Configurable topological textures in strain graded ferroelectric nanoplates

Abstract

The scanning probe microscope is a surface measuring instrument with an atomic resolution, which is widely used in exploring surface physical properties such as morphology, electric, electronic, magnetic, thermal, and optical properties. The scanning probe microscope measures various physical quantities of local sample area underneath a tip with the typical apex curvature of several nanometers. In this dissertation, three major topics regarding configurable topological textures and electronic conduction in strain graded ferroelectrics and magnetostriction measurement in a ferromagnetic film are addressed. First, ferroelectric domain textures are analyzed in terms of topology. In mathematics, topology is concerned about space properties that are conserved by continuous deformations. Topological isomorphism means two objects are transformed to each other without changing the topological number by stretching, wrinkling, and bending. The concept of topology has been used in various fields. In solid state physics, topology is related to various properties of materials such as electric conductivity, spin textures, and optical transmission. Topological defects are singular points where order parameters are discontinuous such as domain walls or vortex points. Recently, topological defects have attracted much attention for applications of the high-density information storage medium because vortex cores and skyrmions have a few nanometer sizes. In the case of magnetic materials, many theoretical and experimental studies have been made. Although theoretical studies on ferroelectric materials have reported that topological defects can be stabilized in small sizes and controlled by external stimuli with a smaller energy compared to ferromagnetic ones, experimental observations of vortex structures are relatively little progressed in ferroelectric materials. In order to make a topological vortex in a ferroelectric material, a curing structure of ferroelectric polarization is demanded leading to a considerable energy consumption in deforming the lattice. It is the reason why ferroelectric vortex structures are rarely observed in usual cases. In this dissertation, an inhomogeneous strain field associated with misfit strain relaxation is introduced to stabilize topologically non-trivial textures in ferroelectrics. The ferroelectric vortex structures are emergent in ferroelectric nanoplates which undergo a strong compressive strain at the bottom interface while the side and top surfaces are stress-free. In this nanoplate structure, a radial compressive strain relaxation occurs exhibiting a quadrant in-plane ferroelectric domain texture as well as stabilizing a ferroelectric vortex structure. It is directly observed by using angle-resolved piezoresponse force microscopy and local winding number analysis. The topological invariant of the nanoplate can be configured from −1 to 3 by selective non-local domain switching. These findings offer a useful concept for the multi-level topological defect memory. Electrical conduction in the ferroelectric nanoplates is also investigated. Controllable conduction level has been suggested by arranging two opponent polarizations in a head to head (or tail to tail) configuration. However, the charged domain walls in ferroelectric materials are rarely observed due to the electrostatically unstable feature. The ferroelectric nanoplate provides a promising opportunity into stabilization and manipulation of electronic conduction by their highly anisotropic mechanical boundary condition. Electronic conduction near the sidewalls is reversibly adjustable up to about 20 times by $180^Wcirc$ ferroelectric switching that is ferroelastically protected. The findings provide a pathway into electronic conduction modulation applicable to nanoscale ferroelectric logic devices. Finally, a new conceptual magnetostriction force microscopy is introduced. Magnetostriction is a property whereby the volume of the material changes in response to an external magnetic field due to the reorientation of magnetization. Magnetostriction property has been applied to ultrasonic cleaning, fishfinder, water depth meter, pressure gauge, vibration meter. The local magnetostriction measurement has become an emerging issue because strain-mediated nanocomposites have received considerable attention due to their potential applications for high sensitivity sensors and high density energy harvesters. Here, magnetostriction measurement based on scanning probe microscopy on a cobalt thin film is reported. The distribution of magnetostrictive response is mapped by atomic force microscopy in contact mode with a few nanometer lateral resolution and lock-in detection of second harmonic sample vibration in response to oscillating magnetic field. The phase and amplitude signals observed in each domain are interpreted in aspect of the magnetic easy axis. Our findings provide a specific pathway to understand the local magnetostrictive response based on scanning probe microscopy.