Abstract
Skyrmions in ultrathin ferromagnetic metal (FM)/heavy metal (HM) multilayer systems produced by conventional sputtering methods have recently generated huge interest due to their applications in the field of spintronics. The sandwich structure with two correctly-chosen heavy metal layers provides an additive interfacial exchange interaction which promotes domain wall or skyrmion spin textures that are Néel in character and with a fixed chirality. Lorentz transmission electron microscopy (TEM) is a high resolution method ideally suited to quantitatively image such chiral magnetic configurations. When allied with physical and chemical TEM analysis of both planar and cross-sectional samples, key length scales such as grain size and the chiral variation of the magnetisation variation have been identified and measured. We present data showing the importance of the grain size (mostly < 10 nm) measured from direct imaging and its potential role in describing observed behaviour of isolated skyrmions (diameter < 100 nm). In the latter the region in which the magnetization rotates is measured to be around 30 nm. Such quantitative information on the multiscale magnetisation variations in the system is key to understanding and exploiting the behaviour of skyrmions for future applications in information storage and logic devices.
Highlights
Magnetic skyrmions are 2D topological spin textures of significant current interest having been predicted to occur in materials with non-centro-symmetric crystal structures, resulting in a strong Dzyaloshinskii-Moriya interaction (DMI)[1]
Using the technique of spin polarised scanning tunnelling electron microscopy (SP-STM) it has been possible to image at low temperature, compact skyrmions as small as a few lattice parameters, in Fe monolayers on Ir(111)
Lorentz microscopy is the study of magnetic structure in the transmission electron microscope (TEM) and has been used to confirm the structure of Bloch skyrmions in B20 and Fe/Pd ML materials[2,21,22] and study chiral Néel walls in films with interfacial DMI23,24
Summary
This TEM study reveals crucial structural parameters: continuity of the magnetic layers throughout the full structures and the distribution of grains sizes peaking around 4 nm in diameter. This is explored in the Supplementary Information (S3) whereby image calculations of wall structures are made for simulated one dimensional wall with the beam at oblique angles of incidence. It has recently been reported that surface states may be present in ML systems where the Néel components are of opposite signs at the top and bottom layers of the stack[35] If such a configuration were present in the MLs here the divergent Néel wall component would likely remain invisible in LTEM, even at oblique incidence. The functionality of skyrmions is being realised by the intense effort worldwide in this area, studies providing high resolution information on the physical, chemical and magnetic structure are vital to our understanding of these complex magnetic phenomena
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