J. V. Yakhmi

A Three-Dimensional Ferrimagnet with a High Magnetic Transition Temperature (TC) of 53 K Based on a Chiral Molecule

The transparent, double bridged-(R)-spiral three-dimensional polymeric complex K0.4 [Cr(CN)6 ][Mn(S)-pn](S)-pnH0.6 ((S)-pn=(S)-1,2-diaminopropane) has been synthesized and characterized (see X-ray structure; Cr: brown, Mn: red, C: gray, N: blue, K: green). Magnetic measurements on the complex show that the MnII and CrIII ions interact ferrimagnetically and magnetic transition occurs at 53 K (Curie temperature).

Stabilizing the high-Tc superconductor Bi2Sr2Ca2Cu3O10+x by Pb substitution

Abstract The higher-Tc phase of the BiSrCaCu oxide high-Tc superconductor has been stabilized by partially replacing Bi by Pb. The resistance of our best Pb-doped samples falls sharply to zero at 108 K. AC susceptibility data show a single transition to bulk superconductivity with an onset temperature of 108 K. On the basis of X-ray diffraction and EDX data, it is concluded that Pb substitution promotes the growth of the (Bi, Pb)2Sr2Cu3O10+x phase.

Temperature- and magnetic-field-controlled magnetic pole reversal in a molecular magnetic compound

A highly reversible (bipolar) switching of magnetization in a Prussian blue type molecular magnet Cu0.73Mn0.77[Fe(CN)6]⋅zH2O using low magnetic fields is demonstrated. The studied molecular compound also shows both positive and negative magnetocaloric effects below its magnetic ordering temperature. A molecular field theory calculation has also been done to explain the observed temperature dependent magnetization reversal behavior. Possible applications of the magnetic pole reversal phenomenon in magnetoelectronic and magnetocaloric devices such as magnetic memory and magnetic cooling/heating based constant temperature bath have been revealed.

Hepta/octa cyanomolybdates with Fe2+: influence of the valence state of Mo on the magnetic behavior

Two FeII-based molybdenum polycyanides, Fe2[MoIII(CN)7]·8H2O and [Fe2(H2O)4][MoIV(CN)8]·4H2O, have been synthesized. The compound [Fe2(H2O)4][MoIV(CN)8]·4H2O crystallizes in the tetragonal system, space group I422. There are two molybdenum sites in a distorted square antiprism arrangement, each site being surrounded by eight CN–Fe linkages. The distorted octahedral iron site is formed by four NC–Mo linkages and two water molecules in apical positions. The structure is three-dimensional and highly symmetrical. The magnetic characteristics of these two compounds were compared. Fe2[MoIII(CN)7]·8H2O orders below 65 K as a ferrimagnet, but [Fe2(H2O)4][MoIV(CN)8]·4H2O shows no evidence of long-range magnetic order, obviously due to the contribution of the diamagnetic MoIV which suppresses the propagation of magnetic interaction between adjacent iron(II) ions through CN bridges. The results are discussed in the light of the single-crystal structural features of [Fe2(H2O)4][MoIV(CN)8]·4H2O.

ZnO-nanowires modified polypyrrole films as highly selective and sensitive chlorine sensors

Room temperature sensing characteristics of the polypyrrole (PPy) films modified with different weight percentages of ZnO nanowires (ZnO-NWs) have been studied for various oxidizing (NO2 and Cl2) as well as reducing (H2S, NH3, CH4, and CO) gases. We demonstrate that ZnO-NW (50 wt %):PPy composite films are highly selective to Cl2 along with high-sensitivity (40 at 10 ppm), fast-response (55 s), and highly reproducible response curves. It has been shown that negatively charged O2− surfaces of ZnO-NW transfer electrons to PPy, making it in a highly reduced form. A strong localization of charge carriers in the reduced PPy makes composite film highly resistive (>1010 Ω cm) as well insensitive to interaction with most of the gases except Cl2. Cl2 being highly oxidizing gas interacts with composite films and causes a sharp reduction in its resistivity.

