Session D53: Invited Session: Skyrmions in Thin Magnetic Films

Manipulation of magnetic skyrmions with spin-polarized STM

Spin textures of ultra-thin magnetic layers exhibit a surprising variety. The loss of inversion symmetry at the interface of magnetic layer and substrate gives rise to the so-called Dzyaloshinskii-Moriya interaction (DMI) which favors non-collinear spin arrangements with unique rotational sense [1]. An ideal tool to investigate such systems down to the atomic scale is spin-polarized scanning tunneling microscopy (SP-STM), which has enabled the discovery spin spirals with unique rotational sense at surfaces [2-4]. Recently, different interface-driven skyrmion lattices have been found, that either exist without external magnetic field [5,6] or are induced by it [7]. A tuning of the magnetic properties can be realized by tiny variations of the electronic structure due to stacking and hybridization of the magnetic layer. Isolated skyrmions can be stabilized in a wide magnetic field range [7] and the high lateral resolution of SP-STM together with its magnetic sensitivity enables a precise characterization of the evolution of size and shape of single skyrmions with field. A comparison to micromagnetic theory yields the material parameters including the DMI which is responsible for skyrmion formation. The writing as well as the deletion of individual skyrmions based on local spin-polarized current injection has been demonstrated [7]. A new mechanism to detect skyrmions using non-spin-polarized currents has been discovered and can be understood based on the mixing of spin-up and spin-down bands. These interface-induced non-collinear magnetic states offer new exciting possibilities to study fundamental physical properties on the atomic-scale and to tailor material properties for spintronic applications.\\[4pt] [1] K. von Bergmann et al., J. Phys.: Condens. Matter 26, 394002 (2014).\\[0pt] [2] M. Bode et al., Nature 447, 190 (2007).\\[0pt] [3] P. Ferriani et al., Phys. Rev. Lett. 101, 27201 (2008).\\[0pt] [4] M. Menzel et al., Phys. Rev. Lett. 108, 197204 (2012).\\[0pt] [5] K. von Bergmann et al., Phys. Rev. Lett. 96, 167203 (2006).\\[0pt] [6] S. Heinze et al., Nature Phys. 7, 713 (2011).\\[0pt] [7] N. Romming et al., Science 341, 636 (2013).   

Electrical Creation and Manipulation of Magnetic Skyrmion Bubbles

Magnetic skyrmions are topologically stable spin textures, which exhibit many fascinating features including an emergent electromagnetic field and efficient manipulation. Nevertheless, until now this has been challenging to achieve at room temperature, which is a bottleneck for technological implementation of skyrmion-based spintronics. Towards this end, room-temperature electric-current creation of skyrmions in two different (metallic and insulating) commonly accessible materials system will be discussed. First, the experimental creation of magnetic skyrmions triggered by an electric current in Ta/CoFeB/TaO$_{\mathrm{x}}$ trilayers is demonstrated. The skyrmion generation is enabled by laterally inhomogeneous current-induced spin-orbit torques. This process is analogous to the spontaneous droplet formation in surface-tension driven fluid flows. We establish a novel phase diagram that summarizes the dependence of skyrmion generation on the external magnetic fields, and the strength of in-plane currents. Furthermore, we reveal the efficient manipulation of these skyrmions by electric currents. More importantly, a prototype skyrmion racetrack memory device will be experimentally demonstrated. Secondly, the manipulation of skyrmion bubbles by using spin Hall spin torques in (Pt or W)/(Y,Bi)$_{3}$Fe$_{5}$O$_{12}$ (YIG:Bi) bilayers will be discussed. Using MOKE imaging, we have identified a hexagonal lattice of skyrmion bubbles (1.8-$\mu $m diameter). Subsequent current pulses through the Pt layer results both in the motion of some of the skyrmions and a reduction in size of others, which is consistent with different wall structures and resultant skyrmion numbers. Furthermore, we have observed distinct anomalous Hall signals associated with the underlying magnetization textures, which may indicate topological Hall effects in bilayers.\footnote{Financial support for the work at Argonne came from DOE, Office of Science, BES, Materials Sciences and Engineering Division, work at UCLA was supported by TANMS.}$^,$\footnote{This work was performed in collaboration with W. Zhang, M. B. Jungfleisch, F. Y. Fradin, J. E. Pearson, O. Heinonen, S. G. E. te Velthuis, and A. Hoffmann (Argonne National Laboratory), P. Upadhyaya, G. Yu, Y. Tserkovnyak, K. L. Wang (UCLA), Q. H. Yang, Q.Y. Wen, and H. W. Zhang (UESTC, China).}   

