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A hydrothermal method was used to prepare three nano-crystalline samples of TiO2 (S1), N-doped TiO2 (S2), and (N, Ta)-codoped TiO2 (S3) with average crystallite sizes (D) of 13-25 nm. X-ray diffraction studies confirmed a single phase of the samples with a tetragonal/anatase structure. A slight increase in the lattice parameters was observed when N and/or Ta dopants were doped into the TiO2 host lattice. Detailed analyses of extended X-ray absorption spectra indicated that N- and/ or Ta-doping into TiO2 nanoparticles influenced the co-ordination number and radial distance (R) of Ti ions in the anatase structure. Concerning their absorption spectra, (N, Ta)-doping narrowed the band gap (Eg) of TiO2 from 3.03 eV for S1 through 2.94 eV for S2 to 2.85 eV for S3. Such results revealed the applicability of these nanoparticles in the photocatalytic field working in the ultraviolet (UV)-visible region. Among these, photocatalytic activity of S3 was the strongest. By using S3 as a catalyst powder, the degradation efficiency of methylene blue solution was about 99% and 93% after irradiation of UV-visible light for 75 min and visible-light for 180 min, respectively. [ABSTRACT FROM AUTHOR]

We studied the influence of the magnetic phase transition on the transport properties of La0.7Ca0.3-xBaxMnO3 compounds with x = 0.05, 0.075, and 0.10. Our experimental results demonstrated the ferromagnetic-paramagnetic and metal-insulator transitions taking place at temperatures TC = 265-300K and TMI = 280-310 K, respectively, which increase with increasing Ba-doping content. While the x = 0.05 sample undergoes a first-order magnetic phase transition (FOMT), x = 0.10 undergoes a second-order magnetic phase transition (SOMT). The other sample with x = 0.075 is considered as a threshold concentration of the FOMT-SOMT transformation. Further, q(T) data in different temperature regions were fitted to different models. The activation energy Ep and density of states at the Fermi level N(EF) were accordingly determined. Notably, N(EF) increases while Ep decreases in the case of applying an external field. We also have found that N(EF) increases when materials transfer from the FOMT to the SOMT, which becomes smallest for the sample having the coexistence of the FOMT and SOMT (i.e., x = 0.075). [ABSTRACT FROM AUTHOR]

We have studied the magnetic and spin-glass (SG) properties of La0.7Ca0.3MnO3 single-crystalline nanoparticles, which were prepared by the mechanical milling method with different milling times (tm). Analyzing the susceptibility data in the paramagnetic region indicates both ferromagnetic (FM) and anti-FM interactions coexisting in nanoparticles. Additionally, there is a peak associated with the freezing temperature (Tf) appearing on the real part curve of the ac susceptibility, χ'(T). The Tf value increases with increasing frequency as expected for SG systems. The SG behavior was also checked by using the criterion parameter c=ΔTf/TfΔ(log10f), and the power law τ=τ0(T/Tg - 1)-zv. The obtained values of c≈5×10-2, τ0 ≈ 10-5 s and zv ≈ 2-3 are consistent with those expected for SG-like systems, suggesting an existence of a SG phase transition at Tg below Tf, which decreases with decreasing . Basing on ln(f) versus Tf data, and the Néel-Arrhenius model [ln(f)=ln(f0) - Ea/kBT] and Vogel-Fulcher law [ln(f)=ln(f0) - Ea/kB(T - T0)], the Larmor frequency (f0), activation energy (Ea) and effective temperature (T0) for the samples with different {D} were determined. Obtained results indicate the existence a strong interaction between nanoparticles. [ABSTRACT FROM AUTHOR]

A systematic study of the inverse magnetocaloric effect (IMCE) in Ni50Mn35.8Sn14.2 and Pr0.5Sr0.5MnO3 has been performed to understand the impacts of first-order phase transition (FOPT) and phase coexistence on the universality of the temperature-dependent magnetic entropy change, ▵SM(T). We show that for Ni50Mn35.8Sn14.2-a system exhibiting IMCE associated with FOPT-it is possible to construct a universal master curve to describe ▵SM(T) in different applied fields without rescaling a temperature axis. However, the universality of IMCE does not hold for Pr0.5Sr0.5MnO3-a system with coexisting ferromagnetic and antiferromagnetic phases. The proposed universal curve provides a simple method for extrapolating DSM in a wide range of fields and temperatures, thus giving useful guidance to the design of magnetocaloric materials for active magnetic refrigeration technology. [ABSTRACT FROM AUTHOR]

Understanding the impact of reduced dimensionality on the magnetic and magnetocaloric responses of a material is vital in incorporating it as an active magnetic refrigerant in cooling devices. By contrasting the magnetic and magnetocaloric behaviors of bulk polycrystalline, sol-gel derived nanocrystalline, and pulsed laser deposited thin film forms of the La0.7Ca0.3MnO3 system, we show that reducing the dimensionality of a ferromagnetic material tends to broaden and shift the paramagnetic to ferromagnetic transition to lower temperatures, while decreasing the saturation magnetization and the magnitude of the magnetic entropy change. Relative to its bulk counterpart, a pronounced broadening of the magnetic entropy change peak in the thin film leads to enhanced refrigerant capacity-an important figure-of-merit for active magnetic refrigeration technology. With reduced dimensionality, universal curves based on re-scaled entropy change curves tend toward collapse, indicating a weakening of the first order nature of the transition in the nanocrystalline samples and a crossover to second order in the thin film. [ABSTRACT FROM AUTHOR]

We report a comparative study of the universal behaviors of conventional and inverse magnetocaloric effects (CMCE and IMCE, respectively) that coexist in Heusler alloys of Ni50Mn50-xSnx (x ∼ 13, 14) and Ni50-xPrxMn37Sn13 (x ∼ 1, 3). In contrast to CMCE associated with a first-order phase transition (FOPT), we show that it is possible to construct a universal master curve to describe the temperature dependence of magnetic entropy change ΔSM(T) in different applied fields without rescaling the temperature axis for IMCE associated with FOPT. This universal behavior is shown to be different from that observed for CMCE associated with a second-order phase transition. The proposed universal curves provide an effective method of extrapolating ΔSM of materials associated with CMCE and IMCE in any range of temperatures and magnetic fields, giving useful guidance on the development of active magnetic refrigerants. [ABSTRACT FROM AUTHOR]