Magnetoresistance of (Co40Fe40B20)x(SiO2)100-x and (Co84Nb14Ta2)x(Al2O3)100-x nanocomposites below the percolation threshold in pulsed magnetic fields

We present results of experimental studies of magnetic properties, resistivity and magnetoresistance (MR) focusing on MR in high magnetic fields of (Co40Fe40B20)(x)(SiO2)(100-x) and (Co84Nb14Ta2)(x)(Al2O3)(100-x) nanocomposites near the percolation threshold. The nano-composite films with x = 47-58 at.% were deposited onto a glass-ceramic substrate by the ion-beam sputtering. The samples consist of metallic nanogranules embedded into the nonstoichiometric matrix. According to the structural and magnetic data a large amount of metallic atoms are dispersed between magnetic nanogranules. With decreasing metal volume fraction the temperature dependence of conductivity changes from InT behavior, matches a strong tunnel coupling between nanogranules, to the T-1/2 dependence below the metal-insulator transition. MR was studied in pulsed magnetic fields up to 20 T at T = 65-300 K. The pulse duration was 11 ms. Negative MR almost saturates with increasing magnetic field but slowly increases or decreases in high magnetic fields. There is an evidence of linear positive contribution to MR above saturation at 65-300 K with the slope (3-9) 10(-3)% T-1. It was shown that possible reason for positive MR is influence of high magnetic field on the tunnel barrier height and as a result on the tunnel transparency.