University of Alabama MRSEC William H. Butler DMR Theory of Tunneling Magnetoresistance Leads to New Discoveries with Potential Technological Impact If two metallic electrodes are separated by a thin insulating barrier, and a small voltage is applied between the electrodes, quantum mechanical tunneling of electrons can occur between the electrodes through the barrier. If the two metallic electrodes are made of ferromagnetic metals, a phenomenon called tunneling magnetoresistance (TMR) can occur in which the current of tunneling electrons depends on the magnetic configuration of the electrodes. The TMR phenomenon has taken on increased importance in recent years as scientists and engineers have searched for ways to replace the volatile dynamic random access memory currently in use with a memory that is equally fast and dense but which can remember the stored information without using electrical power. Memory chips called magnetic random access memory (MRAM) have been developed which utilize the relative direction of the magnetic moments in two magnetic layers to store a bit of information. TMR is used to “read” the bit by determining if the magnetic moments of the electrodes are parallel or anti-parallel. Another important application of TMR is as the read sensor for a disk drive. In order to make MRAM and TMR read sensors a reality, a very large change in the resistance between parallel and anti-parallel is desirable. In 2001, a new theory of the TMR effect was developed [1] which pointed out that very high values of TMR could be obtained by taking advantage of the differences in the spatial symmetry of the wave functions in the electrodes associated with electrons spinning in opposite directions. The theory was further developed [2-4]. Very recently, this theory has been confirmed by the discovery that the materials combinations predicted to show TMR [5-7] in fact do. The figure shows the experimental results obtained by Yuasa et al. [6] for the tunneling resistance as a function of thickness for crystalline Fe(100)-MgO(100)-Fe(100) films. The resistance change between aligned moments and anti- aligned moments was nearly 200% at room temperature and even larger at low temperature. The figure also shows the values of the resistance calculated without adjustable parameters years before the measurement. The agreement is remarkable given the exponential dependences of the resistance on thickness and other details of the calculations Product of magnetic tunnel junction resistance and area as a function of barrier thickness. The large green circles are the calculated values for parallel alignment of the moments of the electrodes. The open circles are the measured values. References: [1] W. H. Butler, X.-G. Zhang, T. C. Schulthess, and J. M. MacLaren, “Spin Dependent Tunneling Conductance of Fe|MgO|Fe Sandwiches,” Physical Review B 63, (2001). [2] W. H. Butler, X.-G. Zhang, T. C. Schulthess, and J. M. MacLaren, “Reduction of Electron Tunneling Current Due to Lateral Variation of the Wave Function,” Physical Review B 63, (2001). [3] X.-G. Zhang and W. H. Butler, “Band structure, evanescent states and transport in spin tunnel junctions”, J. Phys. Condens. Mater. 15 (2003) R1-R37 [4]X.-G. Zhang and W. H. Butler “Large magnetoresistance in bcc Co/MgO/Co and FeCo/MgO/FeCo tunneling junctions” Phys. Rev. B 70, (2004). [5] W. H. Butler and A. Gupta, “Magnetic Memory: A signal boost is in order” Nature Materials 3, 845–847 (2004). [6] Shinji Yuasa, Taro Nagahama, Akio Fukushima, Yoshishige Suzuki, Koji Ando, “Giant roomtemperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions” Nature Materials 3, (01 Dec 2004) [7] Stuart S. P. Parkin, Christian Kaiser, Alex Panchula, Philip M. Rice, Brian Hughes, Mahesh Samant, See-Hun Yang, “Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers”, Nature Materials 3, (01 Dec 2004).