Multi-bit magnetic memory based on domain wall motion in a vertical nanopillar with two perpendicularly magnetized pinning layers

Magnetic memory devices based on vertical nanopillars are attractive for high-density, nonvolatile storage due to their thermal stability and scalability. However, achieving reliable multi-bit operation remains challenging, particularly in controlling magnetization at the pillar base, motivating the need for new architectures that enable precise domain wall manipulation. We propose a novel, high-density, multilevel magnetic memory structure capable of storing three or more bits per nanopillar. The design comprises a vertically magnetized ferromagnetic nanopillar placed atop two orthogonally magnetized pinning layers. The magnetization state within the pillar is governed entirely by domain wall motion, induced by a spin-polarized current flowing from the pillar into one of the pinning layers. As the two pinning layers are magnetized in opposite directions, the current direction selectively triggers domain wall nucleation and propagation at the base of the pillar. Micromagnetic simulations confirm that this mechanism enables stable and reliable writing of one- and two-bit magnetization states within a single pillar. Furthermore, we demonstrate the feasibility of storing three or more bits per pillar by sequentially generating multiple domain walls through precise modulation of the spin-polarized current. For practical deployment, we propose a random-access circuit architecture that integrates selector transistors or diodes, enabling independent control of each nanopillar in a memory array. These results highlight the potential of the proposed structure as a compact, scalable, and energy-efficient platform for next-generation high-density, multi-bit magnetic memory technologies.