Abstract
The influence of grain boundaries on the magnetoresistance in the perovskite manganitesLa0.7A0.3MnO3 (A = Ca,Sr) and the layeredperovskite manganites La1.36Sr1.64Mn2O7 has been investigated by means of ac and dc magnetic susceptibility and magnetoresistance measurements.The following characteristics were found. (i) The magnetoresistance (MR) of single-crystalLa0.7Ca0.3MnO3 is almost independent of the magnetic field at low temperatures, and the MR of polycrystallineLa0.7A0.3MnO3 (A = Ca,Sr)and La1.36Sr1.64Mn2O7 decreases drastically with the initial increment of the magnetic field up to2.5 × 10−1 T. (ii) The resistivity and the absolute value of the MR of polycrystallineLa0.7Sr0.3MnO3 increase with decreasing grain size at 4.2 K. (iii) The resistivity of these polycrystallinemanganites shows a time-dependent nature, but the single-crystal form does not.
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