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Register a new userVibrational abisotropy and quadrupole interactions of Fe substituted into Mn site of the charge and orbitally ordered and disordered layered manganites LnBaMn1.96Fe0.04O5 and LnBaMn1.96Fe0.04O6 (Ln=Y,Gd,Sm,La)
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A-site ordered manganites LnBaMn1.96Fe0.04O5 and LnBaMn1.96Fe0.04O6 are investigated by x-ray full-profile diffraction and Moessbauer spectroscopy. Powder samples were oriented with preferred orientation of platy crystallites in the plane of sample surface. March-Dollase function of preferred orientation was employed in analysing both the Rietveld patterns and the Mossbauer spectra. Combined effects of preffered orientation and vibrational anisotropy on the line area asymmetry of Mossbauer doublet are analysed. Constructive and destructive interference between the effects of texture and vibrational anisotropy is observed in LnBaMn1.96Fe0.04O6 and LnBaMn1.96Fe0.04O5, respectively. Both series of the manganites show the main axis of electric field gradient perpendicular to layers (Vzz along c) with Vzz>0 in oxygen-poor series and Vzz<0 in oxygen-rich series. Charge-orbital order (COO) melting around Fe dopants explains the single-site spectra observed for several Ln in both O5 and O6 series, except LaBaMn1.96Fe0.04O5. However, the short-range COO persists to be observed in magnetization and in Rietveld patterns.
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Versatile features of impurity doping effects on perovskite manganites, $R_{0.6}$Sr$_{0.4}$MnO$_{3}$, have been investigated with varying the doing species as well as the $R$-dependent one-electron bandwidth. In ferromagnetic-metallic manganites ($R$=La, Nd, and Sm), a few percent of Fe substitution dramatically decreases the ferromagnetic transition temperature, leading to a spin glass insulating state with short-range charge-orbital correlation. For each $R$ species, the phase diagram as a function of Fe concentration is closely similar to that for $R_{0.6}$Sr$_{0.4}$MnO$_{3}$ obtained by decreasing the ionic radius of $R$ site, indicating that Fe doping in the phase-competing region weakens the ferromagnetic double-exchange interaction, relatively to the charge-orbital ordering instability. We have also found a contrastive impact of Cr (or Ru) doping on a spin-glass insulating manganite ($R$=Gd). There, the impurity-induced ferromagnetic magnetization is observed at low temperatures as a consequence of the collapse of the inherent short-range charge-orbital ordering, while Fe doping plays only a minor role. The observed opposite nature of impurity doping may be attributed to the difference in magnitude of the antiferromagnetic interaction between the doped ions.
We prepared the samples K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ (Ln=Sm, Nd and La) with ThCr$_2$Si$_2$-type structure. These samples were characterized by X-ray diffraction, resistivity, susceptibility and thermoelectric power (TEP). Substitution of Ln (Ln=La, Nd and Sm) for K in K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ system raises the superconducting transition temperature to 34-36 K. The TEP measurements indicate that the TEP of K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ is positive, being similar to the case of the Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ system with p-type carrier. In the K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ system, the superconducting $KFe_2As_2$ with $T_csim 3$ K is the parent compound, and no structural and spin-density wave instabilities exist in this system.
The magnetically ordered states of the A-site ordered perovskite manganites LaBaMn2O6 and YBaMn2O6 have been investigated by muon spin relaxation in zero external magnetic field. Our data reveal striking differences in the nature of the magnetically ordered state of these materials. For LaBaMn2O6, the muSR time-spectra in the ferromagnetic state below ~ 330 K reveal a strongly inhomogeneous phase, reminiscent of a Griffiths phase. Within this magnetically inhomogeneous phase, an antiferromagnetic state develops below 150 K, which displays well defined static internal magnetic fields, but reaches only 30% of the volume fraction at low temperatures. A broad distribution of muSR relaxation rates is inferred down to the lowest temperatures. This behavior is similar to the A-site disordered La0.5Ba0.5MnO3. On the other hand, for YBaMn2O6, the muSR time spectra for both (i) the charge and orbital ordered and (ii) the paramagnetic phases reveal rather homogeneous states, namely, an exponential relaxation in the paramagnetic state and well defined muon spin oscillation in the antiferromagnetic state.
We investigate the ultra-sharp jump in the isothermal magnetization and the resistivity in the polycrystalline $Sm_{0.5}(Ca_{0.5-y}Sr_{y})MnO_3$ $(y = 0, 0.1, 0.2, 0.25, 0.3, 0.5)$ compounds. The critical field $(H_{cr})$, required for the ultra-sharp jump, decreases with increase of `Sr concentration, i.e. with increase of average A-site ionic radius $langle r_Arangle$. The magnetotransport data indicate that the phase separation increases with the increase of $langle r_Arangle$, i.e. with $y$. The dependency of $H_{cr}$ with magnetic field sweep rate reveals that the ultra-sharp jump from antiferromagnetic (AFM) state to the ferromagnetic (FM) state is of martensitic in nature. Our two-band double exchange model Hamiltonian calculations show that the `Sr doping induces the ferromagnetic clusters in the antiferromagnetic insulating phase and in turn reduces the critical field. In the end we present a phenomenological picture obtained from our combined experimental and theoretical study.
We evaluated for the first time the use of nanostructured layered perovskites of formulae LnBaCo2O6-d with Ln = Sm and Gd (SBCO and GBCO, respetively) as SOFC cathodes, finding promising electrochemical properties in the intermediate temperature range (~700{deg}C). The synthesis of these nanomaterials, not reported before, was achieved by using porous templates to confine the chemical reagents in regions of about 200 nm and 800 nm. The performance of nanostructured SBCO and GBCO cathodes for the oxygen reduction reaction was analyzed in symmetrical cells using Gd2O3-doped CeO2 (GDC) as electrolyte. For this purpose, nanostructured SBCO and GBCO cathodes were deposited on both sides of the electrolyte by a simple thick-film procedure and evaluated by Electrochemical Impedance Spectroscopy technique under different operating conditions. We found that cathodes synthesized using smaller template pores exhibited better performance. Besides, SBCO cathodes displayed lower area-specific resistance than GBCO ones.
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