Biomedical subjects

A Maignan

Publications and source records attributed to A Maignan.

18 recordsLinked to original sources

PubMed2006

Scaling behavior in thermoelectric misfit cobalt oxides.

We investigate both thermoelectric and thermodynamic properties of the misfit cobalt oxide [Bi1.7Co0.3Ca2O4]RS0.6CoO2. A large negative magnetothermopower is found to scale with both magnetic field and temperature, revealing a significant spin entropy contribution to thermoelectric properties giving rise to a constant S0 approximately 60 microV K-1. Specific heat measurements allow us to determine an enhanced electronic part with gamma approximately 50 mJ (mol K2)-1 attesting to strong correlations. Thereby, the comparison between cobaltites and other materials reveals a universal behavior of the thermopower slope as a function of gamma, testifying to a purely electronic origin. This potentially generic scaling behavior suggests here that the high room temperature value of the thermopower in misfit cobalt oxides results from the addition of a spin entropy contribution to an enlarged electronic one.

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PubMed2004

Structural, magnetic, and transport properties of the SrTi1-xCoxO3-delta perovskite (0 < or = x < or = 0.9).

The series of the SrTi(1-x)Co(x)O(3-delta) polycrystalline samples has been prepared via solid-state reaction in air. The structural study shows that a solid solution exists for all compositions, such as 0 < or = x < or = 0.9, which discards the presence of ferromagnetic and metallic cobalt clusters for the low level of cobalt substitution. The existence of systematic extra peaks on the electron diffraction patterns for x > 0.5 indicates that the oxygen vacancy ordering is responsible for the superstructures (2a(p) x 2a(p) x 4a(p), where the subscript "p" refers to the perovskite subcell). The oxygen nonstoichiometry is confirmed by thermogravimetric analysis. The magnetic properties reveal a maximum in the magnetic moment per mole of cobalt for x approximately 0.10. However, the clear lack of strong ferromagnetism does not suggest that a diluted magnetic semiconductor (DMS)-type behavior is induced in SrTiO(3) as Co is substituted for Ti. The transport properties along the SrTi(1-x)Co(x)O(3-delta) line are explained by considering the substitution of a Co(3+)/Co(4+) mixed valency, which decreases the room-temperature resistivity by at least 5 orders of magnitude as one goes from SrTi(0.9)Co(0.1)O(3-delta) to SrTi(0.1)Co(0.9)O(3-delta).

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PubMed2004

Thermoelectric power of HoBaCo2O5.5: possible evidence of the spin blockade in cobaltites.

Thermoelectric power measurements have been performed for an ordered oxygen-deficient perovskite, HoBaCo2O5.5, in which the alternative layers of CoO6 octahedra and of [CoO(5)](2) bipyramids are occupied by Co3+ species. The T-dependent Seebeck coefficient S shows a clear change of the conduction regime at the metal-insulator (MI) transition (T(MI) approximately 285 K). The sign change of S from S<0 to S>0 can be explained assuming that a spin state transition occurs at T(MI). In the metallic state, Co2+ e(g) electrons are moving in a broad band on the background of high or intermediate spin Co3+ species. In contrast, the insulating behavior may result from the Co3+ spin state transition to a low-spin Co3+ occurring in the octahedra. In this phase the transport would occur by hopping of the low-spin Co(4+)t(2g) holes, whereas the high-spin Co2+ electrons become immobilized due to a spin blockade.

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PubMed2004

Linear antiferromagnetism in Ba2CoS3.

Ba(2)CoS(3) is the first example of an inorganic solid containing one-dimensional corner-sharing [Co(2+)-anion] chains, which leads to one-dimensional cooperative antiferromagnetism.

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PubMed2004

Dome-shaped magnetic phase diagram of thermoelectric layered cobaltites.

Using muon spin spectroscopy we have found that, for both NaxCoO(2) (0.6<or=x<or=0.9) and three- and four-layer cobaltites, a common low temperature magnetic state (which in some cases is manifest as an incommensurate spin density wave) forms in the CoO(2) planes. Here we summarize those results and report an almost dome-shaped relation between the transition temperature into the low-T magnetic state and the composition x for NaxCoO(2) and/or the high-temperature asymptotic limit of thermopower in the more complex three- and four-layer cobaltites. This behavior is explained using the Hubbard model on a two-dimensional triangular lattice in the CoO(2) plane.

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PubMed2002

Ultrasharp magnetization steps in perovskite manganites.

We report a detailed study of steplike metamagnetic transitions in polycrystalline Pr0.5Ca0.5Mn0.95Co0.05O3. The steps have a sudden onset below a critical temperature, are extremely sharp (width <2x10(-4) T), and occur at critical fields which are linearly dependent on the absolute value of the cooling field in which the sample is prepared. Similar transitions are also observed at low temperature in non-Co doped manganites, including single crystal samples. These data show that the steps are an intrinsic property, qualitatively different from either previously observed higher temperature metamagnetic transitions in the manganites or metamagnetic transitions observed in other materials.

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PubMed1996