Atomic resolution imaging of the strength and 3D direction of atomic displacements in functional oxide thin films

TL;DR

This paper demonstrates a novel 4DSTEM technique to map atomic displacements in thin films, revealing details about cation ordering and magnetic properties that were previously inaccessible.

Contribution

It introduces a new method for atomic resolution mapping of atomic displacements in thin films using 4DSTEM, enabling insights into interface effects and cation ordering.

Findings

La displacements mapped at ~30° to imaging plane

Displacement modulation varies with distance from interface

Method reveals interface effects on cation ordering

Abstract

Atomic resolution imaging is key to understanding thin film growth and how a particular set of conditions influences properties. Whilst such imaging in the scanning transmission electron microscope (STEM) has had transformative impact in nanoscience, it forms projection images and provides no direct information about displacements perpendicular to the image plane. In this article we show that it is possible to make atomic resolution maps of the direction and magnitude of La displacements at ~30{\deg} to the imaging plane in a La2CoMnO6 thin film on (111) LSAT (LaAlO3-La(Sr,Ta)O3) using a 4-dimensional STEM (4DSTEM) methodology. This reveals that the La modulation lies preferentially in the interface plane, and is strongly suppressed close to the epitaxial interface, and further reveals how the modulation varies with distance from the interface with unit cell resolution. These details would be completely invisible to all prior techniques in electron microscopy and this sheds light on why this particular substrate in this orientation best promotes double perovskite cation ordering, and the consequent optimal magnetic ordering for this thin film system. The approach used herein of fitting atomic resolution 4DSTEM data to determine crystal parameters opens the door for a new era of atomic-resolution crystallography.