[en] The substitutional effect of Mo on the magnetic and transport properties of double exchange ferromagnets, La_0.5Sr_0.5Co_1-x Mo _x O₃ (0≤x≤0.2) has been investigated. Substitution of 10% Mo at the Co-site of La_0.5Sr_0.5CoO₃ decreases the Curie temperature by ∼60 K than that of the parent compound and the long-range ferromagnetic ordering disappears for x≥0.2. The Mo-doped samples, however, undergo a transition from the parent metallic state to the insulating state below T _c. The insulating state is found to obey Mott's variable range hopping of conduction. The effect of Mo substitution is attributed to the factors namely (i) the dilution of magnetic Co sublattice (ii) the reduction of Co⁴⁺/Co³⁺ ratio resulting in a reduced carrier concentration and (iii) disruption of the intermediate spin structure of Co, namely Co³⁺: t_2g⁵e_g¹

[en] We describe the decoration of multiwalled carbon nanotubes (MCNTs) with Pt–Pd alloy nanoelectrocatalysts of three different compositions and their electrocatalytic performance toward the oxygen reduction reaction (ORR). The decoration of the MCNTs involves polymer-assisted impregnation of metal precursors PtCl₆^2- and PdCl₄^2- and the subsequent reduction of the impregnated precursors by a modified polyol route. The composition of the catalyst was controlled by tuning the molar ratio of the precursors during their impregnation. Electron probe microscopic analysis shows that the catalysts have compositions of Pt₄₆Pd_54, Pt₆₄Pd₃₆ and Pt₂₈Pd₇₂. The Pt₄₆Pd₅₄ and Pt₆₄Pd₃₆ catalysts have truncated octahedral and icosahedral shapes with a size ranging from 8 to 10 nm. On the other hand, the catalyst of Pt₂₈Pd₇₂ composition has a spherical/quasispherical shape with a size distribution of 1–2 nm. The XPS measurement confirms the signature of metallic Pt and Pd. The Pt₄₆Pd₅₄ catalyst has a pronounced electrocatalytic activity toward the ORR with a specific and mass activity of 378 μA cm_Pt-Pd^-2 and 64 μA μg_Pt-Pd^-1, respectively at 0.8 V. Moreover, the Pt₄₆Pd₅₄ nanoelectrocatalyst is highly durable and it retains its initial catalytic activity even after 1000 extensive cycles. Interestingly, this catalyst has a very high tolerance toward methanol and it does not favor the oxidation of methanol in the potential window of 0.1–1.4 V. The electrocatalytic activity of the alloy electrocatalyst is compared with commercially available Pt black and MCNT-supported spherical Pt nanoparticles. The catalytic activity of the Pt₄₆Pd₅₄ nanoelectrocatalyst is higher than the other catalysts. The Pt₄₆Pd₅₄ catalyst outperforms the electrocatalytic activity of all other catalysts. (paper)

[en] X-ray absorption spectroscopic studies of SrRu_0.5Mn_0.5O₃ show that 2p →t_2g related peak at the Ru-L₂ edge is more intense than the Ru-L₃ edge and shift of the 2p →e_g related peak of the Ru-L_2,3 edge by ∼ 0.8 eV to higher energy than that of the Ru (IV) compound, SrRuO₃. Significant differences in the spectral change is due to the existence of the mixed valence ionic pair involving Ru⁺⁴/ Ru⁺³, rather than the crystal field splitting energy (10 Dq) effect, which is confirmed by the crystal field multiplet calculation approach. (author)

[en] Graphical abstract: The correlation of Fe 3s XPS spectrum and the magnetic properties of SrFe_12−xAl_xO₁₉ show that the intensity of majority peak decreased with the doping of Al ion and the spectral features are associated with the magnetic moment. Highlights: ► The coercivity of SrFe₁₂O₁₉ increased from 5420 Oe to 7340 Oe by doping of 10% Al ion. ► The correlation of XPS and magnetic data suggests that the majority peak intensity of Fe 3s XPS spectrum is associated with the magnetization value, and can be used as a spectroscopic diagnostic tool to comprehend the doping effect of Al ion on the magnetic moment of spin up and spin down sublattice of hexaferrite. ► The XPS Fe 3s and Fe 2p data indicate the Al ions occupy the 2a site of the crystal structure up to x = 1.2, and beyond the dopant ions distribute at other occupation sites. - Abstract: We have performed an X-ray photoelectron spectroscopy (XPS) study of SrFe_12−xAl_xO₁₉ (0 ≤ x ≤ 2.4), and the results have been correlated to the magnetic properties measured using VSM technique. The coercivity of the parent compound increased from 5420 Oe to 7340 Oe with increasing the doping concentration of Al ions up to x = 1.2. The XPS data of Fe 3s spectrum show that the intensity of majority peak decreased with the doping of Al ion and it is associated with the magnetic moment. The XPS analysis of Fe 2p data in conjugation with the Fe 3s data suggests that the Al ions occupy the 2a site of the crystal structure up to x = 1.2, and beyond the dopant ions distribute at other occupation sites. The observed correlation between the spectral features of Fe 3s spectrum and the magnetic data suggests that XPS technique can also be used as a spectroscopic diagnostic tool to comprehend the influence of doping ion on the magnetic moment of spin up and spin down sublattice of ferrimagnetic structure.

