Dynamics of Polar Skyrmion Bubbles under Electric Fields

Room-temperature polar skyrmions, which have been recently discovered in oxide superlattice, have received considerable attention for their potential applications in nanoelectronics owing to their nanometer size, emergent chirality, and negative capacitance. For practical applications, their manipulation using external stimuli is a prerequisite. Herein, we study the dynamics of individual polar skyrmions at the nanoscale via in situ scanning transmission electron microscopy. By monitoring the electric-field-driven creation, annihilation, shrinkage, and expansion of topological structures in real space, we demonstrate the reversible transformation among skyrmion bubbles, elongated skyrmions, and monodomains. The underlying mechanism and interactions are discussed in conjunction with phase-field simulations. The electrical manipulation of nanoscale polar skyrmions allows the tuning of their dielectric permittivity at the atomic scale, and the detailed knowledge of their phase transition behaviors provides fundamentals for their applications in nanoelectronics.

Figure 1

Observation of mixed skyrmion bubbles and elongated skyrmions. (a) Cross-sectional MAADF image of the ${({\mathrm{SrTiO}}_3)}_{11}/{({\mathrm{PbTiO}}_3)}_{22}/{({\mathrm{SrTiO}}_3)}_{11}$ trilayer sample, and (b) a dark-field TEM image, displaying an alternating high and low diffraction contrast within the PTO layer. (c) Antiparallel polarization distribution of the cross-sectional view from phase-field simulations (denoted by white arrows). The background color mapping illustrates the polarization orientation. (d) Planar-view MAADF-STEM image showing the coexistence of skyrmion bubbles and elongated skyrmions. For these domains, dark regions possessing out-of-plane polarization are separated by bright domain walls with in-plane polarization. (e) Low-magnification and (f) enlarged view of the white dashed box color mappings constructed from phase-field simulations, depicting the in-plane polarization distribution with hedgehog-like features. The background color saturation is proportional to the in-plane polarization magnitude; i.e., in the dark regions, in-plane polarization is basically null while the out-of-plane polarization is dominant, representing the core or periphery of polar skyrmions. The symbols circle cross and circle dot denote downward and upward polarizations, respectively.

Figure 2

Electrical switching dynamics of polar skyrmions in a trilayer. (a) Schematic showing the experimental setup of in situ STEM. A tungsten tip and SRO conductive layer are used as electrodes to apply an out-of-plane electric field to the film. (b) Selected snapshots showing the evolution of elongated skyrmions and skyrmion bubbles under positive and negative bias loading periods. The threshold electric field is $\sim 5682\mathrm{kV}/\mathrm{cm}$ (10 V). Purple dashed lines outline the characteristic domains. Red circles indicate the drift correction positions. Each frame is processed using principal component analysis for better visualization.

Figure 3

Evolution of the coexisting structure in the trilayer under an external electric field from phase-field simulations. The first five frames display the topological phase transition from polar skyrmions to the trivial monodomain that is induced by an out-of-plane electric field. A uniform saturated monodomain forms under a critical voltage of $U_{[001]}=12.6\mathrm{V}$ ($E_{[001]}=7159\mathrm{kV}/\mathrm{cm}$). The sixth frame displays the relaxation process after the removal of the external field. White dashed boxes highlight domains with fracture characteristics.

Figure 4

Electrical switching dynamics of polar skyrmions in a superlattice. (a) Cross-sectional and (b) planar-view MAADF-STEM images of the superlattice ${[{({\mathrm{SrTiO}}_3)}_{11}/{({\mathrm{PbTiO}}_3)}_{22}/{({\mathrm{SrTiO}}_3)}_{11}]}_6$. The enlarged view of the blue dashed box in (b) is placed in the upper left corner, exhibiting an edge dislocation with $\mathbf{a}[100]$ burgers vector. (c) Evolution of skyrmion bubbles under a positive bias loading along the [001] zone axis. Threshold electric field is about $\sim 3409\mathrm{kV}/\mathrm{cm}$ (6 V). For better visualization, the background in images 1–4 is removed (see Fig. S9 for original images [41]). The characteristic polar skyrmions during the shrinkage process are indicated by orange and yellow arrows.