VP12-02 Thermal stability of skyrmions in polycrystalline thin film.
Creators: Shuto Onaka
Terumitsu Tanaka
Shusuke Kashiwagi and Xiaorui Ya
Publisher: name: IEEE
Year: 2025

In recent years, there has been a great deal of research on skyrmions that can be used as magnetic memories [1-2]. LLG and the nudged elastic band (NEB) calculations estimated magnetic energy for skyrmion configured in magnetic thin film pattern consisted of fine grains where the magnetizations of the neighboring grains coupled through weak inter-gain exchange interactions in the present study. A magnetic thin film pattern was modeled by 0.5-nm-thick and laterally 256 nm square. The averaged values of the perpendicular anisotropy field, the saturation magnetization and Voronoi grain size were 14 kOe, 800 emu/cc and 8 nm, respectively. DMI constant was 1.5 erg/cm2. Figure 1 shows diameters of the skyrmion for relax state as a function of the inter-grain exchange constant, where large area (16% of the film pattern size) initial reverse domain was artificially introduced before the magnetization relaxations. Theoretical diameter of skyrmion for single crystal film is 26 nm[3]. The figure indicates that skyrmion diameter increases with decreasing the inter-grain exchange constant due to non-flat energy landscapes, causing domain wall pinning at grain boundaries in relaxation process. Energy barrier height ΔE was derived using NEB calculations. There are two typical cases need to be considered. One is skyrmion collapse, where magnetizations of the center of the skyrmion switch, and the other is boundary annihilations. Magnetic structure of the skyrmion is topologically stable, so resultant ΔE/kBT were high in the case of the skyrmion collapse. On the other hand, ΔE/kBT for the boundary annihilation case are low especially for the film with large inter-grain exchange constant. Figure 2 shows the thermal stability indexes (ΔE/kBT) for the two typical cases as a function of the diameters of the skyrmions. The figure indicates that ΔE/kBT are proportional to the diameter of skyrmions. Probable reason is that magnetostatic energies are lower when the diameter of skyrmions are larger.References: [1] R. Tomasello, E. Martinez, R. Zivieri, L. Torres, M. Carpentieri, G. Finocchio, Scientific Reports 4, 6784 (2014). [2] J. Ding, X. Yang, and T. Zhu, J.Phys.D:Appl. Phys. 48(11), 115004 (2015). [3] X. S. Wang, H. Y. Yuan, X. R. Wang, Nat. Commun. Phys. 1, 31 (2018).
In recent years, there has been a great deal of research on skyrmions that can be used as magnetic memories [1-2]. LLG and the nudged elastic band (NEB) calculations estimated magnetic energy for skyrmion configured in magnetic thin film pattern consisted of fine grains where the magnetizations of the neighboring grains coupled through weak inter-gain exchange interactions in the present study. A magnetic thin film pattern was modeled by 0.5-nm-thick and laterally 256 nm square. The averaged values of the perpendicular anisotropy field, the saturation magnetization and Voronoi grain size were 14 kOe, 800 emu/cc and 8 nm, respectively. DMI constant was 1.5 erg/cm2. Figure 1 shows diameters of the skyrmion for relax state as a function of the inter-grain exchange constant, where large area (16% of the film pattern size) initial reverse domain was artificially introduced before the magnetization relaxations. Theoretical diameter of skyrmion for single crystal film is 26 nm[3]. The figure indicates that skyrmion diameter increases with decreasing the inter-grain exchange constant due to non-flat energy landscapes, causing domain wall pinning at grain boundaries in relaxation process. Energy barrier height ΔE was derived using NEB calculations. There are two typical cases need to be considered. One is skyrmion collapse, where magnetizations of the center of the skyrmion switch, and the other is boundary annihilations. Magnetic structure of the skyrmion is topologically stable, so resultant ΔE/kBT were high in the case of the skyrmion collapse. On the other hand, ΔE/kBT for the boundary annihilation case are low especially for the film with large inter-grain exchange constant. Figure 2 shows the thermal stability indexes (ΔE/kBT) for the two typical cases as a function of the diameters of the skyrmions. The figure indicates that ΔE/kBT are proportional to the diameter of skyrmions. Probable reason is that magnetostatic energies are lower when the diameter of skyrmions are larger.References: [1] R. Tomasello, E. Martinez, R. Zivieri, L. Torres, M. Carpentieri, G. Finocchio, Scientific Reports 4, 6784 (2014). [2] J. Ding, X. Yang, and T. Zhu, J.Phys.D:Appl. Phys. 48(11), 115004 (2015). [3] X. S. Wang, H. Y. Yuan, X. R. Wang, Nat. Commun. Phys. 1, 31 (2018).