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Dynamics of magnetic skyrmion bags induced by spin-transfer torques
Suyeong Jeong,Hee-Sung Han,Dae-Han Jung,Ganghwi Kim,Ki-Suk Lee 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
Magnetic skyrmions are envisioned to be a good candidate for developing the next generation data processing/storage device due to their high stability [1] from the topological charge invariance [2]. In dynamic motions of skyrmions, the topological charge also plays a significant role: propagation direction of the skyrmions relative to the driving forces such as spin-transfer-torque (STT) or spin-orbit-torque (SOT) varies dramatically upon their topological charge, the skyrmion number. Under the STT or SOT, a skyrmion shows a diagonal motion not a parallel motion to the driving force direction, which is known as the skyrmion Hall effect. [3,4] and its direction depends on the sign of the skyrmion number. Recently, a skyrmionium with the skyrmion number of 0 is indicated to cancel out the skyrmion Hall effect by the combination of two skyrmions with opposite signs of skyrmion number [5]. Which reveals that the change in the skyrmion number of magnetic structures can lead to the variety of the skyrmion motions, which contributes to realizing multi-bit memory devices encoded by the skyrmion number. From this point of view, the concept of a skyrmion bag which contains multiple numbers of skyrmions in a larger skyrmion suggests that we can reconfigure the skyrmion number [6]. In this work, we report micromagnetic simulation results on various types of skyrmion bag dynamics under STT. We found that skyrmion bag dynamics varies dramatically depends on the net skyrmion number of skyrmion bags. We believe that our results can contribute to multi-bit information carriers based on skyrmion bags.
Magnetic field tunability of a skyrmion diode
Dae-Han Jung,Hee-Sung Han,Ganghwi Kim,Suyeong Jeong,Ki-Suk Lee 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
Magnetic skyrmions are topologically stable particle-like objects that are hailed as a potential technology for next-generation information storage and other processing applications. [1-5]. For device applications of magnetic skyrmions, it is essential to control the skyrmion motions by electrical means in geometrically confined boundaries. In this work, we show that a current-driven skyrmion motion can be unidirectionally controlled by forming an asymmetric shape of the device. Furthermore, we demonstrate tunability of the unidirectional motions in which the magnetic field can be used to switch on and off a circuit. A key working principle of the unidirectional motion and its tunability is investigated by micromagnetic numerical and analytical calculations. We believe that our results could provide guidelines to design skyrmion-based logic devices including a diode and a transistor element.
Symmetry-breakings for unidirectional skyrmion motions under spin-orbit and -transfer torques
Dae-Han Jung,Hee-Sung Han,Ganghwi Kim,Suyeong Jeong,Ki-Suk Lee 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
Unidirectional control of magnetic skyrmion motions is an essential prerequisite for developing skyrmion- based spintronic devices. Here, we propose a method to achieve the unidirectional motions in a patterned nanowire with a symmetry-breaking by using spin-orbit torque (SOT) and spin-transfer torque (STT). We confirm the validity of the method by using micromagnetic simulation. The simulation results show that the direction of symmetry-breakings changes depending on the application of SOT and STT. We believe that this method can be used as a basic building block for skyrmion-based memory and logic devices with multiple operations.