Dependence of tunneling magnetoresistance on ferromagnetic electrode thickness and on the thickness of a Cu layer inserted at the Al2O3/CoFe interface

The dependence of the spin tunneling magnetoresistance (TMR) on the ferromagnetic electrode thickness, t(CoFe), and on the thickness of a Cu layer inserted at barrier/ferromagnetic (Al2O3/CoFe) interface is reported. Junctions were fabricated through contact shadow mask or microlithography. Junction TMR increases sharply from zero to a maximum value of 16.4% (shadow masks) with increasing t(CoFe) from 10 to 30 Angstrom, or to 22% (microlithography) with increasing t(CoFe) from 6 to 20 Angstrom, and then varies slightly up to t(CoFe) of 500 Angstrom. The initial increase of TMR with t(CoFe) up to 30 Angstrom results from the spin-polarization increase in CoFe. This is supported by the magnetization measurement of (Ta 150 Angstrom/CoFe t(CoFe))(xn) multilayers, where the magnetization of CoFe increases from near zero to its bulk value with increase of CoFe thickness from 10 to 40 Angstrom. The incorporation of Cu layers at the Al2O3/CoFe (top electrode) interface decreases the TMR. However, junctions with a 40 Angstrom thick Cu layer added at the Al2O3/CoFe interface still maintain a TMR signal of 1%. The bias voltage dependence of TMR does not vary with the Cu layer thickness added at Al2O3/CoFe interface, implying that it is not significantly dependent on the barrier/ferromagnetic interface. (C) 1999 American Institute of Physics. [S0021-8979(99)35808-4].