The NQPI Seminar Series presents Jay Gupta of Ohio State University, on “Spin Polarized STM Studies of Nanoscale Skyrmions and Chiral Magnetic Textues in Oxide and B20 Heterostructures”, on Thursday, Oct. 10, at 4:10 p.m. in Clippinger Labs 194.
Jay Gupta
Abstract: Magnetism in materials and heterostructures with large spin orbit coupling and/or broken inversion symmetries produces a variety of textures that are of interest for next-generation storage. For example, magnetic Skyrmions can be realized on the ~ 1nm scale, which is promising for higher density memory than is possible with conventional ferromagnetic bits. Characterization of such small magnetic textures exceeds the spatial resolution of conventional magnetic imaging methods such as MOKE and MFM, which motivates the development of higher resolution techniques such as spin polarized STM. In this talk I will review our recent progress on SPSTM imaging of magnetic Skyrmions in bulk thin film materials and heterostructures.
First I will discuss our STM and SPSTM studies of FeGe and MnGe thin films grown by Molecular Beam Epitaxy. The ‘B20’ crystal structure of these films is non-centrosymmetric, and the unit cell comprises repeating stacks of Fe,Mn and Ge lattices. The lack of inversion symmetry leads to 8 possible surfaces, depending on the terminating layer and the stacking order. We have developed a methodology for uniquely determining the surface termination and stacking sequence, based on atomic resolution STM imaging and comparison with DFT calculations. This sequence not only impacts the electronic structure we observe on the surface, but also the magnetic properties that can be resolved with SPSTM imaging. In MnGe, we have observed a spin spiral phase with a 6-8 nm period and a propagation direction that is influenced by step edges, surface termination, and strain. We have also discovered isolated ‘target’ Skyrmions with a bullseye texture that likely form due to local modulations in the atomic structure. The target Skyrmions have a triangular shape which appears to be set by the in-plane lattice vectors, and a core size of approximately 15 nm. We observe that the target state is significantly more sensitive to magnetic fields than the spiral phase, and that local voltage and current pulses with the STM tip indicate that the texture can be ‘switched’ between states with different topological charge.
We have also imaged isolated <5 nm Skyrmions in oxide heterostructures that rely on interfacial symmetry breaking and strong spin orbit coupling. We fabricated bilayers of 2 unit cells of SrIrO3 atop of 10 unit cells of ferromagnetic SrRuO3 via off-axis sputtering. We observed a granular morphology of SrIrO3 mounds with rare patches of exposed SrRuO3, distinguished in spectroscopy maps of the local density of states. Concurrent images with magnetic contrast reveal a variety of magnetic features < 10nm in size, including a characteristic double-lobed structure that inverts with applied magnetic field that is associated with individual Skrymions. Inversion, annihilation and creation of these Skyrmions with STM tip perturbations suggests a low switching energy and a correlation with SrIrO3 grains.
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