Abstract
Néel-type magnetic skyrmions in multilayers are promising candidates for ultra-low power spintronic devices. To image the Néel-type skyrmions using Lorentz transmission electron microscopy (L-TEM), the samples must be tilted. Thus, the external magnetic field consists of both in-plane and out-of-plane components. To date, it is still not well known on the effect of the in-plane magnetic field on the L-TEM images, leading to ambiguities in retrieving the structure of Néel-type skyrmions. Here, Néel-type skyrmions in three [Pt/Co/Ta]20 multilayer samples, with the easy magnetization axis being tuned from the out-of-plane to the in-plane direction by increasing the Co thickness from 1.8 to 2.2 nm, are imaged. When using a smaller defocus value (−2 mm) and a higher magnification (×9100) of L-TEM, a surprising dark-bright-dark-bright double contrasted pattern, instead of the previously reported dark-bright contrasted pattern, is observed. The additional dark-bright contrasted pattern becomes more evident for thicker Co layer samples in which the magnetization axis tilts more toward the in-plane direction. Further analysis, via a combination of magnetic force microscopy experiments, micromagnetic simulations, and micromagnetic analysis to Lorentz TEM simulation, shows that the additional dark-bright features originate from the deformation of the Néel-type skyrmions within an in-plane magnetic field.
Highlights
N (DMI), dipole interaction, magnetic anisotropy, exchange interaction, and the external magnetic field.[2,3,18,19,20,21] In particular, the Dzyaloshinskii-Moriya interaction (DMI) from the strong spin–orbit interaction at the interfaces plays a key role in the formation of skyrmions.[22,23] the typical DMI strength in multilayer stacks is usually less than $2 mJ/m2 and difficult to be further increased.[7,15,21,24,25] in actual applications, the best tunable parameter would be the magnetic anisotropy, which could be controlled by changing the thickness of the ferromagnetic layer.[26–28]
Via a combination of magnetic force microscopy experiments, micromagnetic simulations, and micromagnetic analysis to Lorentz TEM simulation, shows that the additional dark-bright features originate from the deformation of the Neel-type skyrmions within an in-plane magnetic field
To investigate the intriguing physics of skyrmions and to explore the potential applications, many techniques have been employed such as magnetic force microscopy (MFM),[14,21,29] X-ray magnetic circular dichroism (XMCD),[3] polar magneto-optical Kerr effect (MOKE) microscopy,[8,18] and Lorentz transmission electron microscopy (LTEM).[11,27,30]
Summary
N (DMI), dipole interaction, magnetic anisotropy, exchange interaction, and the external magnetic field.[2,3,18,19,20,21] In particular, the DMI from the strong spin–orbit interaction at the interfaces plays a key role in the formation of skyrmions.[22,23] the typical DMI strength in multilayer stacks is usually less than $2 mJ/m2 and difficult to be further increased.[7,15,21,24,25] in actual applications, the best tunable parameter would be the magnetic anisotropy, which could be controlled by changing the thickness of the ferromagnetic layer.[26–28]. Neel-type skyrmions in three [Pt/Co/Ta]20 multilayer samples, with the easy magnetization axis being tuned from the out-of-plane to the in-plane direction by increasing the Co thickness from 1.8 to 2.2 nm, are imaged.
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