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.