Probing local order in multiferroics by transmission electron microscopy

Abstract

<jats:title>Abstract</jats:title> <jats:p>The ongoing trend toward miniaturization has led to an increased interest in the magnetoelectric effect, which could yield entirely new device concepts, such as electric field-controlled magnetic data storage. As a result, much work is being devoted to developing new robust room temperature (RT) multiferroic materials that combine ferromagnetism and ferroelectricity. However, the development of new multiferroic devices has proved unexpectedly challenging. Thus, a better understanding of the properties of multiferroic thin films and the relation with their microstructure is required to help drive multiferroic devices toward technological application. This review covers in a concise manner <jats:italic>advanced</jats:italic> analytical imaging methods based on (scanning) transmission electron microscopy which can potentially be used to characterize complex multiferroic materials. It consists of a first broad introduction to the topic followed by a section describing the so-called phase-contrast methods, which can be used to map the polar and magnetic order in magnetoelectric multiferroics at different spatial length scales down to atomic resolution. Section 3 is devoted to electron nanodiffraction methods. These methods allow measuring local strains, identifying crystal defects and determining crystal structures, and thus offer important possibilities for the detailed structural characterization of multiferroics in the ultrathin regime or inserted in multilayers or superlattice architectures. Thereafter, in Section 4, methods are discussed which allow for analyzing local strain, whereas in Section 5 methods are addressed which allow for measuring local polarization effects on a length scale of individual unit cells. Here, it is shown that the ferroelectric polarization can be indirectly determined from the atomic displacements measured in atomic resolution images. Finally, a brief outlook is given on newly established methods to probe the behavior of ferroelectric and magnetic domains and nanostructures during in situ heating/electrical biasing experiments. These in situ methods are just about at the launch of becoming increasingly popular, particularly in the field of magnetoelectric multiferroics, and shall contribute significantly to understanding the relationship between the domain dynamics of multiferroics and the specific microstructure of the films providing important guidance to design new devices and to predict and mitigate failures.</jats:p>

Domains and domain walls in multiferroics

“…The rapid progress that has been made in the understanding of magnetic and electric domains relies on the recent developments in microscopy techniques with high sensitivity and unprecedented spatial resolution. Nowadays, even atomic level resolution is readily available with transmission electron microscopy (TEM) (see, e.g., DOI:10.1515 /PSR.2019.0068 or [64], as well as references therein). In this review, we discuss different microscopy methods that allow for studying the formation and interaction of ferroic domains in spatially resolved measurements on nano-to microscopic length scales.…”

Ferroelectric domain walls for nanotechnology

“…Different from the neutral 109° head-to-tail walls in BiFeO3, accumulation of oxygen vacancies was reported to play the key role for enhanced electronic conduction at charged 109° walls in tail-to-tail configuration. 99,144 The accumulation is facilitated by the tensile strain present at the walls, reflecting a strong correlation between local deformations and defect accumulation, similar to the case of SrMnO3 but with opposite consequences for the local conductance.…”

“…10,97,98 At the same time, we have seen a rapid evolution in characterization techniques, allowing researchers to investigate ferroelectric domain walls in a much more systematic way and across all relevant length scales. 11 The crystallographic 99 and electronic structure 100,101 , as well as the local stoichiometry at domain walls can now be resolved with pico-meter precision and surface-analysis techniques provide access to the domain wall dynamics, local electrostatics and electronic transport. 11 This level of accuracy and the concerted application of complementary state-of-the-art imaging techniques is crucial for moving beyond the utilization of only intrinsic conduction phenomena and benefit from the additional degrees of freedom that arise from extrinsic contributions and the interactions of domain walls with point defects in general.…”

11 Domains and domain walls in multiferroics

“…Nowadays, even atomic level resolution is readily available with transmission electron microscopy (TEM) (see, e.g., DOI:10.1515 /PSR.2019.0068 and references therein (ref. [70]). In this review, we discuss different microscopy methods that allow for studying the formation and interaction of ferroic domains in spatially resolved measurements on nano-to microscopic length scales.…”