Electronic structure of Fe and magnetism in the 3d/5d3d/5d double perovskites Ca2_2FeReO6_6 and Ba2_2FeReO6_6

The Fe electronic structure and magnetism in (i) monoclinic Ca$_2$FeReO$_6$ with a metal-insulator transition at $T_{MI} \sim 140$ K and (ii) quasi-cubic half-metallic Ba$_2$FeReO$_6$ ceramic double perovskites are probed by soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). These materials show distinct Fe $L_{2,3}$ XAS and XMCD spectra, which are primarily associated with their different average Fe oxidation states (close to Fe$^{3+}$ for Ca$_2$FeReO$_6$ and intermediate between Fe$^{2+}$ and Fe$^{3+}$ for Ba$_2$FeReO$_6$) despite being related by an isoelectronic (Ca$^{2+}$/Ba$^{2+}$) substitution. For Ca$_2$FeReO$_6$, the powder-averaged Fe spin moment along the field direction ($B = 5$ T), as probed by the XMCD experiment, is strongly reduced in comparison with the spontaneous Fe moment previously obtained by neutron diffraction, consistent with a scenario where the magnetic moments are constrained to remain within an easy plane. For $B=1$ T, the unsaturated XMCD signal is reduced below $T_{MI}$ consistent with a magnetic transition to an easy-axis state that further reduces the powder-averaged magnetization in the field direction. For Ba$_2$FeReO$_6$, the field-aligned Fe spins are larger than for Ca$_2$FeReO$_6$ ($B=5$ T) and the temperature dependence of the Fe magnetic moment is consistent with the magnetic ordering transition at $T_C^{Ba} = 305$ K. Our results illustrate the dramatic influence of the specific spin-orbital configuration of Re $5d$ electrons on the Fe $3d$ local magnetism of these Fe/Re double perovskites.Comment: 7 pages, 3 figure

Antimicrobial activity of secondary metabolites from endophytic fungi isolated from Nerium oleander L.

Endophytic fungi found inside the plant tissue are endosymbionts, protecting their host from pests, pathogens, etc. Twenty eight endophytic fungi were isolated from different parts of Nerium oleander L., out of which, 54 of isolates were from flower, 39 from stem and 7 from leaf parts. Thirty six percent of the isolates showed antimicrobial activity against tested pathogens. The potential isolates such as Fusarium semitectum (Nof-3), Colletotrichum gloeosporioides (Nof-7), Alternaria alternata (Nof-8) and Mycelia Sterilia sp.1 (Nos-6) were subjected to the production and extraction of secondary metabolites. All the four fungal extracts inhibited Staphylococcus aureus and Bacillus cereus at 20 μg/mL (MIC). Extracts of C. gloeosporioides and Mycelia Sterilia sp.1 showed activity against Pseudomonas aeruginosa at MIC of 20 μg/mL. The growth of Escherichia coli was suppressed by all the tested extracts at MIC of 20 μg/mL except F. semitectum. A. alternata & Mycelia Sterilia sp.1 extracts were active against Salmonella typhimurium at 20 μg/mL. The growth of Candida albicans was inhibited by Mycelia Sterilia sp.1 at 20 μg/mL. The zones of inhibition were statistically significant with respective positive controls

Amylase production by endophytic fungi Cylindrocephalumsp. isolated from medicinal plant Alpinia calcarata (Haw.) Roscoe.

Amylases are among the most important enzymes used in modern biotechnology particularly in the process involving starch hydrolysis. Fungal amylase has large applications in food and pharmaceutical industries. Considering these facts, endophytic fungi isolated from the plant Alpinia calcarata (Haw.) Roscoe were screened for amylolytic activity on glucose yeast extract peptone agar (GYP) medium. Among thirty isolates of endophytic fungi, isolate number seven identified as Cylindrocephalum sp. (Ac-7) showed highest amylolytic activity and was taken for further study. Influence of various physical and chemical factors such as pH, temperature, carbon and nitrogen sources on amylase production in liquid media were studied. The maximal amylase production was found to be at 30(º)C and at pH 7.0 of the growth medium. Among the various carbon and nitrogen sources tested, maltose at 1.5% and Sodium nitrate at 0.3% respectively gave optimum amylase production

Neuroprotective Effects of Bikaverin on H2O2-Induced Oxidative Stress Mediated Neuronal Damage in SH-Sy5y Cell Line

