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

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.

Figure 1

HAADF-STEM image showing an overview of the MTJ multilayer structure.

Figure 2

(Color online) (a) High-resolution STEM image of the interface between the bottom Fe and MgO substrate, obtained along the MgO [110] direction; (b) two line scans of the intensity of the HAADF signal obtained from atomic columns along the directions denoted schematically by arrows A and B. Inset: the atomic arrangement of the Fe and MgO layers.

Figure 3

A HRTEM image from the bottom Fe electrode and MgO barrier. The zone axis is MgO [100]. The interface in region A is coherent while a dislocation is observed in region B. The two insets are images reconstructed from the reflections of MgO (100) and Fe (110) in power spectra calculated from regions A and B, respectively.

Figure 4

(Color online) (a, center) A HRTEM image from a focal series recorded at near ${\text{C}}_5$ (fifth-order aberration) balanced conditions, Gaussian defocus, compared with simulated images of 6-nm-thick samples: (a, right) sharp interface and (a, left) oxidized interface. The inset shows the atomic arrangement at the interface. (b) An intensity scan along the first MgO layer above the bottom Fe electrode over a width of 0.1 nm from the region denoted by a box in (a).

Figure 5

(Color online) Structural input for HRTEM multislice image simulations in the case of (a) an oxidized interface with an FeO layer at the Fe and MgO interface; and (b) a sharp Fe/MgO interface. The interplanar distance at the interface is 0.235 nm for the oxidized model (Ref. 24) and 0.220 nm for the sharp interface (Ref. 31).

Figure 6

(Color online) (a) Reconstructed phase of the exit wave function from a through-focal series of HRTEM images in the JEOL 2200FS with Cs at $-5\mu \text{m}$. (b) An intensity scan across the MgO barrier region denoted by arrow A, (c) an in-plane intensity scan from the first Fe layer denoted by arrow B, and (d) an intensity scan along the growth direction denoted by arrow C. The width of the line scan is 0.08 nm. The zone axis is MgO [110]. Note: the term intensity refers to the reconstructed phase.

Figure 7

$Z$-contrast HAADF-STEM image from the MgO barrier. (The position of steps at the top interface are denoted by arrows.) The zone axis is MgO [110].

Figure 8

(Color online) (a) $Z$-contrast STEM image from the barrier region; (b) the atomic ratio of O and Fe across the MgO and Fe layers derived from EEL spectra measured along the direction denoted schematically by the rectangle shown in (a). The inset is a spectrum from the middle of the MgO barrier apparently showing a large Fe content (c) the composition profiles for O and Fe derived from the EEL spectra data of (b) and applying MSA.