A transmission electron microscope study of Néel skyrmion magnetic textures in multilayer thin film systems with large interfacial chiral interaction

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.

Quantitative imaging of hybrid chiral spin textures in magnetic multilayer systems by Lorentz microscopy

“…Using this detector as opposed to a standard quadrant detector allows a more precise measurement of the shifts of the unscattered central diffraction disk due to the Lorentz deflection of the electron beam. In the case of multilayer films with out-of-plane domains, experience shows that the pixelated detector is necessary to get good magnetic contrast, especially for polycrystalline films with small Lorentz deflection angles [30]; the difficulties related to imaging the multilayer samples specific to this study are discussed in supplementary information S5. The beam shifts measured by DPC are converted into quantitative integrated induction maps which allow quantification of the contributions from the out-of-plane domains and Bloch wall components.…”

“…The technique of Lorentz TEM is used in this study and has previously been utilised in a number of investigations into the structure and magnetic behaviour of domain walls and skyrmions in chiral systems [26][27][28]. Specifically, it has been used to identify whether walls in multilayer materials with chiral texture are of Néel type [29] and also to spatially resolve domain wall widths which may only be ten nanometres or less [30]. CD-XRMS, X-PEEM, SPLEEM and (SP)STM are all surface-sensitive techniques revealing magnetization texture in the top layer(s) only.…”

Quantifying chiral exchange interaction for Néel-type skyrmions via Lorentz transmission electron microscopy

“…45,46 Lorentz TEM has been extensively utilized for studying the non-collinear spin textures. 3,5,8,9,[47][48][49][50][51][52][53][54][55][56][57] Here we first discuss theoretical expressions for, and calculate, the magnetic contrast in the Lorentz TEM images for Néel-type skyrmions ( 1). This will subsequently be used to demonstrate the experimental detection of Néel-type skyrmions with Lorentz TEM and investigate the dependence of skyrmion size as a function of magnetic field in a Pt/Co/W multilayer.…”

Chiral magnetic skyrmions with arbitrary topological charge

“…For the case of magnetic multilayers and heterostructures with interface-induced DMI [49], the demonstration of the stability of the solutions displayed further might suggest that isolated π-domains (also known as bubble-skyrmions) [50,51,52,53] belong to the category of chiral skyrmions. In the eventuality where such a scenario fails, one can conclude that the magnetic dipoledipole interaction represents the core mechanism for the stabilization of those domains.…”