High field magnetoresistance of nanocomposites (Co84Nb14Ta2)X(Al2O3)100-X near the percolation threshold

Abstract

We present results of experimental studies of magnetic properties, resistivity and magnetoresistance (MR) of (Co84Nb14Ta2)x(Al2O3)100-x films deposited onto a glass-ceramic substrate by the ion-beam sputtering, focusing on MR in high magnetic fields for compositions close to the percolation threshold (x=47-57 at.%). The samples consist on Co-Nb-Ta metallic nanogranules size of 2-5 nm which are embedded into the non-stoichiometric Al-O matrix. Magnetization was measured by SQUID magnetometer at T=4.2-350 K. MR was studied in the pulsed magnetic fields μ0H up to 20 T at T=70-300 K in three geometries: magnetic field in plane parallel and perpendicular to current, magnetic field perpendicular to plane. The pulse duration was 11-12 ms. For the sample with x=57 at.% the temperature dependence of conductivity follows the lnT behavior that matches a strong tunnel coupling between nanogranules. With decreasing metal volume fraction lnT behavior gradually changes to the T1/2 dependence at 47 at.%. For all samples MR is small (<1%) and negative. For x<57 at.% it is slightly anisotropic at μ0H<1.0 T and almost saturates with increasing magnetic field. There is an evidence of small positive contribution to MR at μ0H=20T. Accordingly to structural and magnetic data a large amount of metallic atoms are located between magnetic nanogranules that diminish the tunnel barrier height and make tunnel MR small and weakly dependent on temperature.