No abstract

No abstract

A negative-positive-negative switching behavior of magnetoresistance ͑MR͒ with temperature is observed in a ferromagnetic shape memory alloy Ni 1.75 Mn 1.25 Ga. In the austenitic phase between 300 and 120 K, MR is negative due to s-d scattering. Curiously, below 120 K MR is positive, while at still lower temperatures in the martensitic phase, MR is negative again. The positive MR cannot be explained by Lorentz contribution and is related to a magnetic transition. Evidence for this is obtained from ab initio density-functional theory, a decrease in magnetization and resistivity upturn at 120 K. Theory shows that a ferrimagnetic state with antiferromagnetic alignment between the local magnetic moments of the Mn atoms is the energetically favored ground state. In the martensitic phase, there are two competing factors that govern the MR behavior: a dominant negative trend up to the saturation field due to the decrease in electron scattering at twin and domain boundaries and a weaker positive trend due to the ferrimagnetic nature of the magnetic state. MR exhibits a hysteresis between heating and cooling that is related to the first-order nature of the martensitic phase transition.

Effects of boron doped in TiO 2 at a) interstitial site (B int ); b) substitutional site (B sub ) and combination of both the sites (B int+sub ), have been investigated experimentally and theoretically to understand the origin of enhanced photocatalytic activity and stability. Bdoped TiO 2 powder was synthesized by sol-gel method with different concentrations of boron. XPS results indicate that boron first prefers B int site when doped with low concentration (upto 1 at.% B), but as the concentration increases (2 at.% and above) B also occupies substitutional O position in addition to B int to form TiO 2 containing B int+sub (TiO 2 -B int+sub ). Higher absorption of visible light is achieved for TiO 2 -B int+sub due to the presence of two absorption edges (2.4 and 2.2 eV) as observed in the absorption spectra, while insignificant narrowing of band gap is observed for TiO 2 -B int . Electronic structure calculated by DFT for TiO 2 with B int , B sub , and B int+sub revealed that the two localized deep levels are formed in the mid gap which are responsible for these optical transitions for TiO 2 -B int+sub .Photoluminescence (PL) emission spectra showed that the shallow level (as inferred from the DFT calculations) created below the conduction band is able to decrease the radiative recombination process in TiO 2 -B int by trapping electrons and prolonging the lifetime of charge carriers as observed in the time resolved PL decay curve. Furthermore, lower effective mass ratio of charge carriers calculated using DFT for TiO 2 -B int also suggests better charge mobility and low recombination rate. Photocatalytic degradation rate of organic pollutant in water was significantly higher after B-doping with higher performance obtained with TiO 2 containing B int as compared to B int+sub . By imposing the destabilizing circumstances it was established that TiO 2 -B sub is metastable and collapses under mild conditions, whereas TiO 2 -B int is highly stable and retains all its properties. All these unprecedented findings disclose that higher activity of TiO 2 -B int as compared to that of TiO 2 -B int+sub is mainly because of the delayed recombination processes even though the optical band gap is not significantly varied.

No abstract