Using Magnetic Force Microscopy to Investigate Hosts for Anti-Skyrmions

Skyrmions and antiskyrmions can be understood as quasiparticles which are based on topological spin arrangements.

Their high stability and small size makes these quasiparticles a popular subject of research in spintronics applications neuromorphic computing, high density data storage and high density data transfer.^1,2

This article looks at work based on two key publications in this field: Sukhanov et al. https://doi.org/10.1103/PhysRevB.102.174447, and Zuniga Cespedes et al. https://doi.org/10.1103/PhysRevB.103.184411.

Each of these studies is focussed on the impact of geometrical confinement on the formation of antiskyrmions in Mn_1.4PtSn – a tetragonal Heusler compound.^3,4

The researchers employed magnetic force microscopy (MFM) to resolve a thickness-dependent formation of either a spin helix pattern or a fractal ferromagnetic domain pattern which acted as a template for antiskyrmion nucleation.

Benefits and Applications of Skyrmions and Antiskyrmions

The world is becoming increasingly digitalized, and it has been estimated that this results in the production of 2.5⋅10¹⁸ bytes of data every day.⁵ The sheer amount of data that must be accommodated has increased demand for new data storage technologies able to contain and manage this ever-expanding tide of information.

Skyrmions and antiskyrmions represent an innovative solution for smaller, more rapid and more energy-efficient data storage.⁶

Skyrmions and antiskyrmions are topological spin structures that are arranged in small vortices. They are typically between 100 µm and 1 nm in size,⁸ and they function as quasiparticles in the magnetic texture of some host materials.⁷

Their size, stability and response to external magnetic or electric fields has led to an extensive interest in the application of skyrmions and antiskyrmions in spintronics, particularly in terms of their potential for high density data storage, data transfer and neuromorphic computing.^1,2

Studies have shown that antiskyrmions are present in some tetragonal Heusler compounds, most notably Mn_1.4PtSn.

It is necessary to employ an imaging method with high magnetic sensitivity and nanoscale spatial resolution to locally resolve magnetic patterns on such compounds, particularly when looking to explore their dependence on the geometry of the sample.

All Park Systems NX research Atomic Force microscopes (AFM) feature magnetic force microscopy (MFM) capabilities. These capabilities enable simultaneous and real-space imaging of a sample’s surface topography and its magnetic structure.

MFM makes use of magnetized AFM cantilevers, which enable the accurate imaging of magnetic nanostructures – imaging which is possible due to the cantilevers’ discrimination and sensitivity towards magnetic tip-sample and van der Waals (vdW) interactions.

Magnetic and vdW forces can both be measured via differences in the cantilever’s oscillation properties. These properties are reflected by changes in oscillation amplitude, phase or resonance frequency.

MFM employs the varying distance dependence of van der Waals and magnetic force in order to extricate vdW from magnetic contributions.

Van der Waals interactions tend to dominate the tip-sample force at close proximity, meaning it is possible to accurately image sample topography. When working at more extended distances of 50-500 nm, however, van der Waals interactions are minimal, and magnetic forces will primarily dictate tip-sample interaction.

Each scan line is traced twice during MFM analysis. The initial run images the sample topography, while the second run records the sample’s magnetic texture information (Figure 1).

Figure 1. Dual pass MFM mode with the detection of the sample topography in the first pass and the detection of the magnetic structure in the second pass⁹. Image Credit: Park Systems Europe

The cantilever follows the previously recorded topographic profile in order to maintain a consistent tip-sample distance between passes. This ensures that magnetic interactions are recorded at the same nominal distance.

The resulting oscillation phase shift provides information on the magnetic moment’s local magnetic polarization while also resolving ferromagnetic domain patterns, for example, the antiskyrmion lattice present in the sample.

Experimental

In order to demonstrate this principle, a Park Systems NX10 AFM was utilized in an investigation into the magnetic properties of a single crystal and thin lamellas of Mn_1.4PtSn.

The investigation was conducted in ambient conditions, while the sample was mounted on a non-magnetic sample holder in order to minimize any risk of magnetic crosstalk.

A cantilever featuring a magnetic coating (PPP-MFMR, Nanosensors) was magnetized prior to operation. This was then used for all MFM experiments.

MFM measurements were performed in dual pass scanning mode (Figure 1).

The initial pass saw the topography imaged in Park’s True Non-Contact™ Mode before the cantilever was lifted by between 100 nm and 150 nm in order to detect the magnetic stray field via cantilever oscillation demodulation. This was done while retracing the topographic profile (Figure 1).

