Precession torque induced dynamic skyrmion creation on a circularly confined magnetic nanostructure

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-Precession torque induced dynamic skyrmion creation on

a circularly confined magnetic nanostructure

June-Seo Kim1,2,3,*

1_{DGIST-LBNL Research Center for Emerging Materials,}

Daegu Gyeongbuk Institute of Science and Technology, Daegu 42988, Republic of Korea

2_{Intelligent Devices and Systems Research Group,}

3_{Global Center for Bio-Convergence Spin System,}

1. Introduction

Skyrmionics, the nucleation and manipulation of individual skyrmions in magnetic structures for future logic applications and non-volatile memory devices, has been demonstrated by J. Sampaio and colleagues in 2013 and they have introduced a new concept of the magnetic racetrack memory based on the individual skyrmion motion induced by means of spin-polarized currents [1]. After this remarkable progress, various approaches to manipulate individual skyrmions for many other applications are extensively investigated [2,3]. However, extremely low energy consumption and ultrafast operation are strongly required for all kinds of applications based on skyrmions.

In this study, we numerically demonstrate a totally new method to nucleate magnetic skyrmion states on a nanoscale disk by applying in-plane magnetic field pulses with varying the Dzyaloshinskii-Moriya interaction (DMI) energy density [4-7] and the uniaxial anistropy energy density.

2. Simulation Details

The simulations are carried out by performing the object-oriented micromagnetic framework (OOMMF) simulator including DMI. The material parameters are chosen as follows: The saturation magnetization Ms = 1.1 MA/m, the exchange stiffness Aex=16 pJ/m, the uniaxial magnetic anisotropy KU=1.0 MJ/m3, the DM energy

density D = +5.0 mJ/m. The profile of the in-plane field pulse is following: pulse amplitude Bx = 10 mT, rise

time RT = 10 ps, duration time DT = 10 ps, fall time FT = 10 ps and the total simulation time = 10 ns. For systematic investigation of the precessional torque induced skyrmion creation, we vary the magnetic damping constant from 0.05 to 0.20 and the uniaxial magnetic anisotropy from 0.7 MJ/m3 _{to 1.0 MJ/m}3 _{and the DM energy}

density from 2.0 mJ/m to 5.0 mJ/m.

3. Results and Discussion

The precession torque introduced by in-plane magnetic field pulses exert to rotate the magnetization on a confined structure [8,9] and the position dependent precession torque is utilized as the source of skyrmion nucleation. Due to the damping torque, the precession torque is diminished as a function of time and then the topologically stable skrymion state as the final state can be formed. From the systematic simulations, the multiple skyrmion states, which depends on the diameter of nanoscale disk and the magnetic damping constant of the system are observed. We highlight that ultrafast Oersted field pulses in picosecond regime are enough to arise

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-the dynamic skyrmion nucleation process. This new method to create skyrmion states can open a new path to fabricate ultrafast logic devices and non-volatile memory devices based on skyrmions.

4. Conclusion

In conclusion, an ultrafast skyrmion creation by applying an extretemly low (< few mT) in-plane external magnetic field pulse is numerically achieved. Due to the stray field as a function of the diameter of the disk, multiple skyrmion state (pi, 2pi, 3pi, and 4pi states) are also observed.

5. References

[1] A. Fert, V. Cros, and J. Sampaio, Nature Nanotech. 8, 152-156 (2013). [2] S. Woo, et al., Nature Mater. 15, 501-506 (2016).

[3] J. Cho, et al., Nature Commun. 6, 7635 (2015).

[4] N.-H. Kim, et al., Appl. Phys. Lett. 107, 142408 (2015). [5] D.-S. Han, et al., Nano Lett. 16, 4438 (2016).

[6] N.-H. Kim et al., AIP Advances 7, 035213 (2017).

[7] F. C. Ummelen, D.-S. Han, J.-S. Kim, H. J. M. Swagten, and B. Koopmans, IEEE Trans. Magn. 51, 6000703 (2015).

[8] J.-S. Kim et al., Nature Commun. 5, 3429 (2014).

[9] M. J. G. Peeters, F. C. Ummelen, M. L. M. Lalieu, J.-S. Kim, H. J. M. Swagten, and B. Koopmans, AIP Advances 7, 055921 (2017).