Unusual thermoelectric behavior of packed crystalline granular metals

in Journal of Applied Physics (2004), 96(12), 7338-7345

Loosely packed granular materials are intensively studied nowadays. Electrical and thermal transport properties should reflect the granular structure, as well as intrinsic properties. We have compacted ... [more ▼]

Loosely packed granular materials are intensively studied nowadays. Electrical and thermal transport properties should reflect the granular structure, as well as intrinsic properties. We have compacted crystalline CaAl-based metallic grains and studied the electrical resistivity and the thermoelectric power as a function of temperature (T) from 15 to 300 K. Both properties show three regimes as a function of temperature. It should be pointed out: (i) The electrical resistivity continuously decreases between 15 and 235 K, (ii) with various dependencies, e.g., similar ~ T(-3/4) at low T, while (iii) the thermoelectric power (TEP) is positive, (iv) shows a bump near 60 K, and (v) presents a rather unusual square root of temperature dependence at low temperature. It is argued that these three regimes indicate a competition between geometric and thermal processes-for which a theory seems to be missing in the case of TEP. The microchemical analysis results are also reported, indicating a complex microstructure inherent to the phase diagram peritectic intricacies of this binary alloy. (C) 2004 American Institute of Physics. [less ▲]

Dibarium zirconium tetraoxalate trihydrate

in Acta Crystallographica Section C-Crystal Structure Communications (2004), 60(Pt 6), 233-234

A new mixed barium zirconium oxalate, triaquatetra-mu-oxalato-dibarium(II)zirconium(IV), Ba2Zr(C2O4)(4).3H(2)O or [Ba2Zr(C2O4)(4)(H2O)(3)](n), has been synthesized. The complex is built from eightfold ... [more ▼]

A new mixed barium zirconium oxalate, triaquatetra-mu-oxalato-dibarium(II)zirconium(IV), Ba2Zr(C2O4)(4).3H(2)O or [Ba2Zr(C2O4)(4)(H2O)(3)](n), has been synthesized. The complex is built from eightfold-coordinated Zr atoms and eleven- and sixfold-coordinated Ba atoms, linked by oxalate groups. The Zr atom, the two Ba atoms and one water O atom lie on crystallographic twofold axes, so that each coordination polyhedron has imposed C2 symmetry. Packing in the crystal is also assumed through hydrogen bonds. [less ▲]

Magnetic and electronic ground states of B-site-substituted LaMnO3: from antiferromagnetism to ferromagnetism

in Journal of Magnetism & Magnetic Materials (2004), 280(1), 75-83

We report about the physical properties of samples obtained by different substitutions on the Mn site of LaMnO3: LaMn0.85Ni0.15O3, LaMn0.85Ga0.15O3 and LaMn0.5Ga0.5O3. It is well known that the ... [more ▼]

We report about the physical properties of samples obtained by different substitutions on the Mn site of LaMnO3: LaMn0.85Ni0.15O3, LaMn0.85Ga0.15O3 and LaMn0.5Ga0.5O3. It is well known that the antiferromagnetic orbitally ordered ground state of LaMnO3 is easily destroyed to give way to ferromagnetism. However, this ferromagnetic behaviour can result from several mechanisms: depending on the nature of the substituting cation and on the substitution level, the physical properties derived from a complex interplay between exchange interactions, orbital ordering and Jahn-Teller distortion. As a result, there are noticeable differences between the physical properties of the different samples, even though all three samples display a ferromagnetic component in zero applied magnetic field. In this paper we compare the crystallographic, electrical and magnetic properties of these samples, with a special attention for the AC susceptibility behaviour. (C) 2004 Elsevier B.V. All rights reserved. [less ▲]

Synthesis of CMR manganate compounds: the consequences of the choice of a precursor method

in Materials Letters (2002), 57(3), 598-603

We have studied the electrical transport properties of manganate samples synthesized by different precursor methods. Differences in the synthesis process of the materials can result in samples with ... [more ▼]

We have studied the electrical transport properties of manganate samples synthesized by different precursor methods. Differences in the synthesis process of the materials can result in samples with similar grain size but very different physical properties, contrary to what happens in samples prepared by the same method. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

Magnetic properties of Ga-doped lanthanum manganite with controlled oxygen stoichometry

in Crystal Engineering (2002), 5

Stoichiometric LaMnO3 is an insulator whose A-type antiferromagnetic structure at low temperature is mainly connected to the Jahn-Teller distortion of the MnO6 octahedra. The substitution of non magnetic ... [more ▼]

Stoichiometric LaMnO3 is an insulator whose A-type antiferromagnetic structure at low temperature is mainly connected to the Jahn-Teller distortion of the MnO6 octahedra. The substitution of non magnetic d(10) Ga3+ ions on the manganese site disrupts the orbital and spin orderings of the manganese ions and has a strong influence on the physical properties. The antiferromagnetic order is progressively destroyed while the ferromagnetic component increases. At the same time, the distortion of the orthorhombic cell decreases and the high temperature structural Jahn-Teller transition disappears. Such studies on manganese site substitutions give insight into the physics of the Mn-O three-dimensional network, which is supposed to be the key feature in the CMR manganites. (C) 2003 Elsevier Science Ltd. All rights reserved. [less ▲]

Magnetotransport properties of a single grain boundary in a bulk La-Ca-Mn-O material

in Journal of Applied Physics (2001), 90(11), 5692-5697

Besides the "intrinsic" colossal magnetoresistance effect observed in single crystals, the polycrystalline manganate compounds also exhibit an "extrinsic" magnetoresistance related to the presence of ... [more ▼]

Besides the "intrinsic" colossal magnetoresistance effect observed in single crystals, the polycrystalline manganate compounds also exhibit an "extrinsic" magnetoresistance related to the presence of grain boundaries. We report electrical transport and magnetic measurements carried out on a bigrain sample extracted from a floating zone method-grown rod of calcium doped lanthanum manganate. Electrical resistance was measured both within a grain and across the grain boundary, between 20 and 300 K and from 0 to 8 T. Magnetoresistance values up to 99% are reached within the grain. The temperature dependence of the resistance across the grain boundary displays a "foot-like" feature towards the bottom of the transition. Low field and high field magnetoresistance effects are examined. We compare our results for a "bulk" grain boundary to those obtained by other authors for bicrystal thin films and bulk polycrystalline materials. (C) 2001 American Institute of Physics. [less ▲]