Energy Loss Near-Edge Structures [chapter]

Guillaume Radtke, Gianluigi A. Botton
2010 Scanning Transmission Electron Microscopy  
The growth of three dimensional spintronic structures has been proposed as a promising route to overcome the limits of current data storage technologies. Among the potential candidates, selfassembled ferromagnetic nanowires (NWs) embedded in an insulating matrix have recently attracted considerable attention (e.g. Ni in SrTiO3, see Fig. 1 ). This interest is mainly motivated by the possibility to tune the magnetic anisotropy either through the control of the chemical composition of the NWs or
more » ... ion of the NWs or through the presence of interfacially-induced strain giving rise to magneto-elastic anisotropy [1]. In particular, the detection of Ni oxidation, which would be highly detrimental, via the potential presence of a NiO layer between the NW and the matrix, is of interest. This is achieved by investigating the Ni-L23 near-edge structures (ELNES), which show substantial differences between metallic Ni and Ni 2+ [2], and the O-K and Ti-L23 ELNES from the matrix to the Ni:SrTiO3 interface [3]. In this work, the local chemistry of the Ni:SrTiO3 interface is probed at the atomic scale in the aberration-corrected scanning transmission electron microscope (STEM). High energy resolution for ELNES (~160 meV), and sub-Å spatial resolution were used in a monochromated Cs-corrected Nion HERMES, operating at 60 kV. The main results are summarized in Fig. 2 . The series of spectra extracted from region P1, on a TiO2 atomic plane located one matrix unit cell (~3.9 Å) from the interface, still displays all the characteristic features of the bulk electronic structure of SrTiO3 without any detectable Ni. Spectra from region P2, located on the last atomic plane of the matrix from the HAADF contrast, are largely modified. The symmetry breaking induced by the presence of the interface, and the pronounced covalent character of the bonding with neighboring Ni atoms lead to strong broadening of the Ti-L23 ELNES, where the t2g-eg splitting is barely resolved. The O-K ELNES still exhibits the signature of hybridization with Ti-3d states through the presence of a sharp peak a, and a broader structure b. Interestingly, the missing SrO plane on the left side of the interface leads to a considerable loss of spectral weight in the region of the double structure c, primarily associated with the Sr-4d empty states. Although a weak Ni signal is already detected in this region, which might be partly related to the delocalization inherent to atomicresolved EELS, the presence of Ni in the last atomic planes of the matrix cannot be ruled out. The modification of the Ti-L23 ELNES is particularly abrupt along this [100]STO||[100]Ni interface, compared to the [110]STO||[110]Ni facets. Region P3 corresponds to the first atomic plane that can be considered structurally part of the nanowire from the HAADF contrast. The Ti-L23 edge in this area is also further broadened to reach a shape that closely resembles that of the Ti-L23 edge in bulk Ti or NiTi alloy. In region P4, the Ni-L23 edge is essentially that of bulk Ni, while very weak Ti and O signals are still detectable. Finally, regions P5 and P6 are characteristic of pure bulk Ni. The Ni-L3 to L2 ratios show no substantial deviation from the bulk Ni value in this series of spectra A-E. This is consistent with the O-K ELNES at the interface, which does not display any characteristic features of NiO. 664
doi:10.1007/978-1-4419-7200-2_5 fatcat:yb3i77ay5fgx3nutby2bo2pbny