Based on micromagnetic simulation and analysis of Thiele's equation, in this work we demonstrate that ratchet motion of skyrmion can be induced by a biharmonic in-plane magnetic field hx(t) = h 1 sin(mt) + h 2 sin(nt + ), provided that integers m and n are coprime and that m + n is odd. Remarkably, the speed and direction of the ratchet motion can be readily adjusted by the field amplitude, frequency and phase, with the maximum speed being over 5 m/s and the direction rotatable over 360. The origin of the skyrmion ratchet motion is analyzed by tracing the excitation spectra of the dissipation parameter D and the skyrmion position R, and it shows that the dissipative force plays a key role in the appearance of ratchet motion. Such a ratchet motion of skyrmion is distinguished from those caused by single-frequency ac drives reported in the literature, and from that driven by pulsed magnetic fields as also predicted in this work. Our results show that skyrmion ratchet effect under biharmonic forces shares some common features of those found in many soliton systems, and the facile controllability of both the skyrmion speed and direction should be useful in practice.PACS number(s): 76.50.+g, 75.10.Hk, 75.78.−n *

Combining thermodynamic calculation and phase-field simulation, we revealed that mechanical switching of polarization can be achieved in ferroelectric by shear stressviaa simple mechanism where the presence of flexoelectricity is not necessary.

No abstract

The electrocaloric effect in ferroelectrics provides a promising solid-state refrigeration technology to meet the urgent demand for heat management in the integrated circuits. Usually, ferroelectrics show a positive electrocaloric effect, i.e. the application of an electric field causes an increase in temperature. In contrast, the negative electrocaloric effect is also possible but much less explored. These two effects, if occur in the same system, could be combined to improve the cooling performance. Here we report the occurrence of both positive and negative electrocaloric effects in ferroelectric/paraelectric PbTiO3/SrTiO3 superlattice based on phase-field simulations. The superlattice is found to stabilize a variety of vortex dipole states (with the different number of vortex) when the PbTiO3 and SrTiO3 layers have suitable thicknesses. The unique response of these topological states to the external electric field gives rise to both positive and negative electrocaloric effect, and the latter is associated with the field-dependent vortex-to-polar state transformation. The multiplicity of the vortex dipole states brings tuning freedom for the optimization of electrocaloric properties. Moreover, the electrocaloric effect is also sensitive to the strength of the screening effect determined by the thickness of the SrTiO3 layer. We thus demonstrate a novel mechanism of negative electrocaloric effect in ferroelectrics with topological dipole states and indicates the screening effect as an effective way to engineer the electrocaloric performance.

The dielectric nonlinearity of stack lead zirconium titanate (PZT) piezoelectric actuator was investigated under the combined uniaxial mechanical and electric loads. In the intermediate electric field region (Rayleigh region), the Rayleigh coefficient increases at less than 50 MPa, beyond which it declines. Similarly, the dielectric nonlinearity identified by P-E hysteresis loop, revealed by both the dielectric measurement and the Rayleigh law fitting, also shows an initial increase at less than 50 MPa and a subsequent decrease when stress is above 50 MPa. In the high electric field region, the dielectric nonlinearity declined continuously with increasing stress, which leads to a fact that a boundary between the Rayleigh region and high field region becomes smeared. These results could be interpreted as the influences of the applied stress on ferroelectric domain-related contributions to dielectric response, which is very important to the practical application of stack piezoelectric actuators.