Zero resistance at 120 K in Bi(Pb)-Ca-Sr-Cu oxide

Abstract Bulk Bi 1.6 Pb 0.4 Ca 2 Sr 2 Cu 3 O y samples synthesised by a precursor matrix reaction method show a zero resistance state at 104 K after long-term sintering at 1140 K. Samples subjected to an additional heating close to the melting point, for short durations, attain a record zero-resistance temperature of 120 K. Superconducting behaviour of these samples is stable and does not show any degradation upon thermal cycling.

Magnetic and electrical properties of La0.67Ca0.33MnO3 as influenced by substitution of Cr

Abstract The compounds La 0.67 Ca 0.33 Mn 1− x Cr x O 3 ( x =0.00, 0.05, 0.10, 0.15, 0.20, 0.35, 0.50 and 0.60) have been studied by X-ray diffraction (XRD), AC and DC magnetization, and DC electrical resistivity measurement techniques. Ferromagnetism is observed up to as high as 60% of Cr substitution. Electrical resistivity behavior of the compounds as a function of temperature could be explained on the basis of adiabatic small polaron model.

Stabilization of superconductivity in TlBa2CuO5−δ at 9.5 K and its enhancement to 43 K in TlBaSrCuO5−δ

Abstract It is shown that the superconducting state can be stabilized ( T c =9.5 K) by slow-cooling TlBa 2 CuO 5 in a nitrogen environment. Interestingly, partial substitution (50%) of Ba 2+ by Sr + 2 yields a zero-resistance state at 33 K in TlBaSrCuO 5 , even for air-quenched samples. T c could be further enhanced to 43 K ( R =0) by a slow cooling treatment in flowing nitrogen. Contraction of the lattice for TlBaSrCuO 5−δ in comparison with TlBa 2 CuO 5 , supports the substitution of Sr 2+ -ions at Ba-sites.

Synthesis of surfactant encapsulated nickel hexacyanoferrate nanoparticles and deposition of their Langmuir-Blodgett film

Sodium hexametaphosphate (HMP) stabilized nickel hexacyanoferrate (NiHCF) nanoparticles were prepared in aqueous solution and were successfully extracted into an organic phase using cetyltrimethylammonium bromide (CTAB) as the surfactant. Dynamic Light Scattering (DLS) studies suggest that the average size of the nanoparticles is retained during the extraction process from the aqueous to the organic phase. X-Ray diffraction, cyclic voltammetry, IR spectroscopy and magnetic measurements carried on the organic phase shows specific signatures of the presence of the surfactant encapsulated NiHCF nanoparticles. Transmission Electron Microscopy (TEM) measurements show that the average size of these surfactant encapsulated nanoparticles in the organic phase is about 22 nm, and as has been suggested by DLS studies, it does not change with respect to repeated evaporation and re-extraction processes of the organic phase. Pressure–area isotherms of the organic phase in a Langmuir–Blodgett (LB) trough, with water as subphase, indicate stable monolayer formation of the surfactant-encapsulated NiHCF nanoparticles at the air–water interface. Multi-layered deposition of the surfactant-encapsulated nanoparticles onto an indium tin oxide (ITO) coated glass slide could also be carried out using the LB technique. Cyclic voltammetry studies on these LB multilayers confirm regular and systematic transfer of the NiHCF nanoparticles on the ITO substrate. The method described here is the first of its kind with respect to the synthesis of surfactant encapsulated molecular magnetic nanoparticles and subsequent deposition of their LB films.

Stability of the layered Sr3Ti2O7 structure in La1.2(Sr1-xCax)1.8Mn2O7

Samples with nominal compositions La1.2 (Sr1-x Cax )1.8 Mn2 O7 (x = 0.00, 0.25, 0.50, 0.65, 0.80 and 1.00) have been prepared and characterized by x-ray diffraction (XRD) and ac susceptibility techniques. Drastic changes in the XRD pattern and in the ac susceptibility versus temperature behaviour at x = 0.65 indicate that single phase compounds with layered Sr3 Ti2 O7 structure are formed only up to x = 0.50. Rietveld refinement of the XRD patterns of x = 1.00 composition suggests that the compositions with x 0.65 form multiphase mixtures comprising hole doped perovskite manganates as majority phases and CaO as the minority phase. These results are at variance with the literature reports that the compound La1.2 Ca1.8 Mn2 O7 forms with layered Sr3 Ti2 O7 structure.