Skyrmion stabilization at the domain morphology transition in ferromagnet/heavy metal heterostructures with low exchange stiffness

Abstract

We report the experimental observation of micron-scale magnetic skyrmions at room temperature in several Pt/Co-based thin film heterostructures designed to possess a low exchange stiffness, perpendicular magnetic anisotropy, and a modest interfacial Dzyaloshinskii-Moriya interaction (iDMI). We find both experimentally and by micromagnetic and analytic modeling that the combined action of low exchange stiffness and modest iDMI eliminates the energetic penalty associated with forming domain walls in thin film heterostructures. When the domain wall energy density approaches negative values, the remanent domain morphology transitions from a uniform state to a labyrinthian stripe phase. A low exchange stiffness, indicated by a reduction in the Curie temperature below 400 K, is achieved in Pt/Co, Pt/Co/Ni, and Pt/Co/Ni/Re structures by reducing the Co thickness to the ultrathin limit (< 0.3 nm). A similar effect occurs in thicker Pt/Co/NixCu1-x structures when the Ni layer is alloyed with Cu. At this transition in domain morphology, skyrmion phases are stabilized when a small (< 1 mT) perpendicular magnetic field is applied and current-induced skyrmion motion including the skyrmion Hall effect is observed. The temperature and thickness-induced morphological phase transitions observed are similar to the well-studied spin reorientation transition that occurs in the ultrathin limit, but we find that the underlying energy balances are substantially modified by the presence of an iDMI.