EXAFS Studies of Local Structure in MTJ MRSEC (Johns Hopkins University) DMR05-20491 Background: CoFeB-MgO magnetic tunnel junctions (MTJs) are leading spintronic devices that relies on quantum Mechanical tunneling of electrons from one magnetic metal (CoFeB) to another across an insulating barrier of MgO. The electrical resistance of such MTJs is dictated by the orientation of the two CoFeB metals, parallel or antiparallel. The performance of MTJ is measured by tunneling magnetoresistance (TMR), which is only20% initially, but greatly increases to 200% after high temperature annealing (Fig. 1). We reveal the evolution of the local structure during annealing. This work: Working with researchers at Brookhaven National Lab, we used extended x-ray absorption fine structure (EXAFS) at special energies (K-edge of Co and Fe) to study subtle structural change within the MTJ after annealing from 20 sec to 36 min. EXAFS, a special x-ray spectroscopy using intense synchrotron radiation, measures the local atomic arrangements of Co and Fe atoms with the so-called first shell Debye-Waller factor 2 , which reveals how tightly bound the atoms are within the materials. We found, as shown in Fig. 2, that as annealing commences the value of 2 dramatically decreases with a concomitant increase of the change of the resistance (TMR). 250 0.014 200 36 minutes 0.013 200 0.012 150 2 (Å ) TMR (%) 0.010 100 100 0.009 TMR(%) 0.011 150 0.008 50 0.007 50 as prepared 0.006 0 0.005 0 -150 -100 -50 0 50 100 150 Magnetic Field (Oe) Fig. 1: TMR curve of an MgO-MTJ before and after annealing at 36 min at 400 C. The inset is the TEM cross section. 400 800 1200 1600 2000 0 2400 Annealing Time (seconds) Fig. 2 First shell Debye-Waller factor 2 and TMR as a function of annealing time. A. K. Rumaiz, J. C. Woicik, W. G. Wang, J. Jordan-Sweet, G. H. Jaffari, C. Ni, J. Q. Xiao, and C. L. Chien, “Effects of annealing on the local structure of Fe and Co in CoFeB-MgO-MTJs, an EXAFS study,” Appl. Phys. Lett. 96, 1125023 (2010).