Electrical Transport and Magnetoresistance in Co–Al–O Granular Films under High Pressure

Abstract

The tunnel magnetoresistance (TMR) and the temperature dependence of electrical resistivity ρ(T) in insulating Co–Al–O granular films have been studied under high pressure. It is found that the temperature dependence of ρ is affected significantly by applying pressure and the behavior of ρ(T) becomes close to the metallic behavior at high pressure. Furthermore, TMR at low temperatures is enhanced up to 5 GPa, while it decreases with increasing pressure above that. The results provide an evidence that the successive onset of higher-order processes in tunneling conduction at low temperatures below about 50 K is enhanced at high pressure lower than 5 GPa. On the other hand, TMR at 77 K and room temperature (RT) decreases monotonously as pressure increases. The large difference in the pressure dependence of TMR between 4.2 K and the higher temperature is consistently explained by taking into account the higher order tunneling theory and the pressure induced percolation. The metal-insulator transition and the pressure dependence of TMR at 4.2 K are also discussed on the basis of the percolation theory.