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

Abstract

The dependence of the spin tunneling magnetoresistance (TMR) on the ferromagnetic electrode thickness, tCoFe, 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 tCoFe from 10 to 30 Å, or to 22% (microlithography) with increasing tCoFe from 6 to 20 Å, and then varies slightly up to tCoFe of 500 Å. The initial increase of TMR with tCoFe up to 30 Å results from the spin-polarization increase in CoFe. This is supported by the magnetization measurement of (Ta 150 Å/CoFe tCoFe)×n multilayers, where the magnetization of CoFe increases from near zero to its bulk value with increase of CoFe thickness from 10 to 40 Å. The incorporation of Cu layers at the Al2O3/CoFe (top electrode) interface decreases the TMR. However, junctions with a 40 Å 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.