Enhanced tunnel magnetoresistance at high bias voltage in double-barrier planar junctions

Single Co/Al2O/NiFe and double Co/Al2O3/Co/Al2O3/NiFe planar tunnel junctions were grown by sputtering and subsequently patterned in a four- step process using optical lithography. The Al2O3 barriers are formed by radio frequency plasma oxidation of 1.5 nm aluminum layers. The double junctions exhibit three clear resistance levels depending on the relative configuration of the magnetizations. Both single and double junctions exhibit maximum magnetoresistance (MR) ratios above 10% at room temperature and 20% at 30 K and a decrease of MR with increasing bias voltage. With regard to its low bias value, the MR is reduced by a factor of 2 at 0.26 V for the single junctions and at values above 0.8 V for the double junctions. The decay of the MR of double junctions with bias voltage is significantly slower than expected from two independent junctions in series. (C) 1998 American Institute of Physics. [S0003-6951(98)03045-9].