Tunnel-type Giant Magnetoresisitance in Co–Al–Ta–O Insulated Granular System

Abstract

We have investigated the tunnel-type giant magnetoresistance (TMR), the related magnetic and electric properties and the microstructure of Co–Al–Ta–O granular thin films which were prepared by the reactive sputter-deposition technique with oxygen and argon. The TMR attained a maximum value of 12.5% at room temperature under a magnetic field of 1 T for Co40Al13Ta3O44 films. The MR ratio is the largest in a granular system similar to Co–Al–O. During the sputtering, O2 gas flow was introduced at 2.2 sccm into the chamber filled with Ar at a total pressure higher than 1×10-4 Torr. The specific electric resistance of the film that showed the maximum TMR is 1.46×105 µΩ·cm. Ta forms an oxide compound similar to Ta2O5 in the insulator 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 fcc-Co, accompanying the decrease of the Co–O fraction and the decrease in ferromagnetic interaction between Co particles. The specimen that shows large TMR consists of nanometer-sized Co particles which are completely isolated by amorphous Al2O3 and Ta2O5 intergranular layers. The standard free energy of oxidation of Co, Al and Ta may play an important role in the formation of the microstructure of Co–Al–Ta–O granular films and in TMR improvement.