Study of grain boundary contribution and enhancement of magnetoresistance in La_0.67Ca_0.33MnO₃/V₂O₅ composites

Electrical transport (90-300 K) and magnetic (5-375 K) properties of the La_0.67Ca_0.33MnO₃ + x% V₂O ₅ (x ≤ 0, 2, 5, 15 and 30) type magnetic-nonmagnetic composite system show appreciable grain boundary contribution. These composites, except for the one with x ≤ 30%, show metal-insulator transitions at T ≤ T _p which decrease with increasing V₂O₅ concentration. The high temperature (T > T_p) semiconducting part of the resistivity (ρ) data follow a small polaron hopping conduction mechanism. The magnetoresistance (MR) in this system increases with increasing V₂O₅ content, showing a maximum value (25% at 93 K and field H ≤ 1.15 T) for the sample with x ≤ 15%. Spin polarization is considered to be responsible for such an increase of MR. Neither thermoelectric power (between 90 and 300 K) nor magnetization data indicate any noticeable change of the Curie temperature (T_C) with change of x, suggesting that V₂O₅ remains at the grain boundary. The complex impedance (Z^*) of the composite system, measured over a frequency range of 0.01 kHz-13 MHz, at 300 K shows strong frequency dependence and the impedance plane plots (Z′-Z″) are semicircular for different V₂O₅ content. An equivalent resistor-capacitor circuit (R_g (R_gb C_gb)) has been proposed to explain the impedance results in this composite system.