Tunable eccentricity of Bloch skyrmions in Fe_1.9Ni_0.9Rh_0.2P

Abstract text (incl. figure legends and references)

1. Introduction

The experimental discovery of antiskyrmions in Mn_1.4PtSn [1] has opened new pathways towards using microscopically small magnetic whirls as information carriers [2]. For this purpose, a thorough understanding of the detailed three-dimensional spin texture of these magnetic solitons, including their stabilization mechanisms is essential. Quite recently, the compound Fe_1.9Ni_0.9Pd_0.2P was found to host antiskyrmions at room temperature, which can be transformed into skyrmions, e.g., by tuning the external magnetic field [3].

2. Objectives

In this study we investigate this elliptical skyrmion phase as a function of the external magnetic field and temperature in a similar material, Fe_1.9Ni_0.9Rh_0.2P, where Pd was substituted by Rh, to gain further insight into the formation mechanisms of topological spin solitons.

3. Materials & methods

The occurrence and nature of skyrmions in a Fe_1.9Ni_0.9Rh_0.2P lamella fabricated using a focused ion beam (FIB) were studied in a JEOL Jem F-200C by means of off-axis electron holography that allows for the quantitative determination of the projected in-plane magnetic induction B_ip. The sample temperature was controlled with a Mel-Build LN₂ Atmos Defend - In-situ double-tilt LN₂ cooling holder.

4. Results

In Fig. 1 the magnitudes of B_ip in the lamella upon exposure to magnetic fields of 170 mT and 330 mT applied along [001] at room temperature are displayed. In zero field, the skyrmions initially possess an elliptical shape with the semi-major axes parallel to the ⟨110⟩ directions due to dipole–dipole interactions [5, 6]. Increasing the external out-of-plane magnetic field leads to (i) a reduction of the skyrmion size, (ii) an enhancement of B_ip and (iii) a deformation of the skyrmions. The initially elliptical skyrmions are transformed into skyrmions with circular shapes until a critical field is reached, where the system undergoes a transition into the field-polarized phase.

5. Conclusion

The current investigations provide a first insight how the balance between the symmetric and antisymmetric exchange and dipole-dipole interaction can tune the skyrmion structure in Fe_1.9Ni_0.9Rh_0.2P. By combining our experimental results with magnetostatic simulations, we can shed light on the complex interplay of these interactions in the stabilization of topological spin textures in three dimensions.

[1] A. K. Nayak et al., Nature 548 (2017), 561 – 566.

[2] B. Göbel et al., Physics Reports 895 (2021), 1 – 28.

[3] K. Karube et al., Nature Materials 20 (2021), 335 – 340.

[4] K. Karube et al., Adv. Mater. 34 (2022), 2108770.

[5] L. Peng et al., Nat. Nanotechnol. 15 (2020), 181 – 186.

[6] J. Jena et al., Nature Communications 11 (2020), 1115.

Fig. 1: Magnitude of B_ip of Bloch skyrmions in Fe_1.9Ni_0.9Rh_0.2P in external fields of 170 mT and 330 mT, respectively.