Magnetic Domain With Straight Domain Walls Passing Through a Ferromagnetic Insulator for a Continuously Looped Racetrack Memory

We demonstrate via micromagnetic simulations that a magnetic domain, which has two straight domain walls (DWs) and is formed in a perpendicularly magnetized ferromagnetic metal (FM), moves through a perpendicularly magnetized ferromagnetic insulator (FI) because of the spin-transfer torque arising from a spin-polarized current flowing in the FM. This phenomenon could be used for realizing racetrack memory with a continuous loop structure built from a nanowire comprising an FM/FI junction. We employ two nanowire structures in which two FMs are separated by a rectangular or a zigzag FI. In the system with a rectangular FI, the domain passes through the insulator when the length of the insulator is shorter than the DW length. However, the domain transforms into an unstable structure with current-induced domain motion after it passes through the insulator. In the system with a zigzag FI, the domain passes through the insulator irrespective of the length of the insulator. When passing through the zigzag FI, the velocity of the domain increases.