The authors fabricated double MgO barrier magnetic tunnel junctions (MTJs) with 3-nm-thick Co40 Fe40 B20 free layer. When annealed at 350 °C, tunnel magnetoresistance (TMR) ratio at room temperature was 130%, much lower than that (297%) of single MgO barrier MTJs processed and annealed under the same condition. The middle CoFeB free layer sandwiched between the two MgO barriers was found to be mostly amorphous. Replacement of the Co40 Fe40 B20 free layer by a highly oriented Co50 Fe50 layer and a composite Co50 Fe50 / Co40 Fe40 B20 layer led to the enhanced TMR ratios up to 165% and 212% at annealing temperature of 350 °C, respectively.
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