropy change in La0.7(Ca0.2Sr0.1)MnO3:Pd composite
(Springer New York LLC, 2014) Ramesh Chandra Bhatt; P.C. Srivastava; S.K. Agarwal; V.P.S. Awana
Magneto-resistive and magneto-caloric properties of polycrystalline La 0.7(Ca0.2Sr0.1)MnO3 (LCSMO) and La0.7(Ca0.2Sr0.1)MnO3:10 %PdO (LCSMO:Pd10) composites sintered at 1400 °C have been investigated. Rietveld refinement of their X-ray diffraction (XRD) patterns confirms the single-phase crystalline structure with orthorhombic Pbnm space group, showing no significant change in their lattice parameters with Pd addition. Disappearance of the low temperature resistivity hump in the Pd composite has been attributed to the suppression of the grain boundary effect and the conducting channels due to the presence of metallic Pd. While the Curie temperature T c remained nearly unchanged, the peak value of the temperature coefficient of resistance (TCR) increased in the composite. TCRMAX increased to 6.4 % (at 308 K) in the composite from 2 % (at 305 K) for the pristine LCSMO. Magneto-resistance (MR) and magnetic entropy change (ΔS M ) also increased markedly in the composite material. This could be ascribed to the observed sharpness of both the magnetic and resistive transitions resulting from better grain connectivity. Maximum MR of 12.9 % (1 T) and 19.6 % (2 T) has been observed close to its T c (309 K) in the pristine LCSMO. These values increased to 24.1 % (1 T) and 33.9 % (2 T) with the addition of PdO. The maximum values of |ΔS M | are found to be 4.4, 8.2 and 11.7 J∈kg-1∈K-1 at field values of 1, 2 and 3 T, respectively, for LCSMO:Pd10 composite, which are far better than those reported for LCSMO:Ag10 composite. © 2013 Springer Science+Business Media New York.
Room temperature magneto-resistance and temperature coefficient of resistance in la0.7Ca0.3-xAgxMnO3 thin films
(2007) P.K. Siwach; V.P.S. Awana; H. Kishan; R. Prasad; H.K. Singh; S. Balamurugan; E. Takayama-Muromachi; O.N. Srivastava
We investigate the magnetotransport properties of La0.7Ca 0.3-xAgxMnO3 (0 ≤ x ≤ 0.3) polycrystalline films prepared by spray pyrolysis. The virgin La 0.7Ca0.3MnO3 (LCMO) (x=0.0) film shows paramagnetic-ferromagnetic [insulator-metal (IM)] transition at TC ∼ 260 K (TIM ∼245 K). Ag doping induces appreciable enhancement in TC/TIM and interestingly TIM of Ag doped films is higher than the corresponding TC value. In the 30% Ag doped film IM transition occurs at ∼325 K which is ∼ 10 K higher than its TC ∼315 K. Strong irreversibility is observed in zero field cooled-field cooled magnetization of all the films and the irreversibility temperature Tirr >TC increases with Ag doping. This suggests the formation of magnetic clusters above TC. The pure LCMO film exhibits small magnetoresistance (MR) ∼20% and rather poor temperature coefficient of resistance (TCR) in the vicinity of the room temperature. In contrast, around room temperature the Ag doped films show large MR which is ∼60% for x=0.15 and decreases to ∼30% for the pure Ag doped film. Ag doped films also have good TCR ∼4%/K in the vicinity of room temperature. © 2007 American Institute of Physics.
Unexplored photoluminescence from bulk and mechanically exfoliated few layers of Bi2Te3
(Nature Publishing Group, 2018) Bipin Kumar Gupta; Rabia Sultana; Satbir Singh; Vijeta Singh; Geet Awana; Anurag Gupta; Bahadur Singh; A.K. Srivastava; O.N. Srivastava; S. Auluck; V.P.S. Awana
We report the exotic photoluminescence (PL) behaviour of 3D topological insulator Bi2Te3 single crystals grown by customized self-flux method and mechanically exfoliated few layers (18 ± 2 nm)/thin flakes obtained by standard scotch tape method from as grown Bi2Te3 crystals. The experimental PL studies on bulk single crystal and mechanically exfoliated few layers of Bi2Te3 evidenced a broad red emission in the visible region from 600-690 nm upon 375 nm excitation wavelength corresponding to optical band gap of 2 eV. These findings are in good agreement with our theoretical results obtained using the ab initio density functional theory framework. Interestingly, the observed optical band gap is several times larger than the known electronic band gap of ∼0.15 eV. The experimentally observed 2 eV optical band gap in the visible region for bulk as well as for mechanically exfoliated few layers Bi2Te3 single crystals clearly rules out the quantum confinement effects in the investigated samples which are well known in the 2D systems like MoS2,WS2, WSe2, and MoSe2 for 1-3 layers. © 2018 The Author(s).