VEELS investigation of perovskite manganite interfaces

Abstract

As promising candidates for magnetic storage and magnetic field sensing applications at room temperature Sr‐doped LaMnO 3 (LSM) perovskite manganites show a very large negative magnetoresistance [1]. These materials exhibit a colossal magnetoresistance (CMR) which is very sensitive to the behaviour of the interface, due to the lattice misfit between substrate and thin film, and hence to the induced strain. The investigated LSM thin film synthesised from La 0.8 Sr 0.2 MnO 3 powder was prepared by pulsed laser deposition (PLD). The 140 nm thick LSM layer was epitaxially grown on a single crystalline LaAlO 3 (LAO) (100) substrate. The preparation of a cross section specimen for TEM analysis was done by focused ion beam (FIB) followed by subsequently Ar + ion polishing. The analytical TEM investigations were performed by using a FEI TECNAI G20 TEM equipped with a Gatan GIF 2001 energy filter and a field‐emission FEI TECNAI F20 TEM equipped with a Gatan GIF Tridiem energy filter. In this work, we will present a study of the microstructure as well as the optical and electronic properties of LaSrMnO 3 ‐LaAlO 3 interfaces at different temperatures and operating voltages. High‐resolution (HR) TEM, high‐angle annular dark field (HAADF) STEM and analytical methods, such as valence electron energy loss spectrometry (VEELS) and electron magnetic chiral dichroism (EMCD) were applied. The bright field (Fig. 1B) and the dark field (Fig. 1C) image convey the impression of a non‐uniform distribution in the LSM layer. This fact is confirmed by means of the HAADF image in Fig. 1A. A columnar structure is distinguished in the thin film. In addition, spot splitting perpendicular to the interface occurs from the lattice mismatch as shown in the Fourier transform (FT) (Fig. 1D) of the HRTEM image (Fig. 1E) recorded at the interface. It was observed that the columnar growth mechanism starts directly at the interface within a heavily strained region as reported in [2]. The EELS investigations of the LaSrMnO 3 ‐LaAlO 3 interface are shown in Fig. 2. In the case of determining optical properties, high beam energy alters the valence EELS (VEELS) spectrum by exciting retardation losses and therefore, we reduced the operation voltage in order to eliminate these effects [3]. The Cerenkov contribution is observed at 200 keV in LAO and LSM in Fig. 2, while these losses are vanished at lower beam energy. The EELS spectrum after zero‐loss peak (ZLP) removal is shown for LAO in Fig. 2A and for LSM in Fig. 2B at 40 keV and 200 keV, respectively. A pre‐measured zero‐loss peak was used for the ZLP removal of the VEELS signal. However, some artefacts at the bandgap can be seen in the insertion above in Fig. 2A. The VEELS spectrum image in Fig. 2C exhibits the bandgap transition at the LaSrMnO 3 ‐LaAlO 3 interface with a beam energy at 60 keV.