Abstract
Recent studies carried out with atomic force microscopy or high-resolution transmission electron microscopy reveal that ferroic domain walls can exhibit different physical properties than the bulk of the domains, such as enhanced conductivity in insulators, or polar properties in non-polar materials. In this review we show that optical techniques, in spite of the diffraction limit, also provide key insights into the structure and physical properties of ferroelectric and ferroelastic domain walls. We give an overview of the uses, specificities and limits of these techniques, and emphasize the properties of the domain walls that they can probe. We then highlight some open questions of the physics of domain walls that could benefit from their use.
Highlights
Introduction and scopeFerroic materials are defined by their ability to exist in several possible states, or domains, that can be controlled and switched by an external field
We provide an overview of the uses of optical techniques for the characterization of the physical and structural properties of ferroelectric and ferroelastic domain walls
It is quite clear that optical techniques in general can rather image domain walls in ferroic materials, in spite of their small size
Summary
Ferroic materials are defined by their ability to exist in several possible states, or domains, that can be controlled and switched by an external field. Following a seminal work in the late 1990s [3], the experimental characterization of specific domain wall properties has enjoyed a renewed interest [4–7], with unusual conductivity in insulators [8–15], or polar properties in non-polar materials [16–25]. We provide an overview of the uses of optical techniques for the characterization of the physical and structural properties of ferroelectric and ferroelastic domain walls. This review does not cover techniques where visible light is used for excitation only, while the imaging itself relies on a different signal This is the case, for example, for atomic force microscopy measurements under illumination, which have revealed the increase of the electrical conductivity of domain walls under UV light in lithium niobate (LiNbO3) [28, 29], or the enhancement of local photovoltaic and photoconductive properties of domain walls in bismuth ferrite (BiFeO3) thin films [30]. This is because the physics of ferromagnetism is very different from ferroelectricity and ferroelasticity, in particular due to the existence of magnetic exchange interactions which have no analog in dielectrics (for a review on magneto-optical microscopy, readers might refer to [43])
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