Ronald I. Dass

Intra- versus intergranular low-field magnetoresistance of Sr2FeMoO6 thin films

Thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited on LaAlO3 and SrTiO3 (001) substrates. Comparison of their transport and magnetic properties with those of polycrystalline ceramic samples shows a metallic versus semiconductor temperature dependence and a saturation magnetization Ms at 10 K of 3.2 μB/f.u. in the film as against 3.0 for a tetragonal polycrystalline sample. However, the Curie temperature TC≈389 K is reduced from 415 K found for the tetragonal ceramic, which lowers Ms at 300 K in the thin films to 2.0 μB/f.u. compared to 2.2 μB/f.u. in the ceramics. A Wheatstone bridge arrangement straddling a bicrystal boundary has been used to verify that spin-dependent electron transfer through a grain boundary is responsible for the low-field magnetoresistance found in polycrystalline samples below TC.

Comparison of the crystal chemistry and electrical properties of La2−xAxNiO4 (A = Ca, Sr, and Ba)

With an objective to understand the influence of the size of the A cations, the crystal chemistry and electrical properties of La2−xAxNiO4 (A = Ca, Sr, and Ba) are compared. The solubility limits of the A cations in La2−xAxNiO4 are 0.6, 1.5, and 1.1, respectively, for A = Ca, Sr, and Ba. At a given value of x, the Ca and Ba systems have lower oxygen content than that of the Sr system. The lower solubility limit and oxygen content for the Ca and Ba systems are due to a preference of Ca2+ for eightfold coordination and a larger degree of tensile stress in the Ni–O bonds of La2−xBaxNiO4 at higher doping levels. For x 0.6, the a parameter increases in the Sr and Ba systems, while the c parameter decreases in the Sr system and remains almost constant in the Ba system with increasing x. The increase in a parameter and the decrease or constancy of c parameter for x > 0.6 is due to a transfer of electron density from the dz2 band to the dx2−y2 band. While the Sr system exhibits a semiconductor-to-metal transition with increasing x, both the Ca and Ba systems are semiconducting for all values of x.

Grain-boundary room-temperature low-field magnetoresistance in Sr2FeMoO6 films

Thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited or LaAlO3 and SrTiO3 (001) substrates by pulsed laser deposition. The deposition conditions were optimized. Single-phase Sr2FeMoO6 was obtained in 100 mTorr 99.999% Ar gas at 825 °C. Transport and magnetic data showed a metallic temperature dependence and a saturation magnetization Ms at 10 K of 3.2μB/f.u. However, the Curie temperature TC≈380 K was reduced from 415 K found for tetragonal polycrystalline best ceramics, which lowers Ms at 300 K in the thin films to 1.5μB/f.u. compared to 2.2μB/f.u. in the ceramics. A low remanence was attributed to the presence of antiphase boundaries. A Wheatstone bridge arrangement straddling a bicrystal boundary was used to verify that spin-dependent electron transfer through a grain boundary and not an antiphase boundary is responsible for the low-field magnetoresistance found in polycrystalline samples below TC.

Synthesis and Characterization of Sr2MgMoO6 − δ An Anode Material for the Solid Oxide Fuel Cell

The double-perovskite Sr 2 MgMoO 6-δ (SMMO) was investigated as an anode material of a solid oxide fuel cell. Via a synthetic method based on thermal decomposition of metal complexes with ethylenediaminetetraacetic acid as the complexant, phase-pure SMMO was readily obtained. Oxygen vacancies are introduced by reduction with 5% H 2 at 800°C. With a 300 μm thick La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 2.815 disk as the electrolyte and SrCo 0.8 Fe 0.2 O 3-δ as the cathode, the SMMO anode showed power densities of 0.84 W/cm 2 in H 2 and 0.44 W/cm 2 in CH 4 at 800°C. Moreover, it performed stably on power cycling and tolerated sulfur and moisture well. Only 1% degradation in the output was observed in H 2 containing 5 parts per million (ppm) H 2 S and 16% degradation in H 2 containing 50 ppm H 2 S compared with the output in pure H 2 . Thermogravimetric analysis showed a drop in mass at around 750°C in the atmospheres of both air and 5% H 2 , indicative of the formation of oxygen vacancies. The mean thermal expansion coefficient was a = 12.7 X 10 -6 K -1 at the operating temperatures. The conductivity strongly depended on the atmosphere, and the electronic activation energies were E a = 0.084 eV in H 2 and 0.126 eV in CH 4 . Our results show that SMMO is a potential anode material for operation with natural gas.

Enhancement of room temperature magnetoresistance in double perovskite ferrimagnets

The double perovskite Sr2FeMoO6 is a half-metallic ferrimagnet exhibiting significant magnetoresistance (MR) at 300 K in a magnetic field H<2 kOe. A preliminary study of the influence of isovalent and aliovalent substitutions for Sr2+ on the magnitude of the room-temperature MR is reported. Of the polycrystalline samples, Sr1.9A0.1FeMoO6 (A=Ca, Ba, La or Pr), the best result was obtained with the Ba(0.1) sample, which showed a factor of 2 enhancement of the MR over that previously reported for Sr2FeMoO6.