The generation of free radicals and oxidative stress has been linked to several neurodegenerative diseases including Parkinson's disease, Alzheimer's disease, Huntington's disease, and Amyotrophic lateral sclerosis. The use of free radical scavenging molecules for the reduction of intracellular reactive oxygen species is one of the strategies used in the clinical management of neurodegeneration. Fungal secondary metabolism is a rich source of novel molecules with potential bioactivity. In the current study, bikaverin was extracted from Fusarium oxysporum f. sp. lycopersici and its structural characterization was carried out. Further, we explored the protective effects of bikaverin on oxidative stress and its anti-apoptotic mechanism to attenuate H2O2-induced neurotoxicity using human neuroblastoma SH-SY5Y cells. Our results elucidate that pretreatment of neurons with bikaverin attenuates the mitochondrial and plasma membrane damage induced by 100 µM H2O2 to 82 and 26 % as evidenced by MTT and LDH assays. H2O2 induced depletion of antioxidant enzyme status was also replenished by bikaverin which was confirmed by Realtime Quantitative PCR analysis of SOD and CAT genes. Bikaverin pretreatment efficiently potentiated the H2O2-induced neuronal markers, such as BDNF, TH, and AADC expression, which orchestrate the neuronal damage of the cell. The H2O2-induced damage to cells, nuclear, and mitochondrial integrity was also restored by bikaverin. Bikaverin could be developed as a preventive agent against neurodegeneration and as an alternative to some of the toxic synthetic antioxidants

Roots of Withania somnifera Inhibit Forestomach and Skin Carcinogenesis in Mice

We evaluated the cancer chemopreventive efficacy of the Withania somnifera root, which has been used in the Indian traditional medicine system for many centuries for the treatment of various ailments. Since, studies showing its mechanism-based cancer chemopreventive efficacy are limited, this was investigated in the present study. We studied the effect of dietary administration of Withania root on hepatic phase I, phase II and antioxidant enzymes by estimation of its level/activity, as well as in attenuating carcinogen-induced forestomach and skin tumorigenesis in the Swiss albino mouse model. Our findings showed that roots of W.somnifera inhibit phase I, and activates phase II and antioxidant enzymes in the liver. Further, in a long-term tumorigenesis study, Withania inhibited benzo(a)pyrene-induced forestomach papillomagenesis, showing up to 60 and 92% inhibition in tumor incidence and multiplicity, respectively. Similarly, Withania inhibited 7,12-dimethylbenzanthracene-induced skin papillomagenesis, showing up to 45 and 71% inhibition in tumor incidence and multiplicity. In both studies, Withania showed no apparent toxic effects in mice as monitored by the body weight gain profile. Together, these findings suggest that W.somnifera root has chemopreventive efficacy against forestomach and skin carcinogenesis and warrants the identification and isolation of active compounds responsible for its anticancer effects, which may provide the lead for the development of antitumor agents

An experimental and computational investigation of structure and magnetism in pyrite Co1−x_{1-x}Fex_xS2_2: Chemical bonding and half-metallicity

Bulk samples of the pyrite chalcogenide solid solutions Co$_{1-x}$Fe$_x$S$_2$ 0 <= x <= 0.5, have been prepared and their crystal structures and magnetic properties studied by X-ray diffraction and SQUID magnetization measurements. Across the solution series, the distance between sulfur atoms in the persulfide (S$_2^{2-}$) unit remains nearly constant. First principles electronic structure calculations using experimental crystal structures as inputs point to the importance of this constant S-S distance, in helping antibonding S-S levels pin the Fermi energy. In contrast hypothetical rock-salt CoS is not a good half metal, despite being nearly isostructural and isoelectronic. We use our understanding of the Co$_{1-x}$Fe$_x$S$_2$ system to make some prescriptions for new ferromagnetic half-metals.Comment: 8 pages including 9 figure

Unconventional magnetism in the 4d4^{4} based (S=1S=1) honeycomb system Ag3_{3}LiRu2_{2}O6_{6}

We have investigated the thermodynamic and local magnetic properties of the Mott insulating system Ag$_{3}$LiRu$_{2}$O$_{6}$ containing Ru$^{4+}$ (4$d$$^{4}$) for novel magnetism. The material crystallizes in a monoclinic $C2/m$ structure with RuO$_{6}$ octahedra forming an edge-shared two-dimensional honeycomb lattice with limited stacking order along the $c$-direction. The large negative Curie-Weiss temperature ($\theta_{CW}$ = -57 K) suggests antiferromagnetic interactions among Ru$^{4+}$ ions though magnetic susceptibility and heat capacity show no indication of magnetic long-range order down to 1.8 K and 0.4 K, respectively. $^{7}$Li nuclear magnetic resonance (NMR) shift follows the bulk susceptibility between 120-300 K and levels off below 120 K. Together with a power-law behavior in the temperature dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic spin correlations with gapless excitations. Electronic structure calculations suggest an $S = 1$ description of the Ru-moments and the possible importance of further neighbour interactions as also bi-quadratic and ring-exchange terms in determining the magnetic properties. Analysis of our $\mu$SR data indicates spin freezing below 5 K but the spins remain on the borderline between static and dynamic magnetism even at 20 mK.Comment: 10 pages, 11 figures. accepted in Phys. Rev.

