Vanadium dioxide (VO₂) is a typical metal-insulator transition (MIT) material, which changes from room-temperature monoclinic insulating phase to high-temperature rutile metallic phase. The phase transition of VO₂ is accompanied by sudden changes in conductance and optical transmittance. Due to the excellent phase transition characteristics of VO₂, it has been widely studied in the applications of electric and optical devices, smart windows, sensors, actuators, etc. In this review, we provide a summary about several phases of VO₂ and their corresponding structural features, the typical fabrication methods of VO₂ nanostructures (e.g., thin film and low-dimensional structures (LDSs)) and the properties and related applications of VO₂. In addition, the challenges and opportunities for VO₂ in future studies and applications are also discussed.

While strain gradients break lattice centrosymmetry, ferromagnetism is a time-reversal symmetry breaking product. Flexomagnetic effect in ferromagnets is usually indirect and weak. In this Letter, we reveal a topologically enhanced flexomagnetic effect in synthetic antiferromagnetic systems based on Dzyaloshinskii-Moriya interaction and the large deformability of skyrmion. Moreover, the synthetic antiferromagnetic skyrmion exhibits an unexpected Hall effect under strain gradient. We propose that this flexo-Hall effect originates from a geometric Magnus force related to the asymmetric deformation of skyrmion. Our results shed new insights into the flexoresponses in systems hosting topological structures and may open up a new field—“flexoskyrmionics”.

While magnetic skyrmions are often modeled as rigid particles, both experiments and micromagnetic simulations indicate their easy-to-deform characteristic, especially when their motion is restricted by defects. Here we establish a theoretical framework for the dynamics of magnetic skyrmions by incorporating the degrees of freedom related to deformation and predict well the current-driven dynamics of deformable skyrmions in the presence of line defects without any parameter fitting, where classical theories based on rigid-particle assumption deviate significantly. Further, we define an emergent property of magnetic skyrmions—flexibility and show that this property strongly modulates the depinning dynamics of skyrmions along a line defect with breaches. Our work explores the emergent mechanics of magnetic skyrmions and extends the current understanding on the dynamics of skyrmions interacted with defects.

Combining micromagnetic simulation and analytical modeling, we perform an in-depth study on the correlation between current-driven transport characteristics of individual skyrmions and deformation under the influences of Dzyaloshinskii-Moriya interaction, perpendicular magnetic anisotropy, temperature, magnetic fields, and electrical currents. How the system parameters affect the configuration of skyrmions and further influence their transport characteristics (e.g., speed, Hall angle, and mobility) is revealed and dynamic “phase diagrams” of skyrmions are presented. Generally, the mobility of a skyrmion increases as its size increases, and the shapes and cores of skyrmions play important roles in their transport behavior. Beyond the small-driving-current regime, a remarkable deformation of skyrmions with expanding size and noncircular shape occur during transport. This leads to an increase of the driving forces and skyrmion mobility, manifesting a nonlinear feature of the skyrmion speed-current curve. Conversely, an increase of the driving forces causes a larger deformation of skyrmions and forms a positive feedback mechanism. Once the current exceeds a critical value, the driving forces are so large that skyrmions cannot maintain their topology and break down. Our work establishes the correlation between transport characteristics and deformation of skyrmions and predicts possible nonlinear and breakdown effects caused by dynamic deformation.

The toroidal order of electric dipoles in ferroelectric materials has attracted attention in the past decade due to fascinating properties and great potential for enabling novel memory devices, and functional devices in general. However, facile manipulation of toroidal order in ferroelectrics remains challenging. Here, using first-principles derived simulations, we demonstrate an efficient scheme to control the polar-toroidal multi-order (PTMO) states in ferroelectric nanowires. Two feasible strategies of controlling PTMO states by a combination of homogeneous electric field and torque are carried out in ferroelectric/paraelectric composite nanowires. This is possible based on trilinear coupling between polarization, toroidization and the twist force. As a result, switching of the toroidization of the nanowire can be readily achieved by reversal of the axial polarization. The torque threshold needed to control PTMO states is also calculated and found to be relatively small, indicating the feasibility of this method. Our study demonstrates facile control of PTMO states, including ferroelectric skyrmions, in ferroelectrics and is a step towards designing ferroelectric devices based on multi-order states.

Recently, the resurgence of interest in flexoelectricity in solids has promoted research on switching of ferroelectric domains via mechanical loads. In this work, combining thermodynamic calculation and phase-field simulation, we revealed that mechanical switching of polarization can be achieved in ferroelectrics by shear stress via a simple mechanism where the presence of flexoelectricity is not necessary. The switching is a consequence of the trilinear coupling between shear stress and the two polarization components that lie in the shear strain plane. Specifically, when the direction of one polarization component is fixed, switching of the other component can be induced by applying a shear stress. Moreover, when the shear stress is fixed, switching of one of the polarization components can lead to the switching of the other component. Phase diagrams of the stable polarization state as a function of shear stress, temperature and electric field are calculated. Furthermore, as demonstrated by phase-field simulation on a ferroelectric thin film, domain switching can indeed be realized by a local shear stress. Importantly, the combining effect of shear stress and electric field can lead to a deterministic writing of charged domain walls, which should be very useful for the fabrication of domain wall devices. The implication of such a shear-stress-modified polarization switching on the design of vortex memory device is also discussed.