Browsing by Author "Gupta, S"

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    High spin polarization in the disordered quaternary Heusler alloy FeMnVGa
    (American Physical Society, 2023-07-25) Gupta, S; Chakraborty, S; Bhasin, V; Pakhira, S; Dan, S; Barreteau, C; Crivello, JC; Jha, SN; Avdeev, M; Greneche, JM; Bhattacharyya, D; Alleno, E; Mazumdar, C
    In this paper, we report the successful synthesis of a Fe-based highly spin-polarized quaternary Heusler alloy FeMnVGa and its structural, magnetic, and transport properties probed through different experimental methods and theoretical techniques. Density functional theory (DFT) calculations performed on different types of structures reveal that the structure with Ga at 4a, V at 4b, Mn at 4c, and Fe at 4d (space group F¯43m) possess minimum energy among all the ordered variants. Ab initio simulations in the most stable ordered structure show that the compound is a ferromagnet having a large spin-polarization (89.9%). Neutron diffraction reveals that the compound crystallizes in disordered type-2 structure (space group Fm¯3m) in which Ga occupies at 4a, V 4b and Fe/Mn occupy 4c/4d sites with 50:50 proportions. The structural disorder is further confirmed by x-ray diffraction, extended x-ray absorption fine structure, 57Fe Mössbauer spectrometry results, and DFT calculations. Magnetization studies suggest that the compound orders ferromagnetically below TC∼293 K and the saturation magnetization follows the Slater-Pauling rule. Mössbauer spectrometry, along with neutron diffraction, suggest that Mn is the major contributor to the total magnetism in the compound, consistent with the theoretical calculations, which also indicates that spin polarization remains high (81.3%), even in the presence of such large atomic disorder. The robustness of the half-metallic ferromagnetic (HMF) property in the presence of disorder is a quite unique characteristic over other reported HMF in literature and makes this compound quite promising for spintronics applications. ©2023 American Physical Society.
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    High spin-polarization in a disordered novel quaternary Heusler alloy FeMnVGa
    (Cornell University, 2023-03-15T12:51:40Z) Gupta, S; Chakraborty, S; Bhasin, V; Pakhira, S; Dan, S; Barreteau, C; Crivello, JC; Jha, SN; Avdeev, M; Greneche, JM; Bhattacharyya, D; Alleno, E; Mazumdar, C
    In this work, we report the successful synthesis of a Fe-based novel half-metallic quaternary Heusler alloy FeMnVGa and its structural, magnetic and transport properties probed through different experimental methods and theoretical technique. Density functional theory (DFT) calculations performed on different types of structure reveal that Type-2 ordered structure (space group: F-43m, Ga at 4a, V at 4b, Mn at 4c and Fe at 4d) possess minimum energy among all the ordered variants. Ab-initio simulations in Type 2 ordered structure further reveal that the compound is half-metallic ferromagnet (HMF) having a large spin-polarization (89.9 %). Neutron diffraction reveal that the compound crystalizes in disordered Type-2 structure (space group: Fm-3m) in which Ga occupy at 4a, V at 4b and Fe/Mn occupy 4c/4d sites with 50:50 proportions. The structural disorder is further confirmed by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS),57Fe Mossbauer spectrometry results and DFT calculations. Magnetisation studies suggest that the compound orders ferromagnetically below TC ~ 293 K and the saturation magnetization follows Slater-Pauling rule. Mossbauer spectrometry, along with neutron diffraction suggest that Mn is the major contributor to the total magnetism in the compound consistent with the theoretical calculations. First principle calculations indicate that spin-polarization remain high (81.3 %) even in the presence of such large atomic disorder. The robustness of the HMF property in presence of disorder is a quite unique characteristic over other reported HMF in literature and make this compound quiet promising for spintronics applications.
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    Observation of charge transfer induced large enhancement of magnetic moment in a structurally disordered inverse Heusler alloy Fe2RuGe
    (American Physical Society (APS), 2023-12-01) Chakraborty, S; Gupta, S; Bhasin, V; Pakhira, S; Barreteau, C; Crivello, JC; Jha, SN; Bhattacharyya, D; Avdeev, M; Paul-Boncour, V; Greneche, JM; Alleno, E; Mazumdar, C
    We report the successful synthesis of a new 4d-based polycrystalline inverse Heusler alloy Fe2RuGe by an arc melting process and have studied in detail its structural, magnetic and transport properties complemented with first-principles calculations. X-ray and neutron diffraction, extended x-ray absorption fine structure, and 57Fe Mössbauer spectroscopic studies confirm the single-phase nature of the system where the Fe and Ru atoms are randomly distributed in the 4c and 4d Wyckoff positions in a ratio close to 50:50. The formation of the disordered structure is also confirmed by the theoretical energy minimization calculation. Despite the random cross-site disorder of Fe and Ru atoms, magnetic measurements suggest not only a high Curie temperature of ∼860 K, but also a large saturation magnetic moment ∼4.9µB per formula unit at 5 K, considerably exceeding the theoretical limit (4 µB per formula unit) predicted by the Slater-Pauling rule. Only a few Fe-based inverse Heusler alloys are known to exhibit such high Curie temperatures. Neutron diffraction analysis coupled with the isothermal magnetization value indicates that the magnetic moments in Fe2RuGe are associated with Fe atoms only, which is also confirmed by Mössbauer spectrometry. Interestingly, in comparison to the cubic or hexagonal phase of the parent compound, Fe3Ge, the Curie temperature of Fe2RuGe has increased significantly despite the substitution of the nonmagnetic yet isoelectronic element Ru in this structurally disordered compound. Our theoretical calculation reveals that the large Fe moment (∼2.8µB/Fe) on the 4b site can be attributed to a charge transfer from this Fe site towards its Ru neighbors while a significant moment (∼2µB/Fe) is kept on the other Fe sites. Instead of expected Slater-Pauling value of 4µB/f.u., the substantially increased observed total magnetic moment of ∼4.9µB/f.u. is due to these electron charge transfers, which have not been previously reported in other ferromagnetic Heusler systems. © 2024 American Physical Society.
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    Temperature- and pH-responsive micelles with collapsible poly(N-isopropylacrylamide) headgroups
    (ACS Publications, 2014-06-18) FitzGerald, PA; Gupta, S; Wood, K; Perrier, S; Warr, GG
    We have studied the micelle formation and phase behavior of a series of temperature- and pH-responsive surfactants prepared by controlled radical (RAFT) polymerization. These C12NIPAMm surfactants consist of a dodecyl tail, a poly(N-isopropylacrylamide) (polyNIPAM) headgroup with average degrees of polymerization of between 7 and 96, and an ionizable carboxylate group. In the un-ionized state, these surfactants phase separate on warming toward a lower critical solution temperature (LCST), which decreases as the length of the NIPAM group is decreased. This is in agreement with the behavior of conventional nonionic poly(ethylene oxide)-based surfactants but is very different from that of polyNIPAM oligomer solutions. Small angle neutron scattering (SANS) shows that these surfactants self-assemble into micelles consisting of a nearly spherical hydrophobic core surrounded by a “hairy” polyNIPAM shell far below their LCST. Upon warming, the micelles undergo a sphere-to-rod transition induced by the collapse of the polyNIPAM shell, causing a reduction in the headgroup area. In the un-ionized state the demixing follows at the LCST, but a single charge on the free polymer end completely suppresses phase separation, allowing micelles to undergo a shape change but remain dissolved. © 2014, American Chemical Society.