Structural characterization of interfaces in epitaxial Fe/MgO/Fe magnetic tunnel junctions by transmission electron microscopy C. Wang, 1 A. Kohn, 2, * ,† S. G. Wang, 3,4 L. Y. Chang, 1 S.-Y. Choi, 1 A. I. Kirkland, 1 A. K. Petford-Long, 5 and R. C. C. Ward 3, * ,‡ 1 Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom 2 Department of Materials Engineering, Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel 3 Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom 4 State Key Laboratory of Magnetism, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 5 Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, USA Received 4 May 2010; revised manuscript received 23 June 2010; published 28 July 2010 We present a detailed structural characterization of the interfaces in Fe/MgO/Fe layers grown by molecular- beam epitaxy using aberration-corrected transmission electron microscopy TEM, scanning TEM, and electron energy-loss spectroscopy. When fabricated into magnetic tunnel junctions, these epitaxial devices exhibit large tunnel magnetoresistance ratios e.g., 318% at 10 K, though still considerably lower than the values predicted theoretically. The reason for this discrepancy is being debated and has been attributed to the structure of, and defects at the interface, namely, the relative position of the atoms, interface oxidation, strain, and structural asymmetry of the interfaces. In this structural study, we observed that Fe is bound to O at the interfaces. The interfaces are semicoherent and mostly sharp with a minor degree of oxidation. A comparison of the two interfaces shows that the top MgO/Fe interface is rougher. DOI: 10.1103/PhysRevB.82.024428 PACS numbers: 87.64.Ee, 75.70.Cn, 72.25.-b I. INTRODUCTION Following theoretical predictions 1,2 of tunneling magne- toresistance TMR in epitaxial Fe/MgO/Fe junctions, TMR ratios of approximately 200% were measured at room tem- perature for CoFe/MgO/CoFe and fully epitaxially Fe/ MgO/Fe junctions. 3–8 Significant progress has been since achieved with sputter-deposited CoFeB/MgO/CoFeB mag- netic tunnel junctions MTJs in which the CoFeB ferromag- netic electrode is amorphous. 9–11 For these MTJ, TMR ratios of 604% at room temperature have been reported, 9 which is of interest for technological applications such as in magnetic random access memory and magnetic sensors. However, these values, and especially the TMR ratio of the model ep- itaxial Fe/MgO/Fe MTJ are still considerably lower than the predictions based on first-principles calculations. 1,2 Conse- quently, revealing the physical origin of this discrepancy may further contribute to understanding the large TMR ratio in MgO-based MTJ, as well as associated experimental ob- servations such as asymmetric bias voltage dependence. 4 Calculations 12 have shown that the TMR value is signifi- cantly reduced if the interfaces are oxidized though recent experimental work shows that the effect of oxidation may not be as significant as expected by those calculations 13 but rather the degree of strain. 14 The asymmetry and decrease in the TMR ratio have been attributed to interface phenomena: dislocations, 15 electronic structure of the Fe/MgO interface, 5 and the formation of an Fe-O layer. 12,16,17 Therefore, to better understand their role on electron tunneling, these interfaces have been characterized by surface x-ray diffraction, Auger electron spectroscopy, x-ray absorption spectra, x-ray mag- netic circular dichroism, x-ray photoelectron spectroscopy, spin-dependent tunneling spectroscopy, and transmission electron microscopy TEM. 12–20 In this work, we report on a detailed structural character- ization of epitaxial Fe/MgO/Fe layers, which when fabri- cated to a MTJ, achieved a TMR value of 170% at room temperature. 6,16 Although fully epitaxial structures will prob- ably not be used in commercial devices, they are model sys- tems to compare experimental results and theoretical calcu- lations and to study spin-polarized coherent tunneling. 4–7 In particular, the aim of this work is to characterize the interface structure of epitaxial Fe/MgO/Fe multilayers, which when fabricated into MTJ devices, demonstrate among the highest TMR ratios to date. To achieve this aim, we use aberration-corrected TEM and scanning TEM STEM. Revealing the atomic structure of the Fe/MgO interface is important because this information is the basis for theoretical calculations. 1,2 Experimental characterization of the interfa- cial structure that has been used as input for such calcula- tions was undertaken by in situ measurements. Urano and Kanaji 20 reported that Fe atoms are adjacent to O ions at the Fe/MgO 001 interface after the first monolayer growth, as measured by low-energy electron diffraction. However, such in situ characterization during growth does not account for structural alteration that may occur after completing the fab- rication of the device, for example, due to strain relaxation. Therefore, an ex situ measurement such as TEM, can be advantageous in characterizing the atomic structure of the actual device. Here, we have investigated the atomic struc- ture across the interface, namely, how the Fe atomic columns are positioned with respect to the Mg or O columns. This was studied by recording high angle annular dark field HAADF-STEM images in which the contrast is related to the atomic number. 21 As the atomic number of Fe is consid- erably larger than that of Mg or O, a HAADF-STEM image PHYSICAL REVIEW B 82, 024428 2010 1098-0121/2010/822/0244289 ©2010 The American Physical Society 024428-1