Imaging and Manipulating Individual Magnetic Skyrmions by Spin-Polarized Scanning Tunneling Microscopy
Pin-Jui Hsu1*
1Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
* Presenter:Pin-Jui Hsu, email:pinjuihsu@phys.nthu.edu.tw
Abstract:
Magnetic skyrmions are topologically protected quasiparticles with whirling spin textures [1-4]. They offer promising perspectives, e.g., robustness against local perturbations [5], motion driven by low current densities [6], and skyrmion Hall effect [7] etc., which are envisioned to be used as magnetic bits in next-generation information technology and skyrmion-based devices. Recently, magnetic ultrathin films [8] and multilayers [9] grown on heavy metal substrates support the formation of individual magnetic skyrmions due to the breaking of inversion symmetry and sizable interfacial Dzyaloshinskii-Moriya (DM) interaction. In this talk, I will report on the studies of magnetic skyrmions in Fe triple-layer (TL) on Ir(111) [10], they show an anisotropic shape due to inherited structural reconstructions followed by Fe double-layer (DL) underneath [11] as resolved by using spin-polarized scanning tunneling microscopy [12-14]. By exploiting the electric fields between the tip and sample, single skyrmions on Fe-TL can be created and annihilated in a controllable fashion. In addition, I will also talk about the isotropic magnetic skyrmions stabilized on Fe-DL by loading the sample with hydrogen atoms. To understand the underlying mechanism, ab initio calculations have been performed and attribute to an increased interlayer distance and consequently modified magnetic interactions, which suggests that hydrogenation provides an unique pathway to engineer crucial magnetic parameters and enables a drastic change of tuning noncollinear magnetism in low-dimensional magnetic materials.

References:
1. Bogdanov, A. N. & Yablonskii, D. Thermodynamically stable “vortices” in magnetically ordered crystals. The mixed state of magnets. Sov. Phys. JETP 68, 101-103 (1989).
2. Rößler, U. K., Bogdanov, A. N. & Pfleiderer, C. Spontaneous skyrmion ground states in magnetic metals. Nature 442, 797–801 (2006).
3. Yu, X. Z. et al. Real-space observation of a two-dimensional skyrmion crystal. Nature 465, 901–904 (2010).
4. Nayak, A. K. et al. Magnetic antiskyrmions above room temperature in tetragonal Heusler materials. Nature 548, 561-566 (2017).
5. Fert, A., Cros, V., & Sampaio, J. Skyrmions on the track. Nat. Nanotechnol. 8, 152–156 (2013).
6. Sampaio, J., Cros, V., Rohart, S., Thiaville, A. & Fert, A. Nucleation, stability and current-induced motion of isolated magnetic skyrmions in nanostructures. Nat. Nanotechnol. 8, 839–844 (2013).
7. Jiang, W. et al. Direct observation of the skyrmion Hall effect. Nat. Phys. 13, 162-169 (2017).
8. Romming, N. et al. Writing and Deleting Single Magnetic Skyrmions. Science 341, 636–639 (2013).
9. Dupé, B., Hoffmann, M., Paillard, C. & Heinze, S. Tailoring magnetic skyrmions in ultra-thin transition metal films. Nat. Commun. 5, 4030 (2014).
10. Hsu, P.-J. et al. Electric-field-driven switching of individual magnetic skyrmions. Nat. Nanotechnol. 12, 123 (2017).
11. Hsu, P.-J. et al. Guiding spin spirals by local uniaxial strain relief. Phys. Rev. Lett. 116, 017201 (2016).
12. Bode, M. Spin-polarized scanning tunneling microscopy. Rep. Prog. Phys. 66, 523 (2003).
13. Wiesendanger, R. Spin mapping at the nanoscale and atomic scale. Rev. Mod. Phys. 81, 1495-1550 (2009).
14. von Bergmann, K., Kubetzka, A., Pietzsch, O. & Wiesendanger, R. Interface-induced chiral domain walls, spin spirals and skyrmions revealed by spin-polarized scanning tunneling microscopy. J. Phys.: Condens. Matter 26, 394002 (2014).


Keywords: Noncollinear magnetism, Magnetic skyrmion, Dzyaloshinskii-Moriya interaction, Spin-polarized scanning tunneling microscopy