Abstract
It is for the first time that in three-dimensional granular cobaltites (La1−x Srx)1−yAgyCoO3 with a grain size of about 1 μm and replacement ratios x = 0.35 and y = 0, 0.05 a “metal–nonmetal” conductivity transition was observed, presumably associated with the AFM ordering of granule moments. This transition manifests itself as a minimum in the temperature dependence of resistance of the samples. An explanation of the nature of the minimum is proposed, based on the intragranular mechanism of electron correlation on the basis of the Zener double-exchange and on the intergranular mechanism of spin-polarized electron tunneling between the nearest neighbors under the antiferromagnetic exchange interaction of granule moments. Numerical calculation of the system conductance is made via the model based on summation of intragranular phase conductivities of the electronic system and on the computation of the total impedance of the system as a sum of resistances of individual granules with a given resistive contribution of intergranular tunneling. It is found that the external magnetic field of up to 10 T does not affect the depth of the minimum and its position on the temperature scale, suggesting that the intergranular antiferromagnetic interaction is stable to external magnetic fields.
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