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We report the results of electrical resistivity measurements carried out on well-sintered La0.7Ca0.3MnO3 / Mn3O4 composite samples with almost constant composition of the magnetoresistive manganite phase (La0.7Ca0.3MnO3). A percolation threshold (fc) occurs when the La0.7Ca0.3MnO3 volume fraction is ~ 0.19. The dependence of the electrical resistivity as a function of La0.7Ca0.3MnO3 volume fraction (fLCMO) can be described by percolation-like phenomenological equations. Fitting the conducting regime (fLCMO > fc) by the percolation power law returns a critical exponent t value of 2.0 +/- 0.2 at room temperature and 2.6 +/-0.2 at 5 K. The increase of t is ascribed to the influence of the grain boundaries on the electrical conduction process at low temperature., Comment: 7 pages, 3 figures, accepted for publication in Phys. Rev. B

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 300K. 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 dependences, e.g. $\simeq$ $T^{-3/4}$ at low $T$, while (iii) the thermoelectric power (TEP) is positive, (iv) shows a bump near 60K, 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., Comment: to be published in J. Appl. Phys.22 pages, 8 figures

We report a detailed set of AC magnetic measurements carried out on bulk large grain La-Ca-Mn-O samples extracted from a floating zone method-grown rod. Three samples with $La_{0.78}Ca_{0.22}Mn_{0.90}O_{x}$ stoichiometry but differing in their microstructure were investigated by electrical resistivity and AC susceptibility measurements: (i) a single grain sample, (ii) a sample containing two grains and (iii) a polycrystalline sample. We show that the superimposition of DC magnetic fields during AC magnetic susceptibility measurements is an efficient way for characterizing the magnetic transition of samples with different microstructures. Whereas both single grain and polycrystalline samples display a single susceptibility peak, an additional kink structure is observed in the case of the double grain sample. The temperature dependence of the AC susceptibility measured with superimposed DC magnetic fields is analyzed in the framework of second-order phase transition ideas. The relations between the critical exponents ($\beta + \gamma$ ~ 1.5, $\delta$ ~ 2.5) are found to be close to those of the mean-field model for all samples. This is attributed to the disordering caused by unoccupied Mn sites., Comment: 25 pages, including 5 figures