Magnetostriction and Tunneling Magnetoresistance of Co/AlO $_{{\rm x}}$ /Co/IrMn Junctions

Cross-strip magnetic tunnel junctions (MTJs) were fabricated following this sequence: Si(100)/Ta(30 A)/Co(75 A)/AlO_x(deltat_o)/Co (75 A)/ IrMn(90 A)/Ta(100 A) was deposited under an in-plane deposition field (h) = 500 Oe, where deltat_o = 12, 17, 22, and 26, and 30 A was the thickness of the AlO _x layer. Tunneling magnetoresistance (TMR) rose initially from 21% to 36% and then fell to 24% as deltat_o increased monotonically. This means the spin tunneling weakened as deltat_o ges 26 A. The magnetostriction (lambda_s) of the Co/AlO_x/Co MTJs was generally in the range of -23 ppm to -12 ppm. Moreover, the lambda_s vs. deltat_o plot shows the concave-up feature. High-resolution sectional transmission electron microscope (HR X-TEM) with nanoprobe energy dispersive x-ray (EDX) spectroscopy was used to analyze the Co, Al, and O concentrations across each AlO_x layer. The different concentration dependences along deltat_o resulted in the lambda_s feature. In addition, HR X-TEM, showed that in the microstructure of each Co/AlO_x/Co MTJ, the interfaces of Co/AlO _x and AlO _x/Co are smooth. We find that the optimal MTJ in this series has the following properties: TMR = 36% and lambda_s = -15 ppm.