J. Androulakis

La0.95Sr0.05CoO3: An efficient room-temperature thermoelectric oxide

We present measurements of electrical resistivity, thermal conductivity, and thermopower of polycrystalline Sr-doped LaCoO3 with composition La0.95Sr0.05CoO3. Our data show that the investigated compound exhibits a very respectable room-temperature thermoelectric figure of merit value of 0.18. Our results not only show that oxides are promising candidates for thermoelectric cooling applications, but also point toward the need for careful theoretical calculations that will serve as a guide in producing the next generation of thermoelectric materials.

Synthesis and physical properties of arc melted NiMnSb

Several polycrystalline samples of the half-Heusler alloy NiMnSb were grown by arc melting of stoichiometric and nonstoichiometric amounts of high-purity constituent elements. The structure and the phase-purity of the prepared samples were examined systematically by powder x-ray diffraction. The transport properties of the best sample, with saturation magnetization M-s(5 K)=4 mu(B)/formula unit, were studied by measuring electrical resistivity, thermal conductivity, and thermopower. Features in both magnetic and transport data are consistent with NiMnSb being in a half-metallic state at low temperatures, i.e., the conduction electrons are fully spin polarized. However, point-contact Andreev reflection measurements on the same sample at 4.2 K demonstrate only similar to45% spin polarization.

Comparative study of the magnetic and magnetotransport properties of a metallic and a semiconducting member of the solid solution LaNixCo1−xO3

We present a comparative study of both the magnetic and magnetotransport properties for two members of the perovskite solid solution LaNixCo1−xO3 (x=0.2, 0.6) located on opposite sides of the chemically induced metal-to-insulator transition. LaNi0.6Co0.4O3 exhibits metallic behavior and small but negative magnetoresistance, whereas LaNi0.2Co0.8O3 exhibits semiconducting behavior and giant negative magnetoresistance at low temperatures. On the other hand, we observe pronounced similarities in the magnetic properties of both compounds. We provide a consistent explanation regarding the origin of the magnetoresistance in the two members of the solid solution.

Thermoelectric properties of LaNi1−xCoxO3 solid solution

We present the results of a systematic investigation of the thermal conductivity, Seebeck coefficient and electrical resistivity for selected members (x=0.3, 0.7, 0.8, and 1) of the LaNi1−xCoxO3 solid solution in the temperature range of 80⩽T⩽320 K. Substitution of Ni by Co drives the system from n-type metallic phase with low thermopower values into p-type semiconducting phase with high thermopower values of the order of hundreds of μV/K. But the thermal conductivity observed varies only slightly with the composition, x, over the temperature range investigated. The latter effect also holds true for the lightly hole doped x=0.8 member (in substituting Sr for La). Our results suggest that semiconducting members of the LaNi1−xCoxO3 solid solution can be classified as “phonon glass and electron crystal” systems. The highest room temperature figure of merit was exhibited by the x=0.8 member (ZT=0.022), which is comparable to that of single crystal NaCo2O4 (ZT=0.03).

Coexistence of ferromagnetic and glassy behavior in semiconducting LaNi0.2Co0.8O3

Systematic dc and ac magnetic susceptibility studies have been performed on single-phase LaNi0.2Co0.8O3 powder samples. Evidence of hysteresis loops supports the existence of ferromagnetic correlations that lead to a rapid increase of the dc susceptibility at T⩽55 K. The frequency dependence of the ac susceptibility is suggestive of a spin-glass-like phase. The temperature dependence of the nonlinear susceptibility components, χ2 and χ3, in zero dc magnetic field demonstrates the existence of spontaneous magnetization, appears to exclude the occurrence of a canonical spin-glass phase and instead indicates that a cluster-glass phase occurs in LaNi0.2Co0.8O3. It is suggested that the spontaneous magnetization in this system is due to the existence of Ni-based clusters with strong intracluster ferromagnetic correlations, embedded randomly in a paramagnetic matrix.

Negative giant longitudinal magnetoresistance in NiMnSb/InSb : Interface effect

We report on the electrical and magneto-transport properties of the contact formed between polycrystalline NiMnSb thin films grown using pulsed laser deposition (PLD) and n-type degenerate InSb (100) substrates. A negative giant magnetoresistance (GMR) effect is observed when the external magnetic field is parallel to the surface of the film and to the current direction. We attribute the observed phenomenon to magnetic precipitates formed during the magnetic film deposition and confined to a narrow layer at the interface. The effect of these precipitates on the magnetoresistance depends on the thermal processing of the system.