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 ferroma gnetic electrode thickness, t(CoFe), and on the thickness of a Cu layer ins erted at barrier/ferromagnetic (Al2O3/CoFe) interface is reported. Junction s 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% (microlithograph y) 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 zer o 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 ad ded 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 Physic s. [S0021-8979(99)35808-4].