Propelled by rapid advances in synthesis, characterization, and computational modeling, materials science is fast progressing toward a “materials-by-design” paradigm. While we can envision a very large number of materials combinations, we are unable to synthesize them in practice because existing characterization and modeling approaches fail to capture the inherent complexities of such systems. This shortcoming is amplified by the fundamental disconnect between highly local and volume-averaged structure-property models resulting from electron microscopy and X-ray diffraction investigations, respectively. Here we show how complementary analysis techniques can reveal previously overlooked local compositional and valence fluctuations around secondary phases in an oxide thin film, yielding powerful new insight into low-pressure thin film synthesis. We explore the model complex oxide, La2MnNiO6 (LMNO), which possesses a hierarchy of structural, chemical, and magnetic ordering across multiple length scales. This material shows great promise for next-generation spintronics and thermoelectrics, but its implementation is hindered by a poor understanding of the underlying structure that governs its macroscale magnetic performance. Using aberration-corrected scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (STEM-EDS), we confirm the onset of cation ordering upon annealing; however, three-dimensional composition mapping using atom probe tomography (APT) reveals a fine distribution of NiO secondary phases, as shown in Figure 1. Electron energy loss spectroscopy (STEM-EELS) mapping of the chemical environment surrounding these particles shows significant fine structure changes, indicating a reduction in Mn valence, as shown in Figure 2. We consider these results in light of ab initio calculations, which show that the NiO phase and a transition region with reduced Mn valence is an inevitable result of low-oxygen pressure growth processes that are commonly used in fabrication of complex oxide heterojunctions. We argue that kinetic limitations on the reincorporation of NiO nuclei “locks” them into the film structure during synthesis; the resulting nanoscale network disrupts the long-range ferromagnetic ordering of the matrix, degrading macroscale magnetic properties. This array of experimental and theoretical techniques allows us to better understand the relationship between structure and magnetic properties, illustrating the need for a correlative, multidimensional approach to thin film characterization.

Figures:

Figure 1: Complementary STEM / APT mapping of NiO secondary phase morphology and composition. (A, B) STEM-HAADF micrograph and corresponding STEM- EDS map of the Ni K peak, revealing the presence of several extended, Ni-rich regions (marked with arrows). Images taken along the [110] pseudocubic zone-axis. Images taken along the [110] pseudocubic zone-axis. (C) APT volume reconstruction of a portion of the annealed film; there is a clear distinction between the NiO regions highlighted by the 15 at % Ni isocomposition surface (green) and the LMNO matrix (colored points).

Figure 2: STEM-EELS mapping of Mn valence in the vicinity of a NiO region. (A) EEL spectrum image overlaid with regions from which the average spectra are extracted. (B) MLLS fitting of two distinct Mn valence components. Green corresponds to a spectrum extracted from the matrix (region 1) and blue to a spectrum extracted from the core (region 6). (C) Plots of the progression of the Mn L23 edge fine structure moving from the matrix (region 1) into the NiO core (region 6). After aligning the spectra to the O K edge to account for any energy drift, the spectra exhibit a significant chemical shift toward lower energy loss, indicating a reduction in Mn valence. The dashed lines have been added as a guide to the eye.

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EMC Abstracts - https://emc-proceedings.com/abstract/multidimensional-analysis-of-local-compositional-and-valence-fluctuations-in-the-model-complex-oxide-la2mnnio6/