Skyrmions


Skyrmions are magnetic bubbles in chiral magnetic materials and multilayers, which can be characterized by a topological winding number. They are promising for new magnetic memory applications because moving magnetic skyrmions with electrical current takes 106 times less current density compared to conventional ferromagnetic domain walls. In addition, the size of the skyrmion can be as small as 1 nm allowing for ultra-high information storage densities. However, to achieve such promising applications, a deep understanding is required of the physical interactions required to make small skyrmions stable at room temperature. In addition, the origin of skyrmion pinning must be understood for energy-efficient writing.

Highlights


Chiral Bobbers in FeGe Epitaxial Films

Skyrmions in bulk magnetic materials are in the form of tubes that lie along the applied magnetic field and span the entire thickness of the crystal. A theoretical calculation predicted the formation of a new type of topological spin texture in thin films, and this new state was named the “chiral bobber.” The chiral bobber state is similar to the skyrmion in that they can form a lattice, but are different because they are localized to the surface. Through a careful study of the magnetization of FeGe thin films as a function of film thickness, we were able to identify the formation of the chiral bobber state. The results are presented in A. S. Ahmed et al., “Chiral bobbers and skyrmions in epitaxial FeGe/Si(111) films” Phys. Rev. Materials (Rapid Comm.) 2, 041401(R) (2018).


Synthesis of B20 Superlattices for Tunable Skyrmion Properties

Most bulk crystals hosting skyrmions such as MnSi and FeGe have the so-called “B20” cubic crystal structure. Because the B20 structure lacks inversion symmetry, it produces a bulk spin-orbit coupling which leads to a bulk Dzyaloshinskii-Moriya interaction (DMI) that causes neighboring magnetic moments to favor a 90-degree orientation. This perpendicular spin coupling is responsible for the formation of the swirling skyrmion pattern. Using MBE, we have grown the first B20 superlattice by depositing layers of FeGe, MnGe, and CrGe in a repeating pattern. The figure shows a cross-sectional transmission electron microscope image of the superlattice, with the color showing the elemental composition. This shows a well formed superlattice with relatively sharp interfaces. Because mirror symmetry is broken at an interface, this is able to generate an interface DMI.  By controlling the thickness of the layers, the total DMI consisting of both bulk and interface contributions can be systematically tuned. The results are presented in A. S. Ahmed et al., “Molecular Beam Epitaxy Growth of [CrGe/MnGe/FeGe] Superlattices: Toward Artificial B20 Skyrmion Materials with Tunable Interactions,” Journal of Crystal Growth 467, 38 (2017).


Atomic-Scale Imaging of Topological Spin Textures in MnGe


Spin-polarized scanning tunneling microscopy (SP-STM) provides the unique ability to measure both the structure and magnetic order with atomic resolution. In collaboration with Profs. Gupta and Randeria, we have studied epitaxial MnGe chiral magnetic films. Interestingly, the observed spin textures differ from the expected “3Q” three-dimensional hedgehog spin texture based on earlier neutron diffraction and Lorentz TEM studies. Instead, we observe helical spin textures with unusual “Target” and “Pi” shaped patterns (panels C and D). Theoretical analysis finds that such patterns arise from the intersection of three domain walls between canted helical phases (panels A and B). Moreover, the calculations allow for a small set of domain walls and intersections, and the SP-STM scans have observed all the possibilities. Detailed STM studies also find that strain has a strong influence on the spin texture and formation of domain walls (panels E and F). The work was reported in Repickly et al., “Atomic-scale visualization of topological spin textures in the chiral magnet MnGe,” Science 374, 1484–1487 (2021).


 


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