In-situ correlation of the anomalous Hall effect with the occurrence of topological and non-topological magnetic phases in Mn_1.4PtSn

Abstract text (incl. figure legends and references)
Introduction

(Anti-)skyrmions are potential future nanoscale information carriers, since they can be electrically manipulated and detected. So far, Hall effect measurements on skyrmionic samples are conducted on samples with different thicknesses and confinements as compared to those used for magnetic imaging in a transmission electron microscope (TEM) [1,2], which has proven extremely valuable for unveiling the details of skyrmionic spin textures even in 3D [3]. Since the stability of (anti-)skyrmions depends sensitively on the sample geometry, a correlation of magneto-transport and TEM data are problematic, if not conducted on identical samples.

Objectives

We have therefore devised an in-situ measurement platform that bridges this gap and allows for the conduction magneto-transport measurements in-situ in a TEM. The current investigation aims at correlating the anomalous Hall effect in the Heusler compound Mn_1.4PtSn with the magnetic field dependent occurrence of non-topological (NT) and topologically protected magnetic phases such as the helical phase, NT bubbles and anti-skyrmions.

Materials & methods

In-situ Lorentz-TEM (L-TEM) investigations were conducted on a JEOL F-200 micro­scope (200 kV, cold FEG) equipped with a Gatan OneView camera. A Protochips Fusion Select holder with gold-plated spring contacts connected to electrical feedthroughs is used to measure the anomalous Hall effect. A TEM lamella was cut from a Mn_1.4PtSn single crystal [4] using a focused ion beam system (FIB), deposited on a measurement chip with a SixN window, and soldered to the pre-defined contact pads by Pt deposition in the FIB.

In Lorentz mode, the objective lens of the microscope was used to apply a magnetic field perpendicular to the sample plane. Acquisition of L-TEM images and concurrent measurements of the Hall voltage as function of the magnetic field were conducted automatically utilizing Python scripting.

Results

The L-TEM-images in Fig. 1 reveal that upon increasing the magnetic field B (i) the density of helices (as manifested by a stripe contrast) is reduced and (ii) the magnetization process is governed by growth of (preferred) helical domains – very alike that of conventional ferromagnets. At fields above roughly B = 300 mT, where the Hall voltage exceeds the otherwise linear increase with B (cf. Fig. 2), we observe (i) a transition from a continuous to a discontinuous helical phase and (ii) finally the formation of NT bubbles and/or anti-skyrmions, which then slow down the approach towards saturation. The formation of anti-skyrmion lattices was, however, only observed after subsequent variations of the magnitude and direction of B. The subtle effect of the latter on the Hall effect of the sample is subject of ongoing research and will be discussed.

Conclusion

Our new setup allows us for the first time to follow in detail the field dependence of the Hall voltage while simultaneously monitoring the magnetic phases in Mn_1.4PtSn, thereby providing valuable insights into the existence and nature of an intensely debated electrical signature of skyrmionics structures.

A.T. and B.R. gratefully acknowledge financial support by the DFG within SPP 2137. M.W. thanks the Max Planck Society for funding through IMPRS-CPQM.

[1] M. J. Stolt et al., Adv. Func. Mater. 2019 (2019) 1805418.

[2] R. Schlitz et al., Nano Lett. 19 (2019) 2366.

[3] D. Wolf et al., Nature Nanotechnology 17 (2021) 250.

[4] P. Vir et al., Chem. Mater. 31 (2019) 5876.