Photo-Thermal Tweezer for Magnetic Skyrmions

Magnetic skyrmions are localized magnetization swirls with non-zero topological charges. Magnetic skyrmions exhibit interesting topological characteristics such as high stability against external perturbations, or various topological phenomena including the skyrmion Hall effect. Furthermore, their small size, ranging from a few nanometers to microns, and electrical mobility with very low threshold current present strong advantages for their potential use as information carriers in future low-power spintronic devices. However, precise and individual control of skyrmions using electrical manipulations remains challenging. Skyrmions deflect due to the skyrmion Hall effect at high current densities and stochastic motion at low current densities. Achieving local and precise control of topological spin textures is crucial for enhancing the reliability of skyrmionic devices and studying interactions between spin textures based on their topologies.<br/>In this research, we present the photo-thermal tweezer for individual control of magnetic skyrmions. Non-uniform spin configurations in ferromagnetic media possess higher energy compared to a uniform state. In the presence of interfacial Dzyaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy (PMA), the domain wall energy density can be formulated as σ=4 √(AK_eff )-πD+(ln 2 /π) μ₀M_s²d, where A is exchange stiffness, K_eff is PMA energy density per volume, D is DMI energy density per surface, and M_s is the saturation magnetization. These magnetic coefficients decrease monotonically with increasing temperature, reducing the total energy density. Consequently, a locally nonuniform temperature distribution creates a potential energy landscape drifting the adjacent spin textures toward the local energy minima. Using photo-thermal effect from focused laser, we can create a micron-scale hot spot which acts as a potential well. The sharp temperature gradient surrounding the beam spot creates a potential barrier that prevents spin textures from escaping. By moving the beam position, we can pinch and drag local spin textures quickly and efficiently.<br/>We used MgO (1 nm) / Ta (0.1 nm) / CoFeB (1.2 nm) / W (3 nm) / TaO_x (3 nm) stack on a Si/SiO₂ wafer. The deposited sample was subjected to post-baking process for 1 hour at 350 under Torr vacuum. Real-time magnetization states were measured using a magneto-optical Kerr effect microscopy setup. Local photo-thermal heating was induced by a focused continuous-wave 638-nm laser. A piezo-electric motor moves the sample, allowing the focused beam to sweep across it. The laser beam is focused onto a magnetic skyrmions, and the sample is moved along in one direction. The skyrmion got trapped within beam spot and dragged along the sweep direction. With this photo-thermal tweezer, we could capture and arrange the skyrmions into arbitrary patterns. Furthermore, we also have demonstrated the photo-thermal trapping of a single skyrmion under external electrical current. Our findings suggest that this photo-thermal approach holds great potential for realizing novel skyrmionic devices and advancing our understanding of interaction between skyrmions.