Nano-scale Skyrmions in Ultrathin Transition Metal Films and Multilayers: From Fundamentals to Potential Applications
Magnetism in ultrathin films can significantly deviate from commonly known bulk magnetism due to low dimensionality, hybridization effects, changes of the lattice constant, stacking dependencies, and broken inversion symmetry at interfaces. This can lead to complex non-collinear spin states such as spin spirals or skyrmions. Especially magnetic skyrmions with their non-trivial topology are interesting objects for both fundamental as well as application-oriented research due to their possible utilization in future magnetic data storage.
Based on the development of atomic-resolution spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy , operated within 3D superconducting magnet systems, we have discovered nanoskyrmion lattices in single atomic layers of transition metals on particular substrates exhibiting a large spin-orbit coupling, such as monolayer (ML) Fe films on Ir(111) [2-5]. In this case, skyrmionic lattices with a periodicity of only one nanometer can be stabilized even in zero external field by the Dzyaloshinskii-Moriya interaction combined with the breaking of inversion symmetry at surfaces and interfaces.
More recently, we have made use of multiple interface engineering in bilayer and multilayer systems in order to demonstrate the direct observation and manipulation of individual skyrmions of single-digit nanometer-scale size . While numerous theoretical and simulation studies have concentrated on individual skyrmions and their physical properties, no high-resolution experimental characterization of the internal spin structure of skyrmions has been reported previously, even though the knowledge about their actual size and shape provides the foundation for predictions about the interactions of skyrmions with spin currents or their manipulation by external fields as envisaged in potential skyrmion-based device concepts. We recently resolved the atomic-scale spin structure of individual isolated skyrmions in real space by SP-STM . Their axial symmetry as well as their unique rotational sense has been revealed by using both out-of-plane and in-plane sensitive SP-STM tips. The size and shape of skyrmions change as a function of magnetic field. An analytical expression for the description of skyrmions has been proposed in order to connect the experimental data to the original theoretical model describing chiral skyrmions .
By locally injecting spin-polarized electrons from an atomically sharp SP-STM tip, we were able to write and delete individual skyrmions one-by-one, making use of spin-transfer torque exerted by the injected high-energy spin-polarized electrons . Switching rate and direction can be controlled by the parameters used for current injection. The creation and annihilation of individual magnetic skyrmions demonstrates their great potential for future nanospintronic devices making use of individual topological charges as information carriers [9,10].
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