Spin-orbit torque characterization techniques generally require Hall cross that generally demands lithography resources and time. As the interest in exploring materials for improved SOT performance surges, an efficient technique to characterize SOT efficiencies with minimal processes, equipment, and time is highly desirable. Here, we demonstrate a lithography-free technique to determine the spin-orbit torque efficiency in perpendicular magnetic anisotropy ferromagnetic heterostructure. By utilizing a customized fzour-point probe in a rhombus geometry, harmonic Hall measurement was performed on a blanket films of Pt/Co/Ti structure to characterize the spin-orbit torque efficiency. Due to the non-uniform current distribution across the blanket film, a correction factor is experimentally evaluated using the ratio of the damping-like field measured on the blanket film to that of a fabricated Hall device with identical material heterostructure. Additionally, this correction factor is analytically derived and experimentally shown to be determined by the configuration of the probes and is independent of the structure material. Our measurement reveals that by performing a single calibration process for the particular set of probes, the same correction factor was validated on a second ferromagnetic heterostructure, Ti/Pt/Co/Ta hence it can be applied to other SOT films stack measurements. Our four-probe harmonic Hall technique provides an alternative and swift way for SOT investigations by eliminating multiple lithography processes necessary in conventional approaches.

Blanket Film Spin Orbit Torque Characterization via Four Probe Measurement

Electrical current remains one of the simplest and most accessible tools for the control of magnetic skyrmions. However, it can be challenging to only perform the intended operation among all the possible current-induced effects such as deformation, nucleation, and propagation [1-3]. While current-induced skyrmion nucleation has been demonstrated [1,2], its equally important complementary operation of current-induced skyrmion annihilation remains elusive. Our investigation on the current-induced skyrmion effects revealed an additional regime at low current densities where skyrmion annihilation can be performed via skyrmion-stripe transformation [4]. In this work, an experimental study of current-induced skyrmion-stripe transformation in a Pt/Co/Fe/Ir magnetic bilayer was performed. Using a Hall cross device shown in Fig. 1(a), a slow evolution of the magnetization after current injection was observed and showed distinct features in the high and low current density regimes. High current density injection induced a sharp negative change in magnetization followed by a continuous decay back to equilibrium while low current density induced a gradual positive change in magnetization from equilibrium before decaying back to equilibrium forming a peak as shown in Fig. 1(b). In situ observation under magneto-optical Kerr (MOKE) imaging showed that at high current densities, additional magnetic skyrmions were nucleated as shown in Fig. 2(e-h). In contrast, at low current densities, skyrmions expanded into stripes. The resulting competition for space annihilates skyrmions. Eventually the stripes contract into skyrmions with a lower overall density as shown in Fig. 2(a-d). The mechanism for the skyrmion-to-stripe transformation was further investigated by analyzing the MOKE images of the stripe domains over many runs. The stripe domains were found to nucleate at predictable spots on the wire which can be attributed to pinning sites. Together with the additional requirement of a long current pulse duration of several milliseconds for the transformation to occur, the transformation was concluded to be occurring as the result of a skyrmion-pinning site interaction and creep motion. A skyrmion at a pinning site undergoes slow creep elongation into a stripe due to the weak propagative being insufficient to fully depin the skyrmion. By utilizing the combination of skyrmion nucleation at high current density and skyrmion annihilation at low current density, skyrmion density control was demonstrated purely by current density modulation. In addition, the volatile nature of these current-induced stripes offers interesting possibilities for neuromorphic computing applications such as artificial leaky-integrate fire neurons which requires a continuous decaying property after an excitation, and artificial synapses with short-term plasticity which requires a temporary retention of state after excitation. In conclusion, our work revealed additional current-induced magnetic texture transformations which can be exploited for skyrmion density control using solely current modulation. In addition, our findings establish an additional requirement of a minimum operating current density in the design of skyrmionic devices to avoid unintended skyrmion deletion.

**References:**

[1] W. Jiang, P. Upadhyaya, W. Zhang, Science 349, 283 (2015). [2] F. Büttner, I. Lemesh, M. Schneider, Nat. Nanotech. 12, 1040 (2017). [3] W. Legrand, D. Maccariello, N. Reyren, Nano Lett. 17, 2703 (2017). [4] C. C. I. Ang, W. Gan, G. D. H. Wong, and W. S. Lew, Phys. Rev. Appl. 14, 054048 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a3717886e8df859abbd62b4fff284cdc.jpg)
Fig. 1 (a) Measurement setup consisting of a Hall cross with widths of 20 and 10 μm. (b) Time evolution of Hall resistance change △RHall after 30 current pulses of 10 ms are injected for a range of current densities. Error bars are omitted from the graph for clarity, and the data have a standard deviation of 6ms or less.

![](https://assets.underline.io/uploads/markdown_image/1/image/a4abf908ca6581b85f8680db6164d0c6.jpg)
Fig. 2 MOKE images of current-induced magnetic states and their relaxation process back to equilibrium. (a) Magnetization state at equilibrium. Magnetization state at (b) t = 0.4 s, (c) t = 12.1 s, and (d) t = 41.4 s after injection of 2.21 × 1010 A/m2 current pulses. (e) Magnetization state at equilibrium with skyrmion density of 0.645 μm-2. Magnetization state at (f) t = 0.4 s, (g) t = 12.1 s and (h) t = 41.3 s after injection of 7.20 × 1010 A/m2 current pulses. Skyrmion density are stated below the respective MOKE images.

 Skyrmion Density Modulation via Current-induced Skyrmion-to-stripe Transformation

EOE: Spin-Orbitronics I

technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

The Conference was held from October 31 to November 4 and offers both in-person and on-demand content.

MMM 2022 is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a tutorial, and several special sessions. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

<iframe width="560" height="315" src="https://s3.amazonaws.com/pf-user-files-01/u-59356/uploads/2022-11-02/un13zp7/MMM%20Welcome%20Video%20v5.mp4" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

For tickets to the event, please register at the following link: https://magnetism.org/registration

Please register for the MMM 2022 conference to join the event.

MMM 2022

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

*Session Chair: Guoqiang Yu, Chinese Academy of Sciences*

These on-demand presentations are available to view at your leisure.

HB: Skyrmions: Control and Manipulation

INTERMAG 2021 Welcome Video



	
<figure class="video_container">
 <iframe src="https://screencast-o-matic.com/player/crflqFVnUpZ?v=6&ff=1&title=0&controls=1" width=640 height=360 frameborder="0" allowfullscreen="true"> </iframe>
</figure>

Want to buy a ticket? 

Register for INTERMAG 2021 here

[![](https://assets.underline.io/uploads/markdown_image/1/image/17ce76e7960db633258024eef8d6da7f.png)](https://intermag.org/register)

Got your ticket, but still can’t access the event?

Contact us and our team will look into it.

Calvin Ching Ian Ang

2

10

SHORT BIO

Presentations

Blanket Film Spin Orbit Torque Characterization via Four Probe Measurement

Skyrmion Density Modulation via Current-induced Skyrmion-to-stripe Transformation

Stay up to date with the latest Underline news!

PRESENTATIONS

CONFERENCES

COMPANY

RESOURCES