Magnetic and electron transport properties of the rare-earth cobaltates, La0.7-xLnxCa0.3CoO3 (Ln = Pr, Nd, Gd and Dy) : A case of phase separation

Magnetic and electrical properties of four series of rare earth cobaltates of the formula La0.7-xLnxCa0.3CoO3 with Ln = Pr, Nd, Gd and Dy have been investigated. Compositions close to x = 0.0 contain large ferromagnetic clusters or domains, and show Brillouin-like behaviour of the field-cooled DC magnetization data with fairly high ferromagnetic Tc values, besides low electrical resistivities with near-zero temperature coefficients. The zero-field-cooled data generally show a non-monotonic behaviour with a peak at a temperatures slightly lower than Tc. The near x = 0.0 compositions show a prominent peak corresponding to the Tc in the AC-susceptibility data. The ferromagnetic Tc varies linearly with x or the average radius of the A-site cations, (rA). With increase in x or decrease in (rA), the magnetization value at any given temperature decreases markedly and the AC-susceptibility measurements show a prominent transition arising from small magnetic clusters with some characteristics of a spin-glass. Electrical resistivity increases with increase in x, showed a significant increase around a critical value of x or (rA), at which composition the small clusters also begin to dominate. These properties can be understood in terms of a phase separation scenario wherein large magnetic clusters give way to smaller ones with increase in x, with both types of clusters being present in certain compositions. The changes in magnetic and electrical properties occur parallely since the large ferromagnetic clusters are hole-rich and the small clusters are hole-poor. Variable-range hopping seems to occur at low temperatures in these cobaltates.Comment: 23 pages including figure

Properties of the ferrimagnetic double-perovskite A_{2}FeReO_{6} (A=Ba and Ca)

Ceramics of A_{2}FeReO_{6} double-perovskite have been prepared and studied for A=Ba and Ca. Ba_{2}FeReO_{6} has a cubic structure (Fm3m) with $a\approx$8.0854(1) \AA whereas Ca_{2}FeReO_{6} has a distorted monoclinic symmetry with $a\approx 5.396(1) \AA, b\approx 5.522(1) \AA, c\approx 7.688(2) \AA$ and $\beta =90.4^{\circ} (P21/n)$. The barium compound is metallic from 5 K to 385 K, i.e. no metal-insulator transition has been seen up to 385 K, and the calcium compound is semiconducting from 5 K to 385 K. Magnetization measurements show a ferrimagnetic behavior for both materials, with T_{c}=315 K for Ba_{2}FeReO_{6} and above 385 K for Ca_{2}FeReO_{6}. A specific heat measurement on the barium compound gave an electron density of states at the Fermi level, N(E_{F}) equal to 6.1$\times 10^{24} eV^{-1}mole^{-1}$. At 5 K, we observed a negative magnetoresistance of 10 % in a magnetic field of 5 T, but only for Ba_{2}FeReO_{6}. Electrical, thermal and magnetic properties are discussed and compared to the analogous compounds Sr_{2}Fe(Mo,Re)O_{6}.Comment: 5 pages REVTeX, 7 figures included, submitted to PR

Investigation Of The Local Fe Magnetic Moments At The Grain Boundaries Of The Ca2 Fere O6 Double Perovskite

