Structure, Energetics, and Deterministic Writing of Skyrmions in Thin Film Ferromagnets

Skyrmions are the smallest non-trivial entities in magnetism with great potential for data storage applications. They were recently observed at room temperature in magnetic multilayer systems [1-4], most of them in materials with sizable Dyzaloshinskii-Moriya interaction (DMI). Despite this experimental breakthrough, our understanding of skyrmions is still limited because existing theories cannot analytically predict how the skyrmion energy changes as a function of its size. In particular, for many decades, the 6-fold integral of the stray field energies was considered unsolvable.

This problem has now been solved. In this talk, I will present a unified theory that analytically approximates the energy, including stray fields, of isolated skyrmions of all sizes with 1% precision [1]. I will show that there are two types of skyrmions, "stray field skyrmions" and "DMI skyrmions", with rigorous and practical distinction criteria. I will furthermore show that our model can predict materials with sub-10 nm skyrmions at zero field and room temperature, where those skyrmions can be moved with velocities exceeding 1000 m/s at reasonable current densities of 10¹² A/m².

Experimentally, I will show that skyrmions can be nucleated by spin-orbit torque current pulses [2]. In contrast to spin-orbit torque switching of uniformly magnetized MRAM cells, skyrmion nucleation does not require any in-plane fields to be applied. The nucleation mechanism is robust, ultra-fast (sub-nanosecond), and extremely easy to implement. I will discuss the mechanism of the skyrmion generation and explain why DMI can replace the need for in-plane fields.

[1] Büttner et al., Nat Phys. 11, 225 (2015).
[2] Woo et al., Nat Mater. 15, 501 (2016).
[3] Moreau-Luchaire et al., Nat Nano. 11, 444 (2016).
[4] Boulle et al., Nat Nano. 11, 449 (2016).
[5] Büttner et al., arXiv:1704.08489
[6] Büttner et al., Nat Nano. 12, 1040 (2017).