Anatomy of Skyrmionic Textures in Magnetic Multilayers

Abstract

Room temperature magnetic skyrmions in magnetic multilayers are considered as information carriers for future spintronic applications. Currently, a detailed understanding of the skyrmion stabilization mechanisms is still lacking in these systems. To gain more insight, it is first and foremost essential to determine the full real‐space spin configuration. Here, two advanced X‐ray techniques are applied, based on magnetic circular dichroism, to investigate the spin textures of skyrmions in [Ta/CoFeB/MgO]n multilayers. First, by using ptychography, a high‐resolution diffraction imaging technique, the 2D out‐of‐plane spin profile of skyrmions with a spatial resolution of 10 nm is determined. Second, by performing circular dichroism in resonant elastic X‐ray scattering, it is demonstrated that the chirality of the magnetic structure undergoes a depth‐dependent evolution. This suggests that the skyrmion structure is a complex 3D structure rather than an identical planar texture throughout the layer stack. The analyses of the spin textures confirm the theoretical predictions that the dipole–dipole interactions together with the external magnetic field play an important role in stabilizing sub‐100 nm diameter skyrmions and the hybrid structure of the skyrmion domain wall. This combined X‐ray‐based approach opens the door for in‐depth studies of magnetic skyrmion systems, which allows for precise engineering of optimized skyrmion heterostructures.

Current-Driven Skyrmion Dynamics and Drive-Dependent Skyrmion Hall Effect in an Ultrathin Film

“…In particular, fast motion (100 m s −1 ) of small skyrmions (100 nm) has been reported in [Pt/CoFeB/MgO] 15 [8,16]. However, in these strayfield-coupled multilayers, the large dipolar fields can outweigh the DMI and stabilize twisted spin structures with a nonuniform chirality across the film thickness [17][18][19]. This leads to a complex current-driven dynamics due to layer-dependent SOTs and topologies [20,21].…”

Stabilizing chiral spin structures via an alternating Dzyaloshinskii-Moriya interaction

“…To compensate for this, the magnetic volume is usually increased to enhance the thermal stability and reduce the skyrmion energy [12][13][14]18]. The concomitant increase of dipolar interactions, however, has been shown both theoretically and experimentally to compete with the DMI, leading to a nonuniform magnetic chirality across the thickness of the layers [19][20][21][22][23][24][25][26][27][28]. This is considered detrimental for applications because most of the functionality relies on the uniform chirality of a skyrmion across the thickness of the multilayer system [29].…”

Depth-Resolved Magnetization Dynamics Revealed by X-Ray Reflectometry Ferromagnetic Resonance

“…Advantageously, reflectivity is not tied to fulfilling a diffraction condition, thereby allowing for a very large variety of materials systems to be investigated. Our ½CoFeB=MgO=Ta 4 multilayers host a variety of topological magnetic phases [22]. Here, we reveal the depth dependence to the dynamics in the field-polarized phase exhibiting a conical precession with a phase lag between the magnetization precession in adjacent layers.…”

“…The multilayer is then able to adopt a depth-dependent dynamic structure where the inclusion of a phase lag between the layers is necessary to explain the experimental data. In fact, the ½CoFeB=MgO=Ta 4 multilayer investigated here are known to host noncollinear magnetic structures stabilized by the Dzyaloshinskii-Moriya interaction, which arises at interfaces between a magnetic layer and a heavy metal with large spinorbit coupling [22]. The combination of the Dzyaloshinskii-Moriya interaction and dipole interaction leads to the formation of complex three-dimensional noncollinear structures, where, e.g., the domain wall configuration continuously evolves throughout the stack, going from Néel-type to Bloch-type and back again to the inverse Néel-type [29].…”