The local Fe ferromagnetic (FM) moment at the grain boundaries of a ceramic sample of Ca2 FeRe O6 double perovskite was investigated by means of x-ray magnetic circular dichroism spectroscopy at the Fe L2,3 edges and compared to the overall bulk magnetization. We found that, at the grain boundaries, the Fe FM moments at H=5 T are much smaller than expected and that the M×H curve is harder than in the bulk magnetization. These results suggest a larger degree of FeRe antisite disorder at the grain boundaries of this sample, shedding light into the intriguing nonmetallic resistivity behavior despite the reported presence of free carriers. © 2007 American Institute of Physics.1019Serrate, D., De Teresa, J.M., Ibarra, M.R., (2007) J. Phys.: Condens. Matter, 19, p. 023201Kobayashi, K.-I., Kimura, T., Sawada, H., Terakura, K., Tokura, Y., (1998) Nature (London), 395, p. 677Kobayashi, K.-I., Kimura, T., Tomoika, Y., Sawada, H., Terakura, K., Tokura, Y., (1999) Phys. Rev. B, 59, p. 11. , 159Prellier, W., Smolyaninova, V., Biswas, A., Galley, C., Greene, R.L., Ramesha, K., Gopalakrishnan, J., (2000) J. Phys.: Condens. Matter, 12, p. 965Gopalakrishnan, J., Chattopadhyay, A., Ogale, S.B., Venkatesan, T., Greene, R.L., Millis, A.J., Ramesha, K., Marest, G., (2000) Phys. Rev. B, 62, p. 9538Maignan, A., Raveau, B., Martin, C., Hervieu, M., (1999) J. Solid State Chem., 144, p. 224Dai, J.M., (2001) Mater. Sci. Eng., B, 83, p. 217Sarma, D.D., Mahadevan, P., Saha-Dasgupta, T., Ray, S., Kumar, A., (2000) Phys. Rev. Lett., 85, p. 2549Fang, Z., Terakura, K., Kanamori, J., (2001) Phys. Rev. B, 63, p. 180407Wu, H., (2001) Phys. Rev. B, 64, p. 125126Szotek, Z., Temmerman, W.M., Svane, A., Petit, L., Winter, H., (2003) Phys. Rev. B, 68, p. 104411Chattopadhyay, A., Millis, A.J., (2001) Phys. Rev. B, 64, p. 024424Aligia, A.A., Petrone, P., Sofo, J.O., Alascio, B., (2001) Phys. Rev. B, 64, p. 092414Kato, H., Okuda, T., Okimoto, Y., Tomoika, Y., Oikawa, K., Kamiyama, T., Tokural, Y., (2002) Phys. Rev. B, 65, p. 144404Westerburg, W., Lang, O., Ritter, C., Felser, C., Tremel, W., Jakob, G., (2002) Solid State Commun., 122, p. 201Oikawa, K., Kamiyama, T., Kato, H., Tokura, Y., (2003) J. Phys. Soc. Jpn., 72, p. 1411Granado, E., Huang, Q., Lynn, J.W., Gopalakrishnan, J., Greene, R.L., Ramesha, K., (2002) Phys. Rev. B, 66, p. 064409Iwasawa, H., Saitoh, T., Yamashita, Y., Ishii, D., Kato, H., Hamada, N., Tokura, Y., Sarma, D.D., (2005) Phys. Rev. B, 71, p. 075106Longo, J., Ward, R., (1961) J. Am. Chem. Soc., 83, p. 2816Alamelu, T., Varadaraju, U.V., Venkatesan, M., Douvalis, A.P., Coey, J.M.D., (2002) J. Appl. Phys., 91, p. 8909Serrate, D., De Teresa, J.M., Algarabel, P.A., Marquina, C., Morellon, L., Blasco, J., Ibarra, M.R., (2005) J. Magn. Magn. Mater., 290-291, p. 843De Teresa, J.M., Serrate, D., Blasco, J., Ibarra, M.R., Morellon, L., (2004) Phys. Rev. B, 69, p. 144401De Teresa, J.M., Serrate, D., Blasco, J., Ibarra, M.R., Morellon, L., (2005) J. Magn. Magn. Mater., 290-291, p. 1043Sikora, M., (2006) Appl. Phys. Lett., 89, p. 062509Azimonte, C., Cezar, J.C., Granado, E., Huang, Q., Lynn, J.W., Campoy, J.C.P., Gopalakrishnan, J., Ramesha, K., (2007) Phys. Rev. Lett., 98, p. 017204Abbate, M., (1992) Phys. Rev. B, 46, p. 4511Regan, T.J., Ohldag, H., Stamm, C., Nolting, F., Lning, J., Sthr, J., White, R.L., (2001) Phys. Rev. B, 64, p. 214422Thole, B.T., Carra, P., Sette, F., Van Der Laan, G., (1992) Phys. Rev. Lett., 68, p. 1943Carra, P., Thole, B.T., Altarelli, M., Wang, X.D., (1993) Phys. Rev. Lett., 70, p. 694Stohr, J., König, H., (1995) Phys. Rev. Lett., 75, p. 3748Teramura, Y., Tanaka, A., Jo, T., (1996) J. Phys. Soc. Jpn., 65, p. 105