The single Mn_1.4PtSn crystals investigated in this study had been grown using the flux method.¹⁰ Once the crystals had been grown correctly, they were then polished along the ab plane using a focused xenon ion beam in order to ensure sufficiently smooth surfaces for imaging (Figure 2).⁴

In order to investigate the thickness dependence on magnetization, it was necessary to create suspended lamellae of thicknesses ranging from 400 nm to 10 µm. This was done by cutting a 100×100 µm² groove into the single crystal’s side (Figure 3a).³

Figure 2. a) Sample topography b)-d) Fractal ferromagnetic domain pattern in the MFM phase of bulk crystals of Mn_1.4PtSn.⁴ Image reproduced with permission: Phys. Rev. B 102, 174447 – Published 30 November 2020. Image Credit: Park Systems Europe

Results and Discussion

Existing studies have demonstrated the presence of ultrathin layers of antiskyrmions in Mn_1.4PtSn,^10,11 but MFM measurements performed on the polished ab plane of a bulk Mn_1.4PtSn single crystal at ambient temperature did not reveal an antiskyrmion lattice.

It was instead noted that the sample exhibited a fractal ferromagnetic domain pattern comprised of lamellar stripes with widths up to 3 µm (Figure 2b).

Examining this pattern at increased magnification (Figure 2c and Figure 2d) revealed further smaller nested domains. These domains had an arrowhead shape and exhibited opposite orientation around domain walls, therefore, introducing chirality to the sample.

Chirality around domain walls would suggest a Dzyaloshinski-Moriya interaction (DMI) is present. This anisotropic magnetic exchange interaction is key to the formation of skyrmions and antiskyrmions.

The study undertaken by Sukhanov et al., therefore, demonstrates that bulk Mn_1.4PtSn single crystals typically favor a ferromagnetic ground state dominated by a lamellar stripe pattern, as opposed to antiskyrmion lattice formation.⁴

Figure 3. a) Mn_1.4PtSn single crystal with groove cut along the ab plane to create a suspended thin plate with defined thickness. b- e) Magnetic patterns at varying plate thicknesses with the appearance of a sinusoidal spin helices pattern.³ Image reproduced with permission: Phys. Rev. B 103, 184411 – Published 12 May 2021. Image Credit: Park Systems Europe

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References

1. Zhang, X. et al. Skyrmion-electronics: writing, deleting, reading and processing magnetic skyrmions toward spintronic applications. J. Phys. Condens. Matter 32, 143001 (2020).
2. Büttner, F., Lemesh, I. & Beach, G. S. D. Theory of isolated magnetic skyrmions: From fundamentals to room temperature applications. Sci. Rep. 8, 4464 (2018).
3. Zuniga Cespedes, B. E., Vir, P., Milde, P., Felser, C. & Eng, L. M. Critical sample aspect ratio and magnetic field dependence for antiskyrmion formation in Mn_1.4PtSn single crystals. Phys. Rev. B 103, 1–7 (2021).
4. Sukhanov, A. S. et al. Anisotropic fractal magnetic domain pattern in bulk Mn_1.4PtSn. Phys. Rev. B 102, 1–9 (2020).
5. Price, D. How much Data is produced every day? https://cloudtweaks.com/2015/03/how-much-data-is-produced-every-day/.
6. Karube, K. et al. Room-temperature antiskyrmions and sawtooth surface textures in a non-centrosymmetric magnet with S4 symmetry. Nat. Mater. 20, (2021).
7. Koshibae, W. & Nagaosa, N. Theory of antiskyrmions in magnets. Nat. Commun. 7, (2016).
8. Nagaosa, N. & Tokura, Y. Topological properties and dynamics of. Nat. Publ. Gr. 8, 899–911 (2013).
9. Pineda, J. P., Newcomb, C., Pascual, G., Kim, B. & Lee, K. Detection of Magnetization Reversal in Magnetic Patterned Array using Magnetic Force Microscopy. Park Syst. Appl. Note 17 (2017).
10. Vir, P. et al. Tetragonal superstructure of the antiskyrmion hosting Heusler compound Mn1. 4PtSn. Chem. Mater. 31, 5876–5880 (2019).
11. Saha, R. et al. Intrinsic stability of magnetic anti-skyrmions in the tetragonal inverse Heusler compound Mn 1.4 Pt 0.9 Pd 0.1 Sn. Nat. Commun. 10, 1–7 (2019).

Acknowledgements

Produced from materials originally authored by B. E. Zuniga Cespedes, A. S. Sukhanov, P. Milde, and L. M. Eng from the Institute of Applied Physics, Technische Universität Dresden; and I. Hermes and A. Klasen from Park Systems Europe GmbH.

This information has been sourced, reviewed and adapted from materials provided by Park Systems Europe.

For more information on this source, please visit Park Systems Europe.