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Register a new userEmergent chirality in a polar meron to skyrmion phase transition
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Polar skyrmions are theoretically predicted to emerge resulting from the interplay of elastic, electrostatic and gradient energies, in contrast to the key role of the anti-symmetric Dzyalozhinskii-Moriya interaction in magnetic skyrmions. With the discovery of topologically stable polar skyrmions reported by Das et al., (Nature 568, 368, 2019), it is of both fundamental and practical interest to understand the microscopic nature and the possibility of temperature- and strain-driven phase transitions in ensembles of such polar skyrmions. Here, we explore the emergence of a two-dimensional, tetratic lattice of merons (with topological charge of +1/2) from a skyrmion state (topological charge of +1) upon varying the temperature and elastic boundary conditions in [(PbTiO$_3$)$_{16}$/(SrTiO$_3$)$_{16}$]$_8$ lifted-off membranes. Such a topological phase transition is accompanied by a change in chirality, e.g. from left-handed to zero-net chirality, as measured by four-dimensional scanning transmission electron microscopy (4D-STEM). We show how 4D-STEM provides a robust measure of the local polarization simultaneously with the strain state at sub-nm resolution, while directly revealing the origins of chirality in each skyrmion. Using this, we demonstrate strain as a crucial order parameter to drive isotropic-to-anisotropic structural transitions of chiral polar skyrmions to non-chiral merons, validated with X-ray reciprocal space mapping and theoretical phase-field simulations. These results provide the first illustration of systematic control of rich variety of topological dipole textures by altering the mechanical boundary conditions, which may offer a promising way to control their functionalities in ferroelectric nanodevices using the local and spatial distribution of chirality and order for potential applications.
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Room-temperature polar skyrmion bubbles that are recently found in oxide superlattice, have received enormous interests for their potential applications in nanoelectronics due to the nanometer size, emergent chirality, and negative capacitance. For practical applications, the ability to controllably manipulate them by using external stimuli is prerequisite. Here, we study the dynamics of individual polar skyrmion bubbles at the nanoscale by using in situ biasing in a scanning transmission electron microscope. The reversible electric field-driven phase transition between topological and trivial polar states are demonstrated. We create, erase and monitor the shrinkage and expansion of individual polar skyrmions. We find that their transition behaviors are substantially different from that of magnetic analogue. The underlying mechanism is discussed by combing with the phase-field simulations. The controllable manipulation of nanoscale polar skyrmions allows us to tune the dielectric permittivity at atomic scale and detailed knowledge of their phase transition behaviors provides fundamentals for their applications in nanoelectronics.
Chirality, an intrinsic handedness, is one of the most intriguing fundamental phenomena in nature. Materials composed of chiral molecules find broad applications in areas ranging from nonlinear optics and spintronics to biology and pharmaceuticals. However, chirality is usually an invariable inherent property of a given material that cannot be easily changed at will. Here, we demonstrate that ferroelectric nanodots support skyrmions the chirality of which can be controlled and switched. We devise protocols for realizing control and efficient manipulations of the different types of skyrmions. Our findings open the route for controlled chirality with potential applications in ferroelectric-based information technologies.
A magnetic skyrmion is a topological object that can exist as a solitary embedded in the vast ferromagnetic phase, or coexists with a group of its siblings in various stripy phases as well as skyrmion crystals (SkXs). Isolated skyrmions and skyrmions in an SkX are circular while a skyrmion in other phases is a stripe of various forms. Unexpectedly, the sizes of the three different types of skyrmions depend on material parameters differently. For chiral magnetic films with exchange stiffness constant $A$, the Dzyaloshinskii-Moriya interaction (DMI) strength $D$, and perpendicular magnetic anisotropy $K$, $kappaequivpi^2D^2/(16AK)=1$ separates isolated skyrmions from condensed skyrmion states. In contrast to isolated skyrmions whose size increases with $D/K$ and is insensitive to $kappall1$ and stripe skyrmions whose width increases with $A/D$ and is insensitive to $kappagg1$, the size of skyrmions in SkXs is inversely proportional to the square root of skyrmion number density and decreases with $A/D$. This finding has important implications in our search for stable smaller skyrmions at the room temperature in applications.
We report spin-current generation related with skyrmion dynamics resonantly excited by a microwave in a helimagnetic insulator $mathrm{Cu_2OSeO_3}$. A Pt layer was fabricated on $mathrm{Cu_2OSeO_3}$ and voltage in the Pt layer was measured upon magnetic resonance of $mathrm{Cu_2OSeO_3}$ to electrically detect injected spin currents via the inverse spin Hall effect (ISHE) in Pt. We found that ISHE-induced electromotive forces appear in the skyrmion phase of $mathrm{Cu_2OSeO_3}$ as well as in the ferrimagnetic phase, which shows that magnetic skyrmions can contribute to the spin pumping effect.
In this work, the current-induced inertial effects on skyrmions hosted in ferromagnetic systems are studied. {When the dynamics is considered beyond the particle-like description, magnetic skyrmions can deform due to a self-induced field. We perform Monte Carlo simulations to characterize the deformation of the skyrmion during its movement}. In the low-velocity regime, the deformation in the skyrmion shape is quantified by an effective inertial mass, which is related to the dissipative force. When skyrmions move faster, the large self-induced deformation triggers topological transitions. The transition is characterized by the proliferation of skyrmions and different total topological charge, which are obtained in terms of the skyrmion velocity. Our findings provide an alternative way to describe the skyrmion dynamics that take into account the deformations of its structure. Furthermore, the motion-induced topological phase transition brings the possibility to control the number of ferromagnetic skyrmions by velocity effects.
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