MgO-Based Double Barrier Magnetic Tunnel Junctions With Synthetic Antiferromagnetic Free Layer

Abstract

CoFeB/Ru/CoFeB has been used as the middle free layer in MgO-based double barrier magnetic tunnel junctions (DBMTJs). The tunneling magnetoresistance (TMR) ratio, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> (bias voltage at half maximum TMR ratio) and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> (output voltage, defined as V multiplied by TMR ratio) have been investigated as a function of annealing temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) and Ru thickness (t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ru</sub> ) in the free layer. Magnetization data reveal that the two CoFeB layers in CoFeB/Ru/CoFeB are antiferromagnetically coupled. Compared with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> of only 0.31 V for single barrier MTJs (SBMTJs) annealed at 375°C, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> for DBMTJs is up to 0.66 V. By increasing Ta, TMR ratio first increases, reaching the highest critical value, and then decreases. The highest TMR ratio is 181% with t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ru</sub> = 0.6 nm, which is much higher than that obtained in the DBMTJs with the pure CoFeB as the free layer. The improved TMR ratio is mainly due to the relatively thorough crystallization of the CoFeB layers in the free layer. The thermal annealing has been proven to be an effective method to remove the dissimilarity of the top and bottom CoFeB/MgO interfaces. V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> in the positive and negative voltage branches follows the same trend as that of TMR ratio with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> .