Delivered by Ingenta to: University of Houston IP : 129.7.143.47 Sun, 15 Apr 2012 14:37:24 REVIEW Copyright © 2012 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 12, 1006–1023, 2012 Interface Characterization of Epitaxial Fe/MgO/Fe Magnetic Tunnel Junctions S. G. Wang 1 2 ∗ , R. C. C. Ward 2 , T. Hesjedal 2 , X.-G. Zhang 3 , C. Wang 4 , A. Kohn 5 , Q. L. Ma 1 , Jia Zhang 1 , H. F. Liu 1 , and X. F. Han 1 1 State Key Laboratory of Magnetism, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 2 Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, UK 3 Center for Nanophase Materials Sciences and Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6493, USA 4 Department of Materials, University of Oxford, Oxford OX1 3PH, UK 5 Department of Materials Engineering, Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Following predictions by first-principles theory of a huge tunnel magnetoresistance (TMR) effect in epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs), measured magnetoresistance (MR) ratios of about 200% at room temperature (RT) have been reported in MgO-based epitaxial MTJs. Recently, a MR ratio of about 600% has been reported at RT in MgO-based MTJs prepared by magnetron sputtering, using amorphous CoFeB as the ferromagnetic electrode. These MTJs show great poten- tial for application in spintronic devices. Fully epitaxial MTJs are excellent model systems that enhance our understanding of the spin-dependent tunneling process as the interface is well defined and can be fully characterized. Both theoretical calculations and experimental results clearly indi- cate that the interfacial structure plays a crucial role in the coherent tunneling across a single crystal MgO barrier, especially in epitaxial MgO-based MTJs grown by molecular beam epitaxy (MBE). Sur- face X-ray diffraction, Auger electron spectroscopy, X-ray absorption spectra, and X-ray magnetic circular dichroism techniques have been reported previously for interface characterization. However, no consistent viewpoint has been reached on the interfacial structures (such as FeO layer formation at the bottom Fe/MgO interface), and it is still an open issue. In this article, our recent studies on the interface characterization of MgO-based epitaxial MTJs by X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and spin-dependent tunneling spectroscopy, will be presented. Keywords: Magnetic Tunnel Junctions, Magnesium Oxide, Interface Characterization, First- Principles Theory, Spin Dependent Tunneling. CONTENTS 1. Introduction ........................................ 1006 1.1. MgO-Based Magnetic Tunnel Junctions ............... 1006 1.2. Coherent Tunneling Across MgO Barrier .............. 1009 1.3. Interfacial Structures in MgO-Based MTJs ............ 1012 2. Interface Characterization ............................. 1012 2.1. Theoretical Models and Experimental Results .......... 1012 2.2. High Resolution Transmission Electron Microscopy ..... 1014 2.3. X-ray Photoelectron Spectroscopy ................... 1016 2.4. Spin Dependent Tunneling Spectroscopy .............. 1018 3. Conclusions ........................................ 1022 Acknowledgments ................................... 1022 References and Notes ................................ 1022 ∗ Author to whom correspondence should be addressed. 1. INTRODUCTION 1.1. MgO-Based Magnetic Tunnel Junctions The magnetic tunnel junction (MTJ) is a key element of next generation spintronic devices 1, 2 such as read heads in hard disk drives, magnetic random access mem- ory (MRAM), and magnetic sensors. A MTJ consist- ing of two ferromagnetic (FM) electrodes separated by a thin insulating barrier exhibits a tunnel magnetoresis- tance (TMR) effect, originating from different electrical resistances in the antiparallel (AP) and parallel (P) con- figurations of the two FM layers according to the direc- tion of an external magnetic field. The TMR effect was first studied by Julliere 3 in Fe/Ge/Co system which showed 1006 J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 2 1533-4880/2012/12/1006/018 doi:10.1166/jnn.2012.4257