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Register a new userEffects of Ni Substitutions on the Critical Behaviors in $Nd_{0.6}Sr_{0.4}Mn_{1-x}Ni_{x}O_{3}$ Manganite
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We investigate the effect of Ni${text -}$substitution on the crystalline structure and the critical behavior of $Nd_{0.6}Sr_{0.4}Mn_{1-x}Ni_{x}O_{3}$ (0.00 $leq$ x $leq$ 0.20) perovskite. X${text -}$ray diffraction patterns revealed that the major phase in all samples is the orthorhombic structure with space group $textit{Pnma}$. Rietveld refinement revealed a linear reduction in the lattice parameters along with monotonic reduction in the O2${text -}$Mn${text -}$O2 angel with increasing Ni concentration. The modified Arrott plots and the Kouvel${text -}$Fisher method have been used to analyze the magnetization isotherms near the paramagnetic to ferromagnetic (PM${text -}$FM) phase transition. The obtained critical exponents ($beta$, $gamma$ and $delta$) revealed that the Ni${text -}$free sample is consistent with 3D${text -}$Heisenberg like behavior. However, upon Ni${text -}$substitution, the critical exponents exhibit a mean field like behavior. The reliability of the obtained critical exponent ($beta$, $gamma$ and $delta$) values have been confirmed by the universal scaling behavior of the isothermal magnetization near the transition temperature.
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In this work, we investigate the effect of Ni concentration on several shielding properties of $Nd_{0.6}Sr_{0.4}Mn_{1-y}Ni_{y}O_{3}$ (0.00 $leq$ y $leq$ 0.20) perovskite ceramic for possible use as radiation shielding materials. X-ray diffraction (XRD) analysis revealed that these ceramics have the orthorhombic structure with group space Pnma over a wide range of Ni-substitutions. Moreover, the analysis showed a nearly linear decrease in the lattice parameters and the unit cell volume (V) causing a gradual increase in the packing density with increasing Ni concentration. The shielding features for photons, neutrons, and protons of all ceramic samples were assessed. The mass attenuation coefficient (MAC) was computed at 0.1, 0.6, 1.25, 5 and 15 MeV by utilizing (MCNP) (version 5.0); the results were compared with the corresponding values obtained using Phy-X and XCOM program. The results obtained showed slight enhancement with increasing Ni contents. The substitution of Ni leads to progressive enhancement in effective removal cross-section of fast neutron (${Sigma}$R) values. Whereas the values of Mass Stopping Power (MSP) and projected range for the protons showed a gradual reduction with increasing Ni concentration. These findings suggest that the current ceramic samples can be useful as radiation shielding materials.
We present the electronic structure of Sr_{1-(x+y)}La_{x+y}Ti_{1-x}Cr_{x}O_{3} investigated by high-resolution photoemission spectroscopy. In the vicinity of Fermi level, it was found that the electronic structure were composed of a Cr 3d local state with the t_{2g}^{3} configuration and a Ti 3d itinerant state. The energy levels of these Cr and Ti 3d states are well interpreted by the difference of the charge-transfer energy of both ions. The spectral weight of the Cr 3d state is completely proportional to the spin concentration x irrespective of the carrier concentration y, indicating that the spin density can be controlled by x as desired. In contrast, the spectral weight of the Ti 3d state is not proportional to y, depending on the amount of Cr doping.
By using laboratory x-ray photoemission spectroscopy (XPS) and hard x-ray photoemission spectroscopy (HX-PES) at a synchrotron facility, we report an empirical semi-quantitative relationship between the valence/core-level x-ray photoemission spectral weight and electrical conductivity in La_{1-x}Sr_{x}MnO_{3} as a function of x. In the Mn 2p_{3/2} HX-PES spectra, we observed the shoulder structure due to the Mn^{3+} well-screened state. However, the intensity at x=0.8 was too small to explain its higher electrical conductivity than x=0.0, which confirms our recent analysis on the Mn 2p_{3/2} XPS spectra. The near-Fermi level XPS spectral weight was found to be a measure of the variation of electrical conductivity with x in spite of a far lower energy resolution compared with the energy scale of the quasiparticle (coherent) peak because of the concurrent change of the coherent and incoherent spectral weight.
We report the new results of exchange bias effect in Nd_{1-x}Sr_{x}CoO_3 for x = 0.20 and 0.40, where the exchange bias phenomenon is involved with the ferrimagnetic (FI) state in a spontaneously phase separated system. The zero-field cooled magnetization exhibits the FI (T_{FI}) and ferromagnetic (T_C) transitions at ~ 23 and sim 70 K, respectively for x = 0.20. The negative horizontal and positive vertical shifts of the magnetic hysteresis loops are observed when the system is cooled through T_{FI} in presence of a positive static magnetic field. Training effect is observed for x = 0.20, which could be interpreted by a spin configurational relaxation model. The unidirectional shifts of the hysteresis loops as a function of temperature exhibit the absence of exchange bias above T_{FI} for x = 0.20. The analysis of the cooling field dependence of exchange bias field and magnetization indicates that the ferromagnetic (FM) clusters consist of single magnetic domain with average size around sim 20 and ~ 40 AA ~ for x = 0.20 and 0.40, respectively. The sizes of the FM clusters are close to the percolation threshold for x = 0.20, which grow and coalesce to form the bigger size for x = 0.40 resulting in a weak exchange bias effect.
We report the characterization of the crystal structure, low-temperature charge and orbital ordering, transport, and magnetization of Pr_{0.6}Ca_{0.4}MnO_{3} films grown on LaAlO_{3}, NdGaO_{3}, and SrTiO_{3} substrates, which provide compressive (LaAlO_{3}) and tensile (NdGaO_{3} and SrTiO_{3}) strain. The films are observed to exhibit different crystallographic symmetries than the bulk material, and the low-temperature ordering is found to be more robust under compressive-- as opposed to tensile-- strain. In fact, bulk-like charge and orbital ordering is not observed in the film grown on NdGaO_{3}, which is the substrate that provides the least amount of nominal and measured, but tensile, strain. This result suggests the importance of the role played by the Mn--O--Mn bond angles in the formation of charge and orbital ordering at low temperatures. Finally, in the film grown on LaAlO_{3}, a connection between the lattice distortion associated with orbital ordering and the onset of antiferromagnetism is reported.
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