Field induced fluctuations in percolation in granular-heterogeneous La-Ag-Mn-O/ MnO

Several materials in the granular form, such as transition metals and manganese perovskites, show a giant magnetoresistance (GMR) that is attributed to field induced percolation effects [1]. We discuss here the observation of GMR in the solid solution La$_{1-x}$Ag$_{x}$MnO$_{3}$ (x = 0.05 - 0.50), possibly due to magnetic field induced fluctuation in percolation (MFP). Samples with x in steps of 0.05 were fabricated by the sol-gel method. We found Ag disassociation and run-off when the sample sintering temperature exceeded 800${^\circ}$, resulting in a composite of La-Ag-Mn-O and manganese oxide. Analysis of x=0.35, for example, revealed a composite composition of (La$_{0.926}$Ag$_{0.074}$MnO$_{3})_{0.698}$(MnO$_{2})_{0..302}$. Studies on such samples showed GMR that could be attributed to field-induced enhancement in percolation [1]. [1] Ning Zhang, Weiping Ding, and Wei Zhong et al., Phys Rev B 56, 8139 (1997). -Work supported by the National Science Foundation (DMR-0302254)