Julian Hagemeister

Atom-specific spin mapping and buried topological states in a homologous series of topological insulators

A topological insulator is a state of quantum matter that, while being an insulator in the bulk, hosts topologically protected electronic states at the surface. These states open the opportunity to realize a number of new applications in spintronics and quantum computing. To take advantage of their peculiar properties, topological insulators should be tuned in such a way that ideal and isolated Dirac cones are located within the topological transport regime without any scattering channels. Here we report ab-initio calculations, spin-resolved photoemission and scanning tunnelling microscopy experiments that demonstrate that the conducting states can effectively tuned within the concept of a homologous series that is formed by the binary chalcogenides (Bi(2)Te(3), Bi(2)Se(3) and Sb(2)Te(3)), with the addition of a third element of the group IV.

Minimal radius of magnetic skyrmions: statics and dynamics

In a broad range of applied magnetic fields and material parameters isolated magnetic skyrmions condense into skyrmion lattices. While the geometry of isolated skyrmions and their lattice counterparts strongly depend on field and Dzyaloshinski-Moriya interaction, this issue has not been adequately addressed in previous studies. Meanwhile, this information is extremely important for applications, because the skyrmion size and the interskyrmion distance have to be tuned for skyrmion based memory and logic devices. In this investigation we elucidate the size and density-dependent phase diagram showing traditional phases in field vs. material parameters space by means of Monte-Carlo simulations on a discrete lattice. The obtained diagram permits us to establish that, in contrast to the continuum limit, skyrmions on a discrete lattice cannot be smaller than some critical size and have a very specific shape. These minimal skyrmions correspond to the micromagnetic configuration at the energy barrier between the ferromagnetic and the skyrmionic states. Furthermore, we use atomistic Landau-Lifshitz-Gilbert simulations to study dynamics of the skyrmion annihilation. It is shown that this procees consists of two stages: the continuous skyrmion contraction and its discontinuous annihilation. The detailed analysis of this dynamical process is given.

Pattern formation in skyrmionic materials with anisotropic environments

Magnetic skyrmions have attracted broad attention during recent years because they are regarded as promising candidates as bits of information in novel data storage devices. A broad range of theoretical and experimental investigations have been conducted with the consideration of rotational symmetric skyrmions in isotropic environments. However, one naturally observes a huge variety of anisotropic behavior in many experimentally relevant materials. In the present work, we investigate the influence of anisotropic environments onto the formation and behavior of the non-collinear spin states of skyrmionic materials by means of Monte-Carlo calculations. We find skyrmionic textures which are far from having a rotational symmetric shape. Furthermore, we show the possibility to employ periodic modulations of the environment to create skyrmionic tracks.

Symmetry breaking in spin spirals and skyrmions by in-plane and canted magnetic fields

The influence of in-plane and canted magnetic fields on spin spirals and skyrmions in atomic bilayer islands of palladium and iron on an Ir(111) substrate is investigated by scanning tunneling microscopy at low temperatures. It is shown that the spin spiral propagation direction is determined by the island’s border which can be explained by equilibrium state calculations on a triangular lattice.Wefind a different response of spin spirals to in-plane magnetic fields for a propagation direction parallel to the applied field as compared to perpendicular, which originates from their cycloidal nature. As a result, the spin spiral propagation direction may be reorientated by in-plane fields. Furthermore, it is demonstrated that also skyrmions are distorted in canted fields which allows to determine the sense of magnetization rotation as enforced by the interfacial Dzyaloshinskii–Moriya interaction.