Creation and Dynamics of Individual Skyrmions in Helimagnets

The physics and future applications of magnetic skyrmions in helimagnet have attracted great attentions in the past years. Now the major focus of this field is the ability of creating single skyrmions in a controlled manner, and exploring novel properties of skyrmions as quasi-particles. In this talk, I will introduce our recent work on creating a single skyrmion via electric current or geometric confinement. The time and position of the skyrmion can be accurately controlled. More importantly, the microscopic mechanism is clearly revealed by the analysis of the topological charge, which also explains the topological origin of the skyrmion stability for the first time. The distinct feature of the skyrmion is an emergent gauge field attached to it. The motion of a single skyrmion is realized by coupling this emergent gauge field to electric current or magnon current. I will show that an electric current or temperature gradient can result in a steady motion of a single skyrmion. \\[4pt] [1] Gen Yin, Yufan Li, Lingyao Kong, Roger Lake, C. L. Chien, and Jiadong Zang, submitted.\\[0pt] [2] Jiadong Zang, Maxim Mostovoy, Jung Hoon Han, and Naoto Nagaosa, Phys. Rev. Lett. 107, 136804 (2011)\\[0pt] [3] Lingyao Kong, and Jiadong Zang, Phys. Rev. Lett. 111, 067203 (2013).\\[0pt] [4] M. Mochizuki, X. Z. Yu, S. Seki, N. Kanazawa, W. Koshibae, J. Zang, M. Mostovoy, Y. Tokura, and N. Nagaosa, Nature Material 12, 241 (2014).\\[0pt] [5] Haifeng Du, \textit{et al}. submitted.   

Dynamics of Skyrmionic Spin Structures

Magnetic skyrmions are topologically protected particle-like spin structures, with a topology characterised by their Skyrmion number. They can arise due to various interactions, including exchange, dipolar and anisotropy energy in the case of bubbles (skyrmion bubbles) and an additional Dzyaloshinskii-Moriya interaction (DMi) in the case of chiral skyrmions. Numerical predictions suggest that they exhibit rich dynamical behaviour governed by their topology, such as the basic gyrotropic and breathing eigenmodes [1,2]. The dynamical experiments are performed on skyrmion bubbles in nanostructures from symmetric CoB/Pt multilayers, tailored for high-frequency dynamics. Asymmetric layers were also fabricated (Co layers in-between 5d-metals) in order to tune the DMi, as expected from recent experiments [4]. Stabilizing chiral skyrmions confined in such nanostructures is highly desirable due to their enhanced stability and smaller size that makes them ideal candidates for integration in recently proposed novel spintronics devices [3]. By investigating the size of magnetic domains in magnetic field cycles, and comparing to micromagnetic simulations, asymmetric multilayers were explored. By performing pump-probe dynamical X-ray imaging on confined skyrmion bubbles the first observation of their basic eigenmode dynamics was demonstrated [5]. In particular, we present picosecond nanoscale imaging data i) of the gyrotropic mode of a single skyrmion bubble in the GHz regime and ii) the breathing-like behaviour of a pair of skyrmionic configurations. The observed dynamics is used to confirm the skyrmion topology and show the existence of an unexpectedly large inertia that is key for describing skyrmion dynamics [1,5]. These results demonstrate new ways for observing skyrmion dynamics and provide a framework for describing their behaviour. The next step is to achieve chiral skyrmion dynamics on a DMi system. \\[4pt] [1] C. Moutafis, S. Komineas, J. A. C. Bland, Phys. Rev. B 79, 224429 (2009).\\[0pt] [2] N. Nagaosa, Y. Tokura, Nat. Nanotech. 8, 899--911 (2013).\\[0pt] [3] A. Fert, V. Cros, J. Sampaio, Nat. Nanotech. 8, 152--156 (2013).\\[0pt] [4] G. Chen, et al. Nat. Comms. \textbf{4}, (2013).\\[0pt] [5] F. Buettner, C. Moutafis, et al., submitted, Nat. Physics (2014).