Ph. Lecoeur

Colossal-magnetoresistive manganite thin films

Mixed-valence perovskite manganites (Re1-xAxMnO3 where Re = rare earth, A = alkaline earth) provide a unique opportunity to study the relationships between the structure and the magnetotransport properties due to an interplay among charge carriers, magnetic coupling, orbital ordering and structural distortion. This makes these compounds very exciting from both the basic research and from the technological viewpoint. As the technology pursued with these materials requires film growth, extensive studies have been made on materials synthesis, structural and physical characterization and device fabrication. In this article, the results from the different experimental techniques and the effects of the deposition procedure of the manganite thin films are first reviewed. Second, the relation between the structural and the physical properties is mentioned, and the influence of strains discussed. Finally, possible applications of manganite thin films in spin electronics are presented.

Spectacular decrease of the melting magnetic field in the charge-ordered state of Pr0.5Ca0.5MnO3 films under tensile strain

An insulator to metal transition below 240K is induced by applying a 7T magnetic field in Pr0.5Ca0.5MnO3 thin films grown by the Pulsed Laser Deposition technique on (100) SrTiO3 substrates. This value of the melting magnetic field, much lower that the one required in bulk (20T), is assumed to be an effect of the tensile stress. These results confirm the importance of the bandwidth in the control of the physical properties of this compound and open the route to get colossal magnetoresistive properties by using strain effects.

Spectacular decrease of the melting magnetic field in the charge-ordered state of tensile Pr0.5Ca0.5MnO3 films

An insulator to metal transition below 240K is induced by applying a 7T magnetic field in Pr0.5Ca0.5MnO3 thin films grown by the Pulsed Laser Deposition technique on (100) SrTiO3 substrates. This value of the melting magnetic field, much lower that the one required in bulk (20T), is assumed to be an effect of the tensile stress. These results confirm the importance of the bandwidth in the control of the physical properties of this compound and open the route to get colossal magnetoresistive properties by using strain effects.

Optimal working conditions for thermoelectric generators with realistic thermal coupling

We study how maximum output power can be obtained from a thermoelectric generator (TEG) with non-ideal heat exchangers. We demonstrate with an analytic approach based on a force-flux formalism that the sole improvement of the intrinsic characteristics of thermoelectric modules including the enhancement of the figure of merit is of limited interest: the constraints imposed by the working conditions of the TEG must be considered on the same footing. Introducing an effective thermal conductance we derive the conditions which permit maximization of both efficiency and power production of the TEG dissipatively coupled to heat reservoirs. Thermal impedance matching must be accounted for as well as electrical impedance matching in order to maximize the output power. Our calculations also show that the thermal impedance does not only depend on the thermal conductivity at zero electrical current: it also depends on the TEG figure of merit. Our analysis thus yields both electrical and thermal conditions permitting optimal use of a thermoelectric generator.

Crystallinity, surface morphology, and magnetic properties of La0.7Sr0.3MnO3 thin films: An approach based on the laser ablation plume range models

We report on a systematic investigation of the influence of pulsed laser deposition (PLD) parameters upon the microstructure, surface morphology, and magnetic properties of La0.7Sr0.3MnO3 (LSMO) thin films grown on (100) SrTiO3 substrates. The optimization of the physical properties requires a careful exploration of the main parameters such as the oxygen pressure P and the target-to-substrate distance D. We show that there is a strong correlation between both these parameters and an optimal distance (D=L0), which can be calculated from a PD3 scaling law (in accordance with a shock wave model). This particular value L0 corresponds to the distance for which all species are thermalized in the plume. In the D–P diagram, L0 defines two distinct regions for the morphology and the microstructure: (i) when D 1 μm) morphology. These films are perfectly epitaxial on the substrate, single-phase and not relaxed. (ii) When D≫L0, the films are colum...

Efficiency at maximum power of thermally coupled heat engines.

We study the efficiency at maximum power of two coupled heat engines, using thermoelectric generators (TEGs) as engines. Assuming that the heat and electric charge fluxes in the TEGs are strongly coupled, we simulate numerically the dependence of the behavior of the global system on the electrical load resistance of each generator in order to obtain the working condition that permits maximization of the output power. It turns out that this condition is not unique. We derive a simple analytic expression giving the relation between the electrical load resistance of each generator permitting output power maximization. We then focus on the efficiency at maximum power (EMP) of the whole system to demonstrate that the Curzon-Ahlborn efficiency may not always be recovered: The EMP varies with the specific working conditions of each generator but remains in the range predicted by irreversible thermodynamics theory. We discuss our results in light of nonideal Carnot engine behavior.

Interface electronic structure in a metal/ferroelectric heterostructure under applied bias

The effective barrier height between an electrode and a ferroelectric (FE) depends on both macroscopic electrical properties and microscopic chemical and electronic structure. The behavior of a prototypical electrode/FE/electrode structure, Pt/BaTiO

Irreversibilities and efficiency at maximum power of heat engines: the illustrative case of a thermoelectric generator.

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Current–voltage characterization of the vortex motion in YBa2Cu3O7−δ microbridges and the implications on the development of superconducting flux flow transistors

/Nb-doped SrTiO

Thermoelectric internal current loops inside inhomogeneous systems

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