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.

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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/