Current-Induced Nucleation and Motion of Skyrmions in Zero Magnetic Field

We study the stabilization and electrical manipulation of skyrmions in magnetic ultrathin films in the absence of an applied magnetic field. We show that this requires an increased magnetic anisotropy, controlled by the sample thickness, as compared to usual skyrmionic samples, so that the uniform state corresponds to the zero-field ground state and the skyrmions to metastable excitations. Although skyrmion stabilization at zero field is demonstrated over a broad range of thicknesses, electrical control appears to be more demanding to avoid skyrmion deformation. In the thinnest samples, the large magnetic anisotropy prevents skyrmion deformations and we show that they can be nucleated progressively by current pulses, which underlines that the only possible transition occurs between uniform and skyrmion states. The solitonic skyrmions in zero applied magnetic field have the same properties as compared to field-stabilized ones, with a long-term stability and high mobility when excited by a spin-orbit torque.