This successful three year NSF proposal was based on the research results of LATTE graduate students Gen Yin and Yizhou Liu working in close collaboration with Dr. Jiadong Zang at Johns Hopkins. A magnetic skyrmion is a topologically protected, circular, swirling spin texture in which the spins on the periphery are polarized vertically, the central spin is polarized in the opposite direction, and, in between, the spins smoothly transition between the two opposite polarizations. Skyrmions exist in certain helimagnetic materials such as the B20 magnets in which broken inversion symmetry gives rise to the Dzyaloshinskii-Moriya interaction. The radius of a skyrmion ranges from 3 nm to 100 nm depending on the ratio of the Dzyaloshinskii-Moriya interaction and the symmetric Heisenberg interaction. Skyrmion lattices and isolated skyrmions have been observed in bulk and in thin films, and the electrical current required to move a skyrmion is 4 to 5 orders of magnitude less than that required to move a more conventional magnetic domain wall. Because of the their small size, their stability, the demonstration of their individual creation and annihilation, and their facile movement with low current, skyrmions are being investigated for magnetic information storage applications. For such applications, an understanding of the process of skyrmion creation, annihilation, decay, and readout is required. This project will investigate these processes, and it will optimize a new method for low-energy skyrmion creation and annihilation using nanosecond current pulses. Read-out using the topological Hall effect will be analyzed. Four different architectures that exploit a skyrmion state variable for information processing are proposed, and open questions concerning the underlying physical mechanisms and fundamental limits of the skyrmion systems are identified. These questions will be answered using theoretical and computational methods including the Landau-Lifshitz-Gilbert equation, ab initio density functional theory, and the non-equilibrium Green function formalism. For the Landau-Lifshitz-Gilbert dynamical simulations of the spin system, a stochastic field will be applied to include the effect of thermal fluctuations at finite temperature. A new method for low-energy creation and annihilation of skyrmions using a nanosecond current pulse will be optimized. The lattice version of the topological charge will be used to analyze the microscopic dynamical processes. It provides a clear picture of the spin trajectories and orientations that locally trigger a topological transition, it reveals the topological origin of a skyrmion’s stability at finite temperatures, and it signals the exact moment at which a skyrmion is created or destroyed. A coupled Landau-Lifshitz-Gilbert / non-equilibrium Green function approach will be used to investigate the use of the topological Hall voltage for skyrmion readout and the interaction of spin waves with skyrmions for non-boolean, holographic information processing. A successful project could lead to new approaches to spin and topological based computing. The exploitation of low-energy, stable, high-temperature, topologically protected states can result in high-payoff in the technologies of memory and computation.
Figure created by UCR graduate student Gen Yin from the submitted manuscript, G. Yin, Y. Li, L. Kong, R. K. Lake, C. L. Chien, and J. Zang, ‘Topological Charge Analysis of Single Skyrmion Creation with a Nanosecond Current Pulse.’