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We present results of experimental studies of magnetic properties, resistivity and magnetoresistance (MR) of (Co 84 Nb 14 Ta 2 )x(Al 2 O 3 ) 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 μ 0  H  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 ln T  behavior that matches a strong tunnel coupling between nanogranules. With decreasing metal volume fraction ln T  behavior gradually changes to the T 1/2  dependence at 47 at.%. For all samples MR is small (&lt;1%) and negative. For x&lt;57 at.% it is slightly anisotropic at μ 0  H &lt;1.0 T and almost saturates with increasing magnetic field. There is an evidence of small positive contribution to MR at μ 0  H =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.

epj web of conferences

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