Spin-glass to ferromagnet transition in LaMn1−xScxO3

Abstract Dilution of LaMn1−xMxO3 with a main-group trivalent M = Sc or Al atom is compared to the case M = Ga in order to confirm the generality of a vibronic ferromagnetic Mn(III)–O–Mn(III) superexchange interaction and stabilization of orbital disorder by a magnetizing field. A spin-glass to ferromagnet transition in a modest applied magnetic field has been found in the single-valent perovskite system LaMn1−xScxO3 as in the analogous system LaMn1−xGaxO3; the coexistence of a ferromagnetic phase and an antiferromagnetic phase at low temperatures extends to compositions where the orthorhombic room-temperature axial ratio is c/ 2 >a . The Sc(III) ion is more resistant to a cooperative, static ordering of the Mn(III) e-orbitals than is Ga(III); the Al(III) ion is less resistant than Ga(III). The data for these insulators confirm the existence of a ferromagnetic vibronic-superexchange interaction between octahedral-site, high-spin Mn(III) with fluctuating e-orbitals and stabilization by an applied magnetic field of the orbitally disordered, ferromagnetic phase relative to the orbitally ordered antiferromagnetic phase. A similar spin-glass to ferromagnet transition in the mixed-valent La1−xSrxMnO3 system at compositions exhibiting the colossal magnetoresistance (CMR) phenomenon supports the coexistence of ferromagnetic vibronic-superexchange and double-exchange interactions in the ferromagnetic phase and correlation bags rather than conventional magnetic polarons in the paramagnetic phase of the CMR manganites.

Synthesis and characterization of Sr2MgMoO6-δ

The double-perovskite Sr 2 MgMoO 6-δ (SMMO) was investigated as an anode material of a solid oxide fuel cell. Via a synthetic method based on thermal decomposition of metal complexes with ethylenediaminetetraacetic acid as the complexant, phase-pure SMMO was readily obtained. Oxygen vacancies are introduced by reduction with 5% H 2 at 800°C. With a 300 μm thick La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 2.815 disk as the electrolyte and SrCo 0.8 Fe 0.2 O 3-δ as the cathode, the SMMO anode showed power densities of 0.84 W/cm 2 in H 2 and 0.44 W/cm 2 in CH 4 at 800°C. Moreover, it performed stably on power cycling and tolerated sulfur and moisture well. Only 1% degradation in the output was observed in H 2 containing 5 parts per million (ppm) H 2 S and 16% degradation in H 2 containing 50 ppm H 2 S compared with the output in pure H 2 . Thermogravimetric analysis showed a drop in mass at around 750°C in the atmospheres of both air and 5% H 2 , indicative of the formation of oxygen vacancies. The mean thermal expansion coefficient was a = 12.7 X 10 -6 K -1 at the operating temperatures. The conductivity strongly depended on the atmosphere, and the electronic activation energies were E a = 0.084 eV in H 2 and 0.126 eV in CH 4 . Our results show that SMMO is a potential anode material for operation with natural gas.

Synthesis and Characterization of Sr[sub 2]MgMoO[sub 6−δ]

The double-perovskite Sr 2 MgMoO 6-δ (SMMO) was investigated as an anode material of a solid oxide fuel cell. Via a synthetic method based on thermal decomposition of metal complexes with ethylenediaminetetraacetic acid as the complexant, phase-pure SMMO was readily obtained. Oxygen vacancies are introduced by reduction with 5% H 2 at 800°C. With a 300 μm thick La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 2.815 disk as the electrolyte and SrCo 0.8 Fe 0.2 O 3-δ as the cathode, the SMMO anode showed power densities of 0.84 W/cm 2 in H 2 and 0.44 W/cm 2 in CH 4 at 800°C. Moreover, it performed stably on power cycling and tolerated sulfur and moisture well. Only 1% degradation in the output was observed in H 2 containing 5 parts per million (ppm) H 2 S and 16% degradation in H 2 containing 50 ppm H 2 S compared with the output in pure H 2 . Thermogravimetric analysis showed a drop in mass at around 750°C in the atmospheres of both air and 5% H 2 , indicative of the formation of oxygen vacancies. The mean thermal expansion coefficient was a = 12.7 X 10 -6 K -1 at the operating temperatures. The conductivity strongly depended on the atmosphere, and the electronic activation energies were E a = 0.084 eV in H 2 and 0.126 eV in CH 4 . Our results show that SMMO is a potential anode material for operation with natural gas.

Low-temperature Kerr spectroscopy on half-metallic Sr2FeMoO6

The polar Kerr rotation and ellipticity spectra of epitaxially grown (001)-oriented half-metallic Sr2FeMoO6 thin films have been determined in the photon energy range from 1.2 to 4.9 eV. The Kerr rotation spectrum shows three maxima at E=1.6, 4.0, and 4.65 eV. The maxima at 4.0 and 4.65 eV are consistent with spin-polarized band structure calculations for interband transitions from the O-2p to the minority-spin π*Mo/Fe and majority-spin Mo-t2g bands, respectively. The overall maximum intrinsic Kerr rotation is ΘK=−0.045° at a photon energy of 4.65 eV. The maximum of ΘK at E=1.6 eV coincides with a minimum in the reflectivity due to the plasma edge of Sr2FeMoO6 and, therefore, is not related to an interband transition.