Scientists from ShanghaiTech University, RIKEN Center, Nanjing University, Chinese Academy of Sciences, the Diamond Light Source, and University of Oxford report in a recent article in Nano Letters on their observations of how magnetic skyrmions bond vertically. Utilising an advanced resonant x-ray scattering technique, they unambiguously revealed that skyrmions can be imprinted from one material to another, forming a bound 3D spin configuration. This offers new opportunities for engineering multidimensional topological structures.
More than a decade ago, a new, magnetically ordered phase was experimentally identified and welcomed into the family of magnetic objects, and dubbed the magnetic skyrmion phase. Being quite different from conventional magnetic order, skyrmions, manifesting themselves as a spin swirl configuration, are said to be topological. Perhaps there are two major aspects that make ‘being topological’ extraordinary. First, the intrinsic structural protection makes a skyrmion a robust entity, analogous to a funnel structure of a tornado that can travel thousands of miles without dissipating. This aspect is greatly appreciated when it comes to information carriers for magnetic information storage schemes.
The other intrinsic property of topological objects is that they can be geometrical deformed, as long as they are not cut or punctured, while leaving their physical properties intact. This means that a skyrmion swirl does not have to be a rigid body quasiparticle. By definition, they can morph into any shape and adapt to any environment, if the perturbation is gentle. For example, it does not matter if the skyrmion size is 200 nm or 10 nm, and it also does not matter if the skyrmion is a convergent vortex or a divergent one. If one can perform such a gentle surgery, a skyrmion can transform in size and shape without dissolving.
So far, this ‘morphing’ aspect of skyrmions has not been exploited or experimentally studied. However, the successful skyrmion transformation can be quite revolutionary. First, their size reduction has been a key challenge, aiming to provide ever smaller bits. Second, their travelling trajectory is highly dependent on their internal spin configuration, called the helicity angle. Both the skyrmion size and helicity angle are governed by the materials’ parameters. In other words, once a material is chosen, it becomes quite difficult to further engineer the skyrmions. On the other hand, if the ‘morphing’ feature can be exploited, the properties of the skyrmion can be independent from the material constraints.
This has been the initial motivation for the team to perform the study. In order to make the skyrmions to morph, they designed a system of two, vertically stacked dissimilar skyrmion species. The axial inter-skyrmion interaction provides a gentle ‘force’ that can persuade them to change size and shape, in order to adapt to each other. To synthesize such a system, they fabricated a heterostructure consisting of a bulk crystal that hosts smaller, convergent skyrmions, and a thin film system that hosts larger divergent skyrmions.
The experimental characterisation of such axial inter-skyrmion interaction poses great challenges. First, one has to distinguish between the two skyrmion species across the interface. Second, one has to reveal the 3D spin configuration of such bonded system. This calls for resonant elastic x-ray scattering (REXS) to come to the stage, and makes the RASOR diffractometer at beamline I10 the star of the show.
The element-specific, surface-sensitive REXS technique, together with RASOR’s high accuracy and highly efficient data acquisition, allows for the tomography-like 3D reconstruction of the skyrmion structure in the heterostructure. The team found that by contacting 200-nm-diameter skyrmions in the thin film to the 60-nm skyrmions in the bulk magnet, the skyrmions in the film smoothly change their size and position, while eventually being glued to the bottom skyrmion, forming a skyrmion tube entity. Meanwhile, both skyrmion species across the interface changed their helicity angle in order to seamlessly fuse together (see figure below).
Their findings revealed a fascinating “imprinting” effect among skyrmions, i.e., if one proximates a smaller skyrmion to a target material that carries larger skyrmions, the larger vortex will morph and adapt to the ‘stamp’. This offers new opportunities for engineering 3D topological magnetic orders towards device applications.
Corresponding author Prof Shilei Zhang explains his excitement:
For years we have been searching for a way to reduce the skyrmion size and change their helicity angles. Now, we have been able to prove that one can obtain skyrmions even in a familiar material, as long as one has a proper stamp.” And Prof Zhang further adds: “Flexible skyrmions are our friends, and we are now in business to make a large variety of interesting 3D topological phases using this concept.
For more information on the subject matter, please contact either Professor Shilei Zhang (ShanghaiTech University): firstname.lastname@example.org, Professor Gerrit van der Laan (Diamond Light Source): email@example.com, or Professor Thorsten Hesjedal (Oxford Physics): Thorsten.Hesjedal@physics.ox.ac.uk.
To find out more about the I10 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Paul Steadman: firstname.lastname@example.org.
Kejing Ran et al. Axially Bound Magnetic Skyrmions: Glueing Topological Strings Across an Interface. Nano Letters (2022). https://doi.org/10.1021/acs.nanolett.2c00689
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