Abstract text (incl. figure legends and references)

Magnetic skyrmions have attracted considerable interest over the last decade, in particular for novel information storage applications. The swirling vortex-shaped magnetic field texture of a skyrmion has topological particle-like properties and is stabilized by the Dzyaloshinskii-Moriya interaction in bulk systems and multilayers of ferromagnets and heavy metals. Co-Zn-Mn with a β-Mn-type structure is a skyrmion-hosting bulk system that has the cubic chiral space group P4132 (or P4332) and allows continuous tuning of magnetic interactions and hence control of the ferromagnetic transition temperature (Tc). However, only Co10-x/2Zn10-x/2Mnx alloys (x = 2-4) exhibit skyrmion lattice formation above room temperature [1]. Here, we present a reproducible growth route for Co-Zn-Mn alloys with Tc close to 100 ºC and microstructural, compositional and magnetic imaging studies performed using various transmission electron microscopy (TEM) methods.

A Co-Zn-Mn alloy was grown using a self-flux method in a closed, tapered quartz crucible placed on a cold finger in a buffered box furnace starting from a melt of composition Co3Zn6Mn1. Following a dedicated temperature transient with final water quenching, a conical ingot was produced, with the tip corresponding to the primarily solidified material (Fig. 1a). TEM specimens were prepared by using focused Ga ion-beam sputtering in a dual-beam scanning electron microscope (FEI Helios). Conventional TEM and electron probe aberration corrected scanning TEM (STEM) were used to study the microstructure of the alloy. Fresnel defocus imaging and off-axis electron holography [2] were carried out in magnetic field-free conditions (Lorentz mode) using aberration-corrected (FEI Titan) TEMs operated at 300 kV.

High-resolution STEM imaging along [100] is consistent with a chiral β-Mn-type structure (Fig. 1b). The chemical composition was measured to be Co8Zn10Mn2 using inductively-coupled optical emission spectroscopy. The magnetic transition temperature was determined by observing magnetic contrast elimination during in situ heating of a TEM specimen in magnetic field-free conditions. The helical spin texture and skyrmions disappeared in Fresnel defocus images at 98 ºC, which was assumed to correspond to Tc. Figure 1c shows a Fresnel defocus image illustrating skyrmion lattice formation in the Co8Zn10Mn2 specimen. Quantitative magnetic characterization of the skyrmion spin structure was carried out by combining phase shift measurements obtained using off-axis electron holography with model-based iterative reconstruction of the projected in-plane magnetization, as well as through comparisons of the recorded contrast with micromagnetic simulations.

This work was supported by the European Union"s Horizon 2020 Research and Innovation Programme (Grant No. 856538, project ""3D MAGiC"") and the Deutsche Forschungsgemeinschaft through CRC/TRR 270 (Project ID 405553726).

Figure 1. (a) Flux-grown Co8Zn10Mn2 ingot. (b) High-resolution HAADF STEM image along [100] with overlaid structure model. (c) Fresnel defocus image of a skyrmion lattice recorded at 86 ºC in the presence of an 89 mT field applied perpendicular to the surface plane.

References

[1] Y. Tokunaga, et al., Nat. Comm. 6 (2015) 7638.

[2] A. Kovács, R.E. Dunin-Borkowski, In Handbook of Magnetic Materials; Brück, E., Ed.; Elsevier: 2018; Vol. 27, pp 59-153.