Structural characterization of interfaces in epitaxial Fe/MgO/Fe magnetic tunnel junctions by transmission electron microscopy

Abstract

We present a detailed structural characterization of the interfaces in Fe/MgO/Fe layers grown by molecularbeam epitaxy using aberration-corrected transmission electron microscopy ͑TEM͒, 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 K͒, 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.

Evidence for boron diffusion into sub-stoichiometric MgO (001) barriers in CoFeB/MgO-based magnetic tunnel junctions

“…Clearly, the structure of the interfaces is crucial for the TMR performance and a particularly sensitive probe of structure is STEM high angle annular dark field (HAADF) imaging (in which the image contrast and intensity is related to the atomic column orientation [channeling] and atomic number of the elements within the specimen [Z-contrast]). 16 A significant outcome of the present work is that HAADF-STEM imaging appears to be more sensitive to the presence of interfacial disorder than phase contrast (S)TEM and this disorder may have important implications for the mobility of boron in such structures.…”

In situsmall-angle x-ray and nuclear resonant scattering study of the evolution of structural and magnetic properties of an Fe thin film on MgO (001)

“…Interfacial imperfections in terms of possible oxidation of transition metal in the interfacial region or oxygen vacancies, etc., are suggested to be possible reasons for deterioration in TMR value [3][4][5]. This has led to extensive study in the literature on characterization of interfaces in MgO based MTJs [6][7][8][9][10][11]. However, understanding of various interfacial imperfections and their correlation with magnetic and magneto-transport properties of magnetic electrodes is still far from satisfactory.…”

Enhanced magnetoresistance by monoatomic roughness in epitaxial Fe/MgO/Fe tunnel junctions

“…Concerning the epitaxial Fe(001)/MgO MTJs, the impact of chemical and structural defects has been experimentally checked. It has been shown that volume defects in the barrier as misfit dislocations are detrimental for TMR [13][14][15][16]. Surprisingly, controlled contamination of the interface with carbon [17,18], or oxygen has been found to have small impact on TMR values (at zero voltage) [19,20].…”