Tunnel-type giant magnetoresisitance in Co-Al-Ta-O insulated granular system

We have investigated the tunnel-type giant magnetoresistance (TR;IR). the r elated magnetic and electric properties and the microstructure of Co-Al-Ta- O granular thin films which were prepared by the reactive sputter-depositio n technique with oxygen and argon. The TMR attained a maximum value of 12.5 % at room temperature under a magnetic field of 1T for Co40Al13Ta3O44 films . The MR ratio is the largest in a granular system similar to Co-Al-O. Duri ng the sputtering, O-2 gas flow was introduced at 2.2 seem into the chamber filled with Ar at a total pressure higher than 1 x 10(-4) Torr. The specif ic electric resistance of the film that showed the maximum TMR is 1.46 x 10 (5) mu Omega . cm. Ta forms an oxide compound similar to Ta2O5 in the insul ator matrix. The effect of Ta addition on the TMR appears at around 2.8 at. %. The MR ratio increases with an increase of the fraction of fee-Co, accom panying the decrease of the Co-O fraction and the decrease in ferromagnetic interaction between Co particles. The specimen that shows large TMR consis ts of nanometer-sized Co particles which are completely isolated by amorpho us Al2O3 and Ta2O5 intergranular layers. The standard free energy of oxidat ion of Co, Al and Ta may play an important role in the formation of the mic rostructure of Co-Al-Ta-O granular films and in TMR improvement.