Enhanced Spin Polarization by An Extra Co or CoFe Layer in FM/Insulator/FM Structures

Abstract

Spin-dependent tunnel magnetoresistance of Co(10nm)/Al2O3(2.6nm)/M(tnm)/ NiFe(10nm) with M = Co or CoFe was studied as a function of the thickness of Mt. The thickness (t) of the doped Co or CoFe was varied between 0 and 3 nm. We have experimentally showed that the polarization of tunneling electrons can be varied by modifying the interfacial condition between the insulator and magnetic layers. The enhancement of the tunneling magnetoresistance in samples of Co(10nm)/Al2O3(2.6nm)/CoFe(tnm)/NiFe(10nm) is greater than that in samples of Co(10nm)/Al2O3(2.6nm)/Co(tnm)/NiFe(10nm) for t between 0.8 and 2.0 nm. The enhanced tunneling magnetoresistance may be attributed to the increase in the effective polarization of the tunnel electrons due to the spin-filtering effect from the additional magnetic layer M. Besides, from the oxidation process study of the Al-O insulator layer on Corning glass, we have shown experimentally that the optimal condition for the oxidation of the Al-O insulator layer in FM/A1-O/FM system can be obtained under the pressure of 5x10-2 torr for 50% of Ar + 50% of O2 environment with both natural and plasma oxidations. For optimal oxidation of the Al-O insulator layer in a spin-dependent tunnel system, its thickness should be less than 3 nm