[en] We have studied the valence and electronic properties of Mn doped SrRuO₃ using electrical transport measurement, X-ray photoelectron spectroscopy (XPS) and local (spin) density approximation plus Coulomb interaction strength calculation (LSDA+U). The resistivity data revealed that the system undergoes transition from metal to insulator at the critical Mn doping level, x∼0.2, which is accompanied by the structural transition from orthorhombic to tetragonal crystal symmetry. Besides, the significant reduction of the spectral weight at the coherent zone (0.8 eV) of the valence band is observed for x>0.2. The core XPS spectra suggest that both the transition elements exist in the mixed ionic pair, Ru⁺⁴/Ru⁺⁵↔Mn⁺³/Mn⁺⁴. The detail analysis of the results suggests that the Coulomb correlation effect in conjugation with localization of the charge carriers predominate over the mixed ionic pair effect and responsible for the metal-insulator transition in the series. -- Graphical Abstract: XPS data and electrical transport data show that doping of Mn in metallic SrRuO₃ induces mixed ionic pair Ru(IV)/Ru(V)↔Mn(III)/Mn(IV) and the system undergoes a transition from metal to insulator at the critical Mn doping level, x∼0.2. The origin for the metal-insulator transition has been discussed. Display Omitted Research highlights: → We have investigated the electronic properties of SrRu_1-xMn_xO₃ spectroscopically, which shows metal-insulator transition at the critical Mn doping concentartion x∼0.2. → We report the valence states of Ru and Mn in the series. → We also report the responsible mechanism for the metal-insulator transition in the series.

[en] Pulsed electron deposited thin films of Ru substituted La_1-xPb_xMn_0.8Ru_0.2O₃ (0.2≤x≤0.4) show an increase in the magneto-resistance ratio by ∼5-15% at the respective metal to insulator transition (T_MIT) temperature when compared to the parent La_0.6Pb_0.4MnO₃ thin film. A systematic decrease in T_MIT is observed from ∼310 to ∼260 K when the hole (Pb) concentration varies from 40 to 20% with constant 20% Ru substitution at the Mn site. The x-ray rocking curve and high-resolution transmission electron microscopy (HRTEM) images of the thin films suggest that Ru occupies the Mn site and shows epitaxial growth of the films on the LaAlO₃ (LAO) substrate. Transport and magneto-resistive properties show that Ru substitution maintains a considerable hole carrier density (due to Mn⁴⁺:t_2g³e_g⁰/Ru⁵⁺:t_2g³e_g⁰) even for La_0.8Pb_0.2Mn_0.8Ru_0.2O₃ (8282) composition, which influences the double exchange interactions

[en] We have studied the magnetic properties of Ru-doped La_1.2Sr_1.8Mn_1.9Ru_0.1O₇ bilayered manganite using neutron diffraction (ND) technique. The ND measurements show that the compound exhibits ferromagnetic ordering with the spins direction parallel to the c-axis. This is in sharp contrast to the parent compound where the spins are oriented along the a-b plane. The large spin-orbit coupling constant of Ru⁺⁴ ion (λ=∼900cm^-1) seems to play an important role on lifting of the orbital degeneracy of the e_g state by which the e_g electron prefers to occupy the d_3z²-r² orbital and therefore, the spins align along the c-axis

[en] Double exchange (DE)-mediated ferromagnetism in manganites, in which the Zener pair (Mn³⁺/Mn⁴⁺) is a key factor, is marked by the coexistence of the magnetic transition and the metal-insulator transition, thus distinguishing itself from superexchange (SE)-mediated ferromagnetism. Through detailed magnetization, transport and XAS studies, it was proposed by some of us that a unique magnetic pair-making effect occurs in Ru-substituted La_0.6Pb_0.4Mn_1-x Ru _x O₃ by virtue of variable valency of Ru and Mn. Ru ion, exhibiting a variable valency of Ru⁴⁺ and Ru⁵⁺, governs the DE-mediated coupling by initiating a magnetic pair-making effect with Mn³⁺ and Mn⁴⁺ states. Neutron diffraction measurements carried out on these compounds in the temperature range 4.2-500 K reveal the microscopic nature of magnetic ordering. Data obtained, in conjunction with data from other experimental techniques above, show that (i) Ru participates in the DE interaction with Mn for x≤0.4 (ii) there is a weakening of DE due to depletion of Ru⁵⁺ for x>0.4, and (iii) SE ferromagnetic interactions are favored for x>0.4