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Low magnification (a) MAADF and (b) HAADF STEM images of the as-deposited CoFeB/Mg + MgO/CoFeB MTJ cross section. MgO crystallites with (001) texture are observed throughout the middle of the barrier layer. (c) High magnification HAADF STEM image of a MgO crystallite from the blue rectangle in (b) confirms its (001) orientation and the disorder at the interfaces above and below it. (d) Oxygen K-edge electron energy-loss-near-edge structure (ELNES) probed at three positions across the MgO barrier marked by the asterisks (the bottom CoFeB/MgO interface; in the middle where the MgO fringes are observed; and the top MgO/CoFeB/ interface), ELNES changes (including pre-edge features at ∼530 eV) at both interfaces suggest the presence of oxygen vacancies. These edges are taken from the STEM-EELS line scan across the MgO barrier with all spectra for the O K-edge region shown in (e) and the Fe L 2,3 and Co L 2,3 edge spectra shown in (f).
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XPS depth profiles of an as-deposited CoFeB/Mg + MgO bilayer: B 1s, Mg 2p, and O 1s spectral regions. The sets of arrows indicate phases identified by peak fitting the spectra and referencing against known binding energies for these phases, we assume the slight shifts in the binding energies of the peak maxima of each phase are due to differential charging. The spectra suggest that the CoFeB electrode has been reached after 12 s etching. MgO (at 50 eV) and boron oxide (at 192 eV) are detected in just the barrier layer, metallic boron (187 eV) is detected throughout the entire bilayer, intermediate metal boride phases at the MgO-CoFeB interface, and Fe and Co oxides across entire the junction. There is also an indication of intermixed MgBxOy at 531.5 eV in the O 1s spectrum at the MgO-CoFeB interface (revealed after 8 s etching).
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STEM analysis of a cross section of the as- annealed CoFeB/Mg + MgO/CoFeB MTJ: (a) a bright field image suggests a fully crystalline MgO with (001) texture across the barrier, whilst (b) a HAADF image shows clear contrast variations across the MgO barrier, a low density amorphous layer is still retained at the bottom and, to a lesser extent, the top interfaces of the barrier. (c) Oxygen K-edges probed at three positions across the junction (the bottom interface of the CoFeB-MgO, in the middle of the MgO layer, and at the top MgO-CoFeB interface) still exhibit significant changes in the edge shape and the indication of edge pre-peaks at both interfaces, confirming the presence of oxygen vacancies and defects in these interfacial layers. (d) Boron K-edge EELS line scan across the junction with the probe positions superimposed on the associated HAADF image in (e) and the boron relative areal density also overlaid by integrating the B-edge intensity over an integration window of 10 eV after background subtraction and normalization by a partial scattering